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InfoMagic Standards 1994 January
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InfoMagic Standards - January 1994.iso
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ccitt
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1988
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troff
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9_1_01.tro
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9_1_01
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1991-12-13
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4,565 lines
.rs
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\fBSeries\ K\ Recommendations\fR \v'2P'
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.OF ''' \ \ \ ^ %'
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\fBMONTAGE:\fR \ PAGE 2 = PAGE BLANCHE
.sp 1P
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\fBPROTECTION\ AGAINST\ INTERFERENCE\fR
.FS
See the CCITT manual
\fIDirectives concerning the protection of telecommunication lines against\fR
\fIharmful effects from electric power and electrified railway lines\fR
, ITU,\fR
Geneva, 1988 (see also Recommendation\ K.26).
.FE
.ce 0
.sp 1P
.sp 2P
.LP
\fBRecommendation\ K.1\fR
.RT
.sp 2P
.ce 1000
\fBCONNECTION\ TO\ EARTH\ OF\ AN\ AUDIO\(hyFREQUENCY\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.1''
.OF '''Volume\ IX\ \(em\ Rec.\ K.1 %'
.ce 0
.sp 1P
.ce 1000
\fBTELEPHONE\ LINE\fR \fB\ IN\ CABLE\fR
.ce 0
.sp 1P
.ce 1000
\fI(New Delhi, 1960)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.LP
\fBIntroduction\fR
.sp 1P
.RT
.PP
The present state of technique is such that cables can now be so
manufactured that the capacitances of the various circuits at
audio\(hyfrequencies, with respect to the sheath, are very exactly balanced.
.PP
This balance of the capacitances is adequate in the case of circuits having
no unbalanced connections to earth.
.PP
On the other hand, every connection to earth, even with apparent
balance, is likely to involve the inductance and resistance unbalances
of each of the circuits to which such an earth connection is made.
.PP
The dielectric strength between the conductors of a cable is
appreciably less than that between the conductors and the sheath and,
consequently, the connection to earth of some of these conductors would
create a danger of breakdown of the dielectric separating the conductors
when the
cable is subjected to severe induction.
.PP
When a loaded cable is subjected to a high induced electromotive
force, the presence of connections to earth would permit a flow of current
the value of which could, in some cases, exceed the limit for avoiding
deterioration of the magnetic properties of
loading coils
.
.RT
.sp 1P
.LP
\fIFor these reasons, the CCITT makes the following unanimous\fR
\fIrecommendations:\fR
.sp 9p
.RT
.PP
No earth connection should be made at any point whatsoever on an
audio\(hyfrequency circuit, unless all the line windings of the transformers
are permanently connected to the sheath by low resistance connections at
one or
both ends of the cable.
.PP
As a general rule, it is desirable not to make any earth connection at
any point whatsoever on an installation (telephone or telegraph) connected
metallically to a long\(hydistance line in cable.
.PP
However, if, for special reasons, an earth connection must be made to an
installation directly connected to audio\(hyfrequency circuits, the following
precautions should be taken:
.RT
.LP
a)
The earth connection must be made in such a manner as not to affect the
balance of the circuits with respect to earth and
with respect to the neighbouring circuits.
.LP
b)
The
breakdown voltage
of all the other conductors of the cable, with respect to the conductors
of the circuit connected to earth,
must be appreciably greater than the highest voltage which, owing
to induction from neighbouring electricity lines, could exist
between these conductors and those of the circuit connected to
earth.
.LP
c)
When the installation connected to the cable is a telegraph installation,
it is also necessary to conform to
CCTT\ Recommendations concerning the conditions for coexistence
of telephony and telegraphy (Series\ H\ Recommendations).
.bp
.sp 2P
.LP
\fBRecommendation\ K.2\fR
.RT
.sp 2P
.ce 1000
\fBPROTECTION\ OF\ REPEATER\ POWER\(hyFEEDING\ SYSTEMS\fR \fB\ AGAINST\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.2''
.OF '''Volume\ IX\ \(em\ Rec.\ K.2 %'
.ce 0
.sp 1P
.ce 1000
\fBINTERFERENCE\ FROM\ NEIGHBOURING\ ELECTRICITY\ LINES\fR
.ce 0
.sp 1P
.ce 1000
\fI(New Delhi, 1960)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
To avoid
interference to the power feeding of repeaters
, either by magnetic induction from a neighbouring electricity line or as the
result of resistance coupling with a neighbouring electricity line, the
CCITT recommends that, whenever possible, the repeater power\(hyfeeding
system should be so arranged that the circuit in which the power\(hyfeeding
currents circulate
(including the units connected to it) remains balanced with respect to the
sheath and to earth.
.sp 1P
.RT
.sp 2P
.LP
\fBRecommendation\ K.3\fR
.RT
.sp 2P
.ce 1000
\fBINTERFERENCE\ CAUSED\ BY\ AUDIO\(hyFREQUENCY\ SIGNALS\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.3''
.OF '''Volume\ IX\ \(em\ Rec.\ K.3 %'
.ce 0
.sp 1P
.ce 1000
\fBINJECTED\ INTO\ A\ POWER\ DISTRIBUTION\ NETWORK\fR
.ce 0
.sp 1P
.ce 1000
\fI(New Delhi, 1960)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
In the event of the use by electricity authorities of
audio\(hyfrequency signals injected into the power distribution network
for the operation of
remote control systems
, such signals may cause
interference to neighbouring telecommunication lines.
.sp 1P
.RT
.PP
Calculation of such interference may be carried out, using the
formulae in the \fIDirectives\fR , and finding the values of the equivalent
disturbing voltages and currents for these audio\(hyfrequency signals.
.sp 2P
.LP
\fBRecommendation\ K.4\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBDISTURBANCE\ TO\ SIGNALLING\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.4''
.OF '''Volume\ IX\ \(em\ Rec.\ K.4 %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
In order to reduce interference to direct current signalling or
to alternating current signalling at mains frequencies on telecommunication
lines on open wires, in aerial or underground cables, or on composite lines,
arising from neighbouring alternating or direct current electricity lines,
the possibility should be examined of adopting one or more of the following
methods in each case where such interference appears liable to be produced
or where it has been observed to exist:
.sp 1P
.RT
.LP
\(em
development and use of telecommunication systems:
.LP
a)
in which the balance to earth of the signalling circuit is maintained
in all circumstances, even during switching
operations (see\ [1]);
.LP
b)
in which, besides being balanced, interference in such
systems due to
longitudinal currents
arising from direct or
indirect earth connections is avoided;
.LP
\(em
choice of site for
telephone exchange earths
so that, as far as possible, they are, in particular, remote from electric
traction
lines and also from the
earth electrodes
of power systems;
.LP
\(em
adoption of measures for reducing induced currents (use of
telephone cables with a low
screening factor
, use of
booster
transformers
in single\(hyphase traction lines, etc.) to
facilitate the use of existing signalling systems;
.LP
\(em
use of
neutralizing transformers
or use of the active reduction system in telecommunication circuits to
compensate currents
produced by induced voltages;
.LP
\(em
use of tuned circuits to provide a high impedance at the
frequency of the interfering current.
.bp
.PP
\fINote\fR \ \(em\ The \fIDirectives concerning the protection of\fR
\fItelecommunication lines against harmful effects from electric power and\fR
\fIelectrified railway lines\fR mention a limit of 60\ V for the voltage
induced
into telecommunication lines. This limit of 60\ V concerns only the safety of
personnel and should not be taken to be a limit for the purpose of ensuring
that there is no interference to signalling systems. In the case of unbalanced
signalling systems, such interference may be caused by much lower voltages,
as is mentioned in\ [2].
.sp 2P
.LP
\fBReferences\fR
.sp 1P
.RT
.LP
[1]
CCITT manual \fIDirectives concerning the protection of telecommunication\fR
\fIlines against harmful effects from electric power and electrified railway\fR
\fIlines\fR ,\fR Vol.\ IX, ITU, Geneva,\ 1988.
.LP
[2]
\fIIbid\fR ., Vol.\ VI.
.sp 2P
.LP
\fBRecommendation\ K.5\fR
.RT
.sp 2P
.ce 1000
\fBJOINT\ USE\ OF\ POLES\ FOR\ ELECTRICITY\ DISTRIBUTION\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.5''
.OF '''Volume\ IX\ \(em\ Rec.\ K.5 %'
.ce 0
.sp 1P
.ce 1000
\fBAND\ FOR\ TELECOMMUNICATIONS\fR
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
Administrations that wish to adopt joint use of the same supports for open\(hywire
or aerial cable telecommunication lines and for electricity lines are recommended,
when national laws and regulations permit such an arrangement, to take
the following general considerations into account:
.sp 1P
.RT
.LP
1)
There are economic and aesthetic advantages to be derived
from the joint use of poles by Administrations and electricity
authorities.
.LP
2)
When suitable joint construction methods are used, there
is, nevertheless, some increased likelihood of danger by comparison with
ordinary construction methods, both to staff working on the telecommunication
line and to the telecommunication installation connected thereto. Special
training of personnel working on such lines is highly desirable and especially
when the electricity line is a high\(hyvoltage line.
.LP
3)
The rules given in the \fIDirectives\fR in connection with
danger, disturbance, and staff safety should be complied with
(see\ [1]).
.LP
4)
Special formal agreements are desirable between the
Administration and the electricity authority in the case of
joint use of poles in order to define responsibilities.
.LP
5)
If joint use is applied on short sections (of the order of 1\ km), in
most cases a few simple precautions may be enough to
ensure that disturbances due to electric and magnetic induction
are tolerable.
.sp 2P
.LP
\fBReference\fR
.sp 1P
.RT
.LP
[1]
CCITT manual \fIDirectives concerning the protection of telecommunication\fR
\fIlines against harmful effects from electric power and electrified railway\fR
\fIlines\fR , Vol.\ II, ITU, Geneva, 1988.
.sp 2P
.LP
\fBRecommendation\ K.6\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBPRECAUTIONS\ AT\ CROSSINGS\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.6''
.OF '''Volume\ IX\ \(em\ Rec.\ K.6 %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.LP
\fBIntroduction\fR
.sp 1P
.RT
.PP
Crossings between overhead telecommunication lines and electricity lines
present dangers for persons and for equipment.
.PP
A number of arrangements have been made by the responsible authorities
in various countries, resulting in national regulations. These regulations
are sometimes rather inconsistent and the effectiveness of the arrangements
made
varies somewhat.
.bp
.PP
Bearing in mind the stage now reached in technique and the experience gained
in the various countries, it now seems possible for the CCITT to issue
a Recommendation advocating the arrangements which seem to be the most
effective, on the basis of which countries might draw up or revise their
national
regulations.
.PP
It is therefore recommended that, when an overhead telecommunication line
has to cross an electricity line, either of two methods may be used:
namely, to route the overhead telecommunication line in an underground
cable at the crossing, or to leave it overhead.
.RT
.sp 2P
.LP
\fB1\fR \fBLine routed underground\fR
.sp 1P
.RT
.PP
This method is not always to be recommended because if a conductor of the
electricity line breaks, the underground cable may be in a region where
the ground potential is high. This situation is dangerous if the cable
has a
bare metallic sheath
; the higher the voltage of the power line, the
shorter the length of the cable section, and the higher the resistivity
of the soil, the greater is the danger. This dangerous situation also arises
whenever an earth fault occurs on a pylon near the cable.
.PP
If circumstances require the overhead line to be routed in a cable,
special precautions will have to be taken at the crossing, for
example:
.RT
.LP
\(em
the use of an insulating covering around the metal sheath
of the cable;
.LP
\(em
the use of a cable with an all\(hyplastic sheath.
.sp 2P
.LP
\fB2\fR \fBLine left overhead\fR
.sp 1P
.RT
.PP
The method whereby the power line is separated from the
telecommunication line by a guard\(hywire or a cradle cannot generally be
recommended.
.PP
In any case, regardless of the circumstances, a minimum vertical
distance has to be kept between telecommunication conductors, in conformity
with national regulations.
.PP
There are, moreover a number of arrangements that could be introduced to
reduce the danger:
.RT
.PP
2.1
\fIUse of a common support\fR at the crossing\(hypoint, provided the insulators
used for the telecommunication line have, if necessary, a high
breakdown voltage.
.sp 9p
.RT
.PP
2.2
\fIInsulation of the conductors\fR , preferably the telecommunication conductors,
provided that such insulation is properly adapted to the conditions existing.
.PP
2.3
\fIReinforcement of the construction\fR of the power line where the
crossing takes place, so as to minimize the risk of a break.
.sp 2P
.LP
\fR
\fB3\fR \fBCircumstances in which the various arrangements in \(sc\(sc\
2.1, 2.2\fR \fBand 2.3\ above are applicable\fR
.sp 1P
.RT
.PP
The application of these methods depends primarily on the voltage of the
power line. The voltage ranges to be taken into account are not related
to the International Electrotechnical Commission (IEC) standardization,
because of the special features of the problem raised.
.RT
.sp 1P
.LP
3.1
\fISystems using voltages of 600\ V or less\fR
.sp 9p
.RT
.PP
Arrangements to be as in \(sc\ 2.1 and/or \(sc\ 2.2.
.RT
.sp 1P
.LP
3.2
\fISystems using voltages of 60\ kV or more\fR
.sp 9p
.RT
.PP
(In particular the \*Qhigh reliability\*U system referred to in\ [1].)
.PP
Arrangements to be as in \(sc\ 2.3, if necessary.
.RT
.sp 1P
.LP
3.3
\fIIntermediate voltage systems\fR
.sp 9p
.RT
.PP
For the 600\(hyV to 60\(hykV range, because of the variety of voltages,
the mechanical characteristics of lines and the operating methods encountered,
it is impossible to issue precise recommendations.
.PP
However, one or more of the arrangements described above might be
applicable, although certain special cases call for thorough examination in
close collaboration with the services concerned.
.RT
.sp 2P
.LP
\fBReference\fR
.sp 1P
.RT
.LP
[1]
CCITT manual \fIDirectives concerning the protection of telecommunication\fR
\fIlines against harmful effects from electric power and electrified railway\fR
\fIlines\fR , Vol.\ VI, ITU, Geneva, 1988.
.bp
.sp 2P
.LP
\fBRecommendation\ K.7\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBPROTECTION AGAINST ACOUSTIC SHOCK\fR
.EF '% Volume\ IX\ \(em Rec.\ K.7''
.OF '''Volume\ IX\ \(em\ Rec.\ K.7 %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1964; modified at Malaga\(hyTorremolinos, 1984)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
In certain unfavourable circumstances, sudden transient voltages of exceptionally
high instantaneous amplitude, of the order of 1\ kV for
example, may occur across a telephone set which is normally connected to a
metal wire line, as a result of electromagnetic disturbances affecting the
line.
.sp 1P
.RT
.PP
If such voltages occur during a telephone call, they are liable to cause,
through the earphone, such strong sound pressure as to endanger the
human ear and the nervous system.
.PP
Such bursts are most likely to occur when lightning protectors are
inserted in the two conductors of a telephone line and do not function
simultaneously, so that a compensating current flows through the telephone.
The CCITT therefore recommends the use, particularly on lines equipped
with vacuum lightning protectors, of protection devices against acoustic
shock arising from inadmissibly high induced voltages (see Chapter\ I/6
of the \fIDirectives\fR ,
page\ 16).
.PP
Such devices consist, for example, of two rectifiers, in parallel and with
opposite polarities, or of other semiconductor components connected
directly in parallel to the telephone receiver.
.PP
For telephone sets of more recent design, sudden voltage bursts liable
to occur in the receiver may be eliminated by ensuring that the electrical
circuits between the access to the line where dangerous voltages originate
and the
earphone
itself have suitable characteristics.
.PP
It is also recommended that the proposed provisions should limit the
aural discomfort
which might be caused by abnormal electrical signals applied to subscriber
systems as a result of erroneous operation or unwanted
actuation of the equipments to which subscriber systems are connected.
.PP
The provisions adopted to provide protection against acoustic shock
should:
.RT
.LP
\(em
be compatible with the technical requirements applicable to the equipment;
.LP
\(em
facilitate performance checks;
.LP
\(em
not noticeably impair telephone transmission quality.
.PP
For this purpose, it is particularly recommended that:
.LP
1)
with regard to specific devices, their dimensions should be such that
they occupy a small space, so that they can be placed in the case of the
subscriber's or operator's
telephone receiver
;
.LP
2)
the electrical characteristics should not show significant changes under
the temperature and humidity conditions to which the device is
subjected in service;
.LP
3)
effectiveness should be checked in conformity with the
provisions of CCITT Recommendation\ P.36.
.sp 2P
.LP
\fBRecommendation\ K.8\fR
.RT
.sp 2P
.ce 1000
\fBSEPARATION\ IN\ THE\ SOIL\ BETWEEN\ TELECOMMUNICATION\ CABLES\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.8''
.OF '''Volume\ IX\ \(em\ Rec.\ K.8 %'
.ce 0
.sp 1P
.ce 1000
\fBAND\ EARTHING\ SYSTEM\ OF\ POWER\ FACILITIES\fR
.ce 0
.sp 1P
.ce 1000
\fI(Mar del Plata, 1968; modified at Melbourne, 1988)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.LP
\fBIntroduction\fR
.sp 1P
.RT
.PP
If a buried telecommunication cable without an insulating layer
around the metal sheath is located in the vicinity of a high voltage earthing
system, part of the earth potential rise (EPR) in the event of an earth
fault in the high voltage system can transfer to the telecommunication
system through resistive coupling.
.bp
.PP
According to CCITT and CIGRE
.FS
CIGRE International Conference on
Large High\(hyTension Electric Systems.
.FE
documents [1\(hy3], EPR from high
voltage
power installations is recognized as a source of dangerous disturbance to
telecommunication systems and a hazard to service personnel.
.PP
\fR It is possible to calculate EPR near power installations following
the methods given in the \fIDirectives\fR \ [1] (see Volumes\ II and\ III),
and this is especially recommended for dealing with switchyard earthing
systems.
.PP
\fR The object of the present Recommendation is to give practical
guidelines in determining safe distances between buried telecommunication
cables and earthing systems of power facilities in the absence of local
measurements or calculated values of EPR.
.RT
.sp 2P
.LP
\fB1\fR \fBScope\fR
.sp 1P
.RT
.PP
Earth fault
in a power system causes earth currents which raise the
earth potential
where the
fault current
leaves and
enters the earth. The magnitude and extension of the EPR depends on the
fault current level, the
earthing resistance
, the
soil resistivity
and the layout of the earthing arrangement. The duration of an earth fault
depends on the type of power network.
.PP
This Recommendation gives information about:
.RT
.LP
a)
locations where EPR may occur;
.LP
b)
duration of EPR in different types of power networks;
.LP
\fR
c)
\*Q
safe distance
\*U between telecommunication cables and power installations;
.LP
d)
measures to be taken if the safe distance is not
achieved.
.sp 2P
.LP
\fB2\fR \fBGeneral considerations\fR
.sp 1P
.RT
.PP
The minimum separation in soil to be recommended between an
earthing system of a power installation and telecommunication cables depends
on a number of factors:
.RT
.LP
\(em
type of power network;
.LP
\(em
fault current level;
.LP
\(em
power earthing system;
.LP
\(em
soil resistivity;
.LP
\(em
local conditions.
.sp 2P
.LP
\fB3\fR \fBType of power network\fR
.sp 1P
.RT
.PP
Power networks are classified according to how the neutral point is connected
to earth. The earthing system affects both the level and duration of the
fault current, and hence the EPR.
.RT
.sp 1P
.LP
3.1
\fINetworks with the neutral point earthed directly or through a\fR
\fIlow impedance\fR
.sp 9p
.RT
.PP
The level of an earth\(hyfault current is high. A relay system will
clear the fault in a short time.
.RT
.sp 1P
.LP
3.2
\fINetworks with the neutral point earthed through an\fR
\fIarc\fR
\fIsuppression coil\fR
.sp 9p
.RT
.PP
The level of an earth\(hyfault current is small, usually not exceeding
100\ amperes for each coil. The duration of an earth fault is relatively
short.
.PP
Such networks may be equipped with delayed tripping to clear
permanent earth faults.
.RT
.LP
.sp 2
.bp
.sp 1P
.LP
3.3
\fINetworks with the neutral point isolated from earth\fR
.sp 9p
.RT
.PP
\fR
The level of an earth\(hyfault current is normally low, but the
fault duration might be very long. Networks of large extent may give rise to
large capacitive fault currents.
.PP
If such networks are equipped with devices for automatic fault
clearing, the fault duration is short to medium.
.RT
.sp 2P
.LP
\fB4\fR \fBLocations where earth potential rise may occur\fR
.sp 1P
.RT
.sp 1P
.LP
4.1
\fIPower stations and sub\(hystations\fR
.sp 9p
.RT
.PP
\fR
Power stations and sub\(hystations are most likely to experience EPR. The
size of the station, the number and construction of power lines attached
to the station, and the earthing arrangement are factors influencing the
level and station, and the earthing arrangement are factors influencing
the level and
zone of EPR. As given in reference\ [4] the layout and structure of the
earthing arrangement depends on regulations, size, age, purpose and location.
If the
power lines entering the station are provided with
earth wires
, they
will be connected to the earthing system in the station.
.RT
.sp 1P
.LP
4.2
\fIPower line towers\fR
.sp 9p
.RT
.PP
Power line towers with footing electrodes are subjected to EPR due to earth\(hyfault
current in the power system, and currents from
lightning
strikes
. If the power line is equipped with earth wires, these will
normally be connected to the tower electrodes. The probability of high EPR
decreases when a power line is equipped with earth wires.
.RT
.sp 2P
.LP
\fB5\fR \fBMagnitude of earth potential rise\fR
.sp 1P
.RT
.PP
The magnitude of the EPR depends on the power system voltage, the power
line construction, the fault current level and the earthing
resistance.
.RT
.sp 2P
.LP
\fB6\fR \fBZone of earth potential rise\fR
.sp 1P
.RT
.PP
EPR is measured as the earth potential referred to a distant
neutral earth. The zone of EPR, near an earthing system, varies from some
tens to some thousands of metres, depending on soil resistivity, the layout
of the earth electrode, and other local conditions. Further information
is found in
reference\ [5]. The zones of EPR in urban areas are small compared to what
can be expected in rural areas. Only EPR zones having a potential higher
than
values given in reference\ [1] are considered as dangerous. Measurements and
calculation of the EPR zones are made by the power distribution
authorities.
.RT
.sp 2P
.LP
\fB7\fR \fBDuration of earth potential rise\fR
.sp 1P
.RT
.PP
The duration of an earth fault and hence the EPR, depends on
the type of power network.
.RT
.sp 1P
.LP
7.1
\fINetworks with the neutral point earthed directly or through a low\fR
\fIimpedance\fR
.sp 9p
.RT
.PP
The duration of an earth fault is generally less
than\ 0.2\(hy0.5\ s.
.RT
.sp 1P
.LP
7.2
\fINetworks with the neutral point earthed through an arc suppression\fR
\fIcoil\fR
.sp 9p
.RT
.PP
The duration of an earth fault is normally less than 0.8\ s, but
may in some cases last for several seconds. Such networks may be equipped
with delayed (a few seconds) tripping to clear permanent earth faults.
.bp
.RT
.sp 1P
.LP
7.3
\fINetworks with an isolated\fR
\fIneutral point\fR
.sp 9p
.RT
.PP
The duration of an earth fault can be very long, and may last until another
earth fault occurs.
.PP
If such networks are equipped with automatic fault\(hyclearing devices,
the fault duration may be as short as in \(sc\ 7.1.
.RT
.sp 2P
.LP
\fB8\fR \fBMinimum separation in soil between buried telecommunication
cables and power earthing systems\fR
.sp 1P
.RT
.PP
The EPR near a high voltage earthing system can be estimated from calculations
based on idealized earth electrodes and a homogeneous soil
resistivity in the EPR zone. In practice it is not possible to make an exact
calculation of the potential transferred from a high voltage earthing system
to an adjacent telecommunication cable. However, by feeding a current into
the
high voltage earthing system from a sufficiently great distance, the voltage
.PP
between the cable sheath and an auxiliary electrode in the area of neutral
potential can be measured. The result must be corrected proportionately
to the actual earth\(hyfault current. (On armoured cables the correction
factor is not
linear, but depends on the magnetic characteristic of the ferromagnetic
cable screen.) In the absence of other experiments, local measurements
or calculated values of EPR, the values in Table\ 1/K.8 for the minimum
separation in soil
between \*Qordinary\*U telecommunication cable with a metal sheath in direct
contact with the soil and a high voltage power earthing system should be
observed.
.RT
.ce
\fBH.T. [T1.8]\fR
.ce
TABLE\ 1/K.8
.ce
\fBSeparation in soil (in metres) between telecommunication cables and
.ce
high voltage earthing systems\fR
.ce
\fBbeyond which no calculation nor
.ce
measurement is necessary \fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(60p) | cw(54p) sw(60p) | cw(54p) , ^ | c | c | ^ .
Earth resistivity Power network system with Location
{
isolated neutral or arc suppression coil
} directly earthed neutral
_
.T&
cw(60p) | cw(54p) | cw(60p) | cw(54p) , ^ | c | c | c.
Less than 50 ohm | (mu | \ 2 \ \ 5 Urban
\ 5 \ 10 Rural
_
.T&
cw(60p) | cw(54p) | cw(60p) | cw(54p) , ^ | c | c | c.
50\(hy500 ohm | (mu | \ 5 \ 10 Urban
10 \ 20 Rural
_
.T&
cw(60p) | cw(54p) | cw(60p) | cw(54p) , ^ | c | c | c.
500\(hy5000 ohm | (mu | 10 \ 50 Urban
20 100 Rural
_
.T&
cw(60p) | cw(54p) | cw(60p) | cw(54p) , ^ | c | c | c.
{
Greater than
5000 ohm | (mu |
} 10 \ 50 Urban
20 100\(hy200 | ua\d\u)\d Rural
.TE
.LP
\ua\d\u)\d
200 metres in areas with extremely severe soil conditions, i.e.
greater than 10 | 00\ ohm | (mu | .
.LP
\fINote 1\fR
\ \(em\ The values in the table normally refer to lines and installations
which have a nominal voltage equal to or greater than 132\ kV.
.LP
\fINote 2\fR
\ \(em\ The hazards due to lightning strokes on electric plants are not
covered and may require taking into considerating the methods of \(sc\ 9
for high keraunic level areas.
.LP
\fINote 3\fR
\ \(em\ In the case of tower earthing, much shorter distances can be used
if the power lines include earth wires.
.LP
\fINote 4\fR
\ \(em\ Hazards for people working on telecommunication lines inside the
zone of\ EPR is not taken into consideration by these values; such hazards
require additional measures or precautions.
.nr PS 9
.RT
.ad r
\fBTable 1/K.8 [T1.8], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 2P
.LP
\fB9\fR \fBMeasures to be taken to avoid hazards from EPR\fR
.sp 1P
.RT
.PP
The primary method to avoid dangerous influence from EPR is to
increase the distance between telecommunication cables and power earthing
systems. If local conditions do not permit sufficient separation to avoid
dangerous EPR, the telecommunication cables should be provided with insulation,
for example by placing the cables in insulating plastic tubes.
.PP
When the magnitude of EPR is extremely high, or the zone of EPR is
of very great extension, optical fibre cables or radio\(hyrelay systems may be
used instead of metallic cables.
.RT
.sp 2P
.LP
\fBReferences\fR
.sp 1P
.RT
.LP
[1]
CCITT manual \fIDirectives concerning the protection of telecommunication\fR
\fIlines against harmful effects from electrified power and electrified
railway\fR \fIlines\fR , Vols.\ II and\ III, ITU, Geneva,\ 1988.
.LP
[2]
CCITT Study Group V \(em Contribution No. 61/1979.
.LP
[3]
CIGRE No. 36\(hy04/1970 \(em Ground potential rise and telecommunication
lines.
.LP
[4]
ELECTRA No. 71/1980 Station grounding \(em Safety and interference
aspects.
.LP
[5]
ELECTRA No. 60/1978 \(em Zone of influence of ground potential rise.
.sp 2P
.LP
\fBRecommendation\ K.9\fR
.RT
.sp 2P
.ce 1000
\fBPROTECTION\ OF\ TELECOMMUNICATION\ STAFF\ AND\ PLANT\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.9''
.OF '''Volume\ IX\ \(em\ Rec.\ K.9 %'
.ce 0
.ce 1000
\fBAGAINST\ A\ LARGE\ EARTH\ POTENTIAL\ DUE\ TO\ A\ NEIGHBOURING\fR
.ce 0
.sp 1P
.ce 1000
\fBELECTRIC\ TRACTION\ LINE\fR
.ce 0
.sp 1P
.ce 1000
\fI(Mar del Plata, 1968)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.LP
\fB1\fR \fBGeneral\fR
.sp 1P
.RT
.PP
From the technical standpoint the precautions taken on
electrified railways
to protect staff and plant may differ according to a number of factors,
the chief of which are:
.RT
.LP
\(em
ground resistivity
;
.LP
\(em
electrical line equipment (track circuits) which, though
necessary for railway safety installations, may prevent the systematic
connection to rail of metal structures near the railway;
.LP
\(em
the characteristics of the protective devices required which, with
a.c. electric traction systems
, may be to some extent affected by the presence (or absence) of
booster\(hytransformers
;
.LP
\(em
the degree of insulation of the contact system, which may
also affect the nature of the protective devices, particularly in the case
of relatively low\(hyvoltage electric systems such as 1500\ V d.c.\ lines;
.LP
\(em
the means to be recommended for linking a metal structure to the rail
in case of overvoltage without making a permanent connection (one
method is to make the connection via a spark gap).
.sp 2P
.LP
\fB2\fR \fBA.c. electric traction lines\fR
.sp 1P
.RT
.PP
It is recommended that neighbouring metal structures, for example all those
within a certain distance from the line, be connected to rail,
provided that there are no safety installations which make this impossible.
.PP
If the structures cannot be connected to rail, it is recommended that
they be earthed to an earth electrode having a sufficiently low
resistance.
.bp
.RT
.sp 2P
.LP
\fB3\fR \fBD.c. electric traction lines\fR
.sp 1P
.RT
.PP
Protective measures should also take account of the need to avoid any risk
of
electrolytic corrosion
. Such measures may amount to
connecting to rail only such metal structures as are sufficiently insulated
from the ground or linking them via a
spark gap
or, in the case of
metal structures carrying an adequately insulated contact system or lines
with a sufficiently low service voltage, connecting neither to rail nor
to
earth.
.RT
.sp 2P
.LP
\fB4\fR \fBTelecommunication cables\fR
.sp 1P
.RT
.PP
In new installations, it is recommended that cables near rails, at the
entry to
substations
or over metal bridges should have an outer
plastic covering, possibly of high
dielectric strength
, where it is
necessary to prevent contact between the cables and such structures.
.PP
If, on the other hand, cables with metal sheaths already exist, a good
solution, at least in the case of large railway stations, may be to connect
the sheaths to rail.
.RT
.sp 2P
.LP
\fB5\fR \fBConditions to be fulfilled by PTT installations in the\fR
\fBneighbourhood of electric traction lines\fR
.sp 1P
.RT
.PP
The following are the main precautions taken to protect such
installations:
.RT
.LP
\(em
placing them outside the danger zone;
.LP
\(em
screening;
.LP
\(em
substituting insulating components for metal components, in particular
the sheaths or covering of cables or in the
construction of repeater cabinets or boxes.
.PP
\fINote\fR \ \(em\ The above recommendations are inspired solely by
technical considerations which are to be carefully weighed up in each case.
It goes without saying that every Administration must comply with the laws
and
regulations in force in its country.
\v'1P'
.sp 2P
.LP
\fBRecommendation K.10\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBUNBALANCE\ ABOUT\ EARTH\ OF\ TELECOMMUNICATION\ INSTALLATIONS\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.10''
.OF '''Volume\ IX\ \(em\ Rec.\ K.10 %'
.ce 0
.sp 1P
.ce 1000
\fI(Mar del Plata, 1968; modified at Malaga\(hyTorremolinos,\fR
\fI1984)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.LP
\fB1\fR \fBUnbalance about earth of telecommunication equipments\fR
.sp 1P
.RT
.PP
In the interests of maintaining an adequate balance of
telecommunication equipments and of the lines connected to them, it is
recommended that the minimum permissible value for the unbalance of
telecommunication installations longitudinal conversion loss (LCL) should
be 40\ dB (from 300 to 600\ Hz) and\ 46\ dB (from 600\ to\ 3400\ Hz). This is a
general minimum value and does not exclude the possibility of higher minimum
values being quoted for particular requirements in other Recommendations
of the CCITT
.FS
See, in particular, Recommendation\ Q.45, and also the outcome of
further studies under Question\ 13/V [1].
.FE
.
.PP
The test arrangement in Figure 1/K.10 should be used to measure
the unbalance of telecommunications equipment.
.RT
.PP
Nomenclature, definition and measurement of unbalance are based on Recommendations\
G.117 and\ O.121.
.RT
.LP
.sp 3
.bp
.LP
.rs
.sp 23P
.ad r
\fBFigure 1/K.10 p.2\fR
.sp 1P
.RT
.ad b
.RT
.PP
The specification \fIZ\fR\d\fIL\fR\\d1\u\ =\ \fIZ\fR\d1\u/4,
\fIZ\fR \fI\fI\d\fIL\fR\\d2\u\ =\ \fIZ\fR\d2\u/4 should apply in the audiofrequency
range. (See Recommendation\ Q.45 and Recommendation\ O.121, \(sc\ 3.2.)
.PP
The following terms are specified:
.RT
.LP
\(em
longitudinal conversion loss (LCL) (applicable for one\(hy and two\(hyport
networks):
\v'6p'
.sp 1P
.ce 1000
20 log
\d10
\u
@ left | { fIE\fR~\d\fIL\fR~1~\u } over { fIV\fR~\d\fIT\fR~1~\u } right | @
dB
.ce 0
.sp 1P
.LP
.sp 1
\(em
longitudinal conversion transfer loss (LCTL) (applicable for
two\(hyport networks only):
\v'6p'
.sp 1P
.ce 1000
20 log
\d10
\u
@ left | { fIE\fR~\d\fIL\fR~1~\u } over { fIV\fR~\d\fIT\fR~2~\u } right | @
dB
.ce 0
.sp 1P
.LP
.sp 1
.sp 2P
.LP
\fB2\fR \fBUnbalance about earth of telecommunication lines\fR
.sp 1P
.RT
.PP
If a long line is tested, essentially the same test circuit and
nomenclature should be used as given in Figure\ 1/K.10. However, both the
longitudinal induction and unbalances are distributed along the line.
Consequently, the
longitudinal conversion losses
and
longitudinal conversion transfer losses
are not only determined by the inherent
parameters but also by the distribution of the wire to earth/sheath
voltages. To obtain the effect of unbalance in practical cases, it is
recommended that measurements be made both with the wire to sheath voltage
of constant polarity (i.e.\ supply at end, see Table\ 1/K.10) and with
the wire to sheath voltage changing in polarity at the midpoint (i.e.\
supply at the middle, see Table\ 2/K.10).
.PP
In Table 3/K.10, conclusions derived from those measurements are
listed.
.bp
.RT
.LP
.rs
.sp 47P
.ad r
\fBTABLE 1/K.10 \ \
(\*`a traiter comme figure MEP), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBTABLE 2/K.10 \ \
(\*`a traiter comme figure MEP), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 40P
.ad r
\fBTABLE 3/K.10 \ \
(\*`a traiter comme figure MEP), p.\fR
.sp 1P
.RT
.ad b
.RT
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation K.10)
.sp 9p
.RT
.ce 0
.ce 1000
\fBExample for calculating\fR
\fBtransverse voltages of a\fR \fBtelecommunications line\fR
.sp 1P
.RT
.ce 0
.LP
A.1
\fIGeneral\fR
.sp 1P
.RT
.PP
The Contribution mentioned in reference\ [2] contains many
calculated values regarding the relationship between the longitudinal voltage
and its conversion into the transverse one. This annex is an extract of
that Contribution. It gives background information about the application of
measurement proposals for lines which are contained in Recommendation\ K.10.
.bp
.PP
The most important results are summarized in Table\ A\(hy1/K.10. They
relate to a symmetric pair composed of paper\(hyinsulated copper wires
of 0.9\ mm in diameter and stranded in star quads with an equivalent mutual
capacitance of 34\ nF/km. In the course of the calculation, only the capacitance
unbalance has been simulated.
.RT
.sp 1P
.LP
A.2
\fIWire to sheath voltages\fR
.sp 9p
.RT
.PP
The distribution of the wire to sheath (earth) voltages are
basically determined by (see column\ 2 of Table\ A\(hy1/K.10 where, for the
sake of simplicity, it is assumed that the total voltage source in the
longitudinal path is\ 100\ V):
.RT
.LP
\(em
the location of the longitudinal source (see column\ 1 in
Table\ A\(hy1/K.10), and
.LP
\(em
the termination of the longitudinal path (see column\ 3 of
Table\ A\(hy1/K.10).
.PP
On the basis of schemes indicated in column\ 2 of Table\ A\(hy1/K.10, the
following tendencies are worth mentioning:
.LP
a)
When the e.m.f. is applied at one of the terminals of the
longitudinal path, the wire to sheath voltages tend to be
uniform with the same polarity along the line. When switch\ S is
closed, the voltages decrease (compare the solid line with
broken ones in the 1st row and 2nd\ column).
.LP
b)
When the e.m.f. is applied at an intermediate section of the
line, e.g.\ concentrated in the middle or distributed uniformly,
then the wire to earth voltages have the same magnitudes but
opposite polarity on each half of the line (see the curves of
broken line in the 2nd and 3rd\ rows). The symmetry of the
distribution is disturbed if only one switch at the terminals
is closed (see the solid lines in the 2nd and 3rd\ rows). The
differences between voltage distributions arising from
terminations of open/closed and closed/closed switch positions
tend to decrease with the increase of both the length of
line and frequency.
.sp 1P
.LP
A.3
\fILongitudinal conversion losses\fR
.sp 9p
.RT
.PP
The longitudinal conversion losses and the longitudinal transfer
losses (defined in Tables\ 1/K.10 and 2/K.10) are basically determined
by:
.RT
.LP
\(em
the distribution of wire to sheath voltages, see \(sc\ A.2, and
.LP
\(em
the magnitude and distribution of
capacitance
unbalance
.
.PP
Regarding the second aspect, three cases have been studied. These
are indicated in Table\ A\(hy1/K.10 as one\(hysided, perfectly equalized
and equalized with additional unbalance. The one\(hysided uniform \(*D\fIC\fR
\ =\ 600\ pF/km tends to
simulate the worst case which in practice does not exist. The perfectly
equalized line (with crossing at each 0.5\ km) can also never be reached.
.PP
The magnitudes of longitudinal conversion losses can be explained by a
consideration of the fact that high transverse voltages are generated as
a
result of capacitance unbalance if the location of an unbalance coincides
with high wire to earth voltages. The unbalance of a subsequent section
tends to
amplify the transverse voltage if both the direction of the unbalance and
polarity of the wire to earth voltage are the same as those of the previous
section. However, if one of them is reversed, the resultant transverse
voltages become lower.
.PP
In the case of a well equalized line, the magnitude of the
longitudinal conversion losses is high and is largely independent of both
the location of the e.m.f. and the position of the switches at the terminals
(see column\ 5 in Table\ A\(hy1/K.10).
.PP
If the conversion losses increase significantly in magnitude with the opening
of switch\ S and depend on the direction of supply, then the presence of
local unbalance may be expected (see column\ 6 of Table\ A\(hy1/K.10).
.PP
The low values of longitudinal conversion losses (i.e.\ less
than\ 60\ dB) might be caused by a one\(hysided nature of the capacitance
unbalance (see column\ 4 of Table\ A\(hy1/K.10). This is the case for Recommendation\
K.10
where the testing method specified in \(sc\ 2 may produce significantly higher
values for longitudinal conversion losses than the actual values in real
conditions of power induction. In this case, more realistic values can be
obtained by the method given in Table\ 2/K.10.
.bp
.RT
.LP
.rs
.sp 47P
.ad r
\fBTableau A\(hy1/K.10 \ \
(\*`a traiter comme figure MEP), p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
The main unbalance on lines is the capacitance unbalance. However, occasionally,
the
resistive unbalance
(series resistance,\ R) is
important as well. As has been pointed out before, when switch\ S\d2\uis
open, the effect of
shunt unbalance
(in case of line\ \fIC\fR ) is emphasized. If the switch\ S\d2\u(or S\d1\uand
S\d2\uindicated in Table\ 2/K.10) is opened and the conversion loss remains
unchanged (or even decreases), it indicates that
series unbalance
may not be the primary cause of the
line
unbalance
. On the other hand, if there is an increase, the series
unbalances are dominant. It should be noted, that while the reason for
having \fIZ\fR\d\fIL\fR\uand S\d2\uis to allow the tester to distinguish
between series and
shunt unbalances of the line, the effectiveness of this feature depends
on the shunt impedance of the line provided by the resultant earth capacitance
of the line (e.g.\ length of line\ [3]).
.sp 2P
.LP
\fBReferences\fR
.sp 1P
.RT
.LP
[1]
CCITT Question 13/V \fIUnbalance of telephone installations\fR .
.LP
[2]
CCITT Contribution COM V\(hy38,
\fIStudy of relation between unbalance and induced transverse voltages\fR ,
1981\(hy1984 (Hungarian Administration).
.LP
[3]
IEEE Std 455 \(em 1976 \fIIEEE Standard test procedure for measuring\fR
\fIlongitudinal balance of telephone equipment operating in the\fR
\fIvoice band\fR . Published by IEEE, Inc., September 30,\ 1976.
.sp 2P
.LP
\fBRecommendation\ K.11\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBPRINCIPLES\ OF\ \fR \fBPROTECTION\ AGAINST\ OVERVOLTAGES |
AND\ OVERCURRENTS\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.11''
.OF '''Volume\ IX\ \(em\ Rec.\ K.11 %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1972; modified at Malaga\(hyTorremolinos, 1984\fR \fIand at
Melbourne, 1988)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.LP
\fBIntroduction\fR
.sp 1P
.RT
.PP
Current CCITT documents recognize lightning and faults on nearby
electrical installations as sources of dangerous disturbances in
telecommunications lines, which may cause damage leading to interruptions in
service and the need for repairs or even hazards to personnel.
.PP
The object of the present Recommendation is to set out principles
which enable the frequency and seriousness of such disturbances to be limited
to levels which take account of quality of service, operating costs and
safety of personnel. These principles are applicable to all parts of a
telecommunications system. More details on certain methods of protection and
for certain parts of the system are given in the References and in the
following Recommendations: K.5, K.6, K.9, K.12, K.15, K.16, K.17. Information
about disturbing phenomena and protection techniques are given in\ [1]
and\ [2] (see also Recommendation\ K.26).
.PP
This Recommendation deals principally with the local exchange, local loop
plant and subscribers equipment, but its contents may have wider
application.
.PP
\fINote\fR \ \(em\ The disturbing phenomena, when they appear, are relatively
rare or of very brief duration (usually of the order of a fraction of a
second) and in framing the present Recommendation, consideration has not
been given to methods of avoiding interruption of the functioning of equipment
during an
actual disturbance. The CCITT is pursuing the study of such methods.
.RT
.sp 2P
.LP
\fB1\fR \fBGeneral considerations\fR
.sp 1P
.RT
.sp 1P
.LP
1.1
\fIOrigin of dangerous \fR \fIovervoltages and overcurrents\fR
.sp 9p
.RT
.sp 1P
.LP
1.1.1
\fIDirect \fR \fIlightning strikes\fR
.sp 9p
.RT
.PP
Such strikes may cause currents of some thousands of amperes to
flow along wires or cables for some microseconds. Physical damage may occur
and overvoltage surges of many kilovolts may apply stress to the dielectrics
of
line plant and terminal equipment.
.RT
.sp 1P
.LP
1.1.2
\fILightning strikes\fR \fInearby\fR
.sp 9p
.RT
.PP
Lightning currents flowing from cloud to earth or cloud to cloud
cause overvoltages in overhead or underground lines near to the strike. The
area affected may be large in districts of high earth
resistivity.
.bp
.RT
.sp 1P
.LP
1.1.3
\fIInduction from fault currents in power lines including\fR
\fIelectric traction systems\fR
.sp 9p
.RT
.PP
Earth faults
in power systems cause large unbalanced
currents to flow along the power line inducing overvoltages into adjacent
telecommunications lines which follow a parallel course. The overvoltages
may rise to several kilovolts and have durations of 200 to 1000\ ms (occasionally
even longer) according to the fault clearing system used on the power
line.
.RT
.sp 1P
.LP
1.1.4
\fIContacts with power lines\fR
.sp 9p
.RT
.PP
Contacts may occur between power and telecommunication lines when local
disasters, e.g.\ storms, fires, cause damage to both types of plant or
when the normal safeguards of separation and insulation are not followed.
Overvoltages rarely exceed 240\ V a.c., r.m.s. above earth in countries where
this is the normal distribution voltage but may continue for an indefinite
period until observed. Where higher distribution voltages, e.g.\ 2\ kV, are
used the power line protection arrangements usually ensure that the voltage
is removed in a short time if a fault occurs. The overvoltage may
cause excessive currents to flow along the line to the exchange earth causing
damage to equipment and danger to staff.
.RT
.sp 1P
.LP
1.1.5
\fIRise of\fR
\fIearth potential\fR
.sp 9p
.RT
.PP
Earth faults in power systems cause currents in the soil which
raise the potential in the neighbourhood of the fault and of the power
supply earth electrode. (See also Recommendation\ K.9.) These earth potentials
may
affect telecommunication plant in two ways:
.RT
.LP
a)
Telecommunication signalling systems may malfunction if the
signalling earth electrode
is in soil whose potential rises by as
little as 5\ V with respect to true earth. Such voltages may be caused
by minor faults on the power system which may remain undetected for long
periods.
.LP
b)
Higher rises of earth potential can cause danger to staff
working in the affected area or, in extreme cases, may be sufficient to
break down the insulation of the telecommunications cable causing extensive
damage.
.sp 1P
.LP
1.2
\fIMethods of protection\fR
.sp 9p
.RT
.PP
1.2.1
Some of the protective measures for lines which are described
in \(sc\ 2 have the effect of reducing overvoltages and overcurrents at their
source and so reduce the risk of damage to all parts of the system.
.PP
1.2.2
Other protective measures which may be applied to specific parts of the
system as indicated in \(sc\(sc\ 2, 3 and 4 fall broadly into 2 classes:
.LP
\(em
the use of protective devices which prevent excessive energy from reaching
vulnerable parts either by diverting it (for example, spark gaps) or by
disconnecting the line (for example, fuses);
.LP
\(em
the use of equipment with suitable dielectric strength,
current carrying capacity and impedance so that it can withstand the conditions
applied to it.
.sp 2P
.LP
1.3
\fITypes of \fR \fIprotective devices\fR
.sp 1P
.RT
.sp 1P
.LP
1.3.1
\fIAir\(hygap protectors\fR \fIwith carbon or metallic\fR
\fIelectrodes\fR
.sp 9p
.RT
.PP
Usually connected between each wire of a line and earth, they limit the
voltage which can appear between their electrodes. They are inexpensive
but their insulation resistance can fall appreciably after repeated operation
and they may require frequent replacement.
.RT
.sp 1P
.LP
1.3.2
\fIGas discharge tubes\fR
.sp 9p
.RT
.PP
Usually connected between each wire of a line and earth or as
3\(hyelectrode units between a pair and earth. Their performance may be
specified to precise limits to meet system requirements. The protectors
are compact and will operate frequently without attention.
.PP
Detailed requirements for gas discharge tubes appear in
Recommendation\ K.12.
.bp
.RT
.sp 1P
.LP
1.3.3
\fISemi\(hyconductor protective devices\fR
.sp 9p
.RT
.PP
Used in a similar way to carbon electrode protectors or
gas\(hydischarge tubes, these will protect equipment from values of overvoltage
as low as 1\ V. They are precise and fast\(hyacting, but may be damaged
by excessive currents.
.RT
.sp 1P
.LP
1.3.4
\fIFuses\fR
.sp 9p
.RT
.PP
These are connected in series with each wire of a line to
disconnect when excessive current flows. Simple fuses have a uniform wire
which melts. Slow\(hyacting fuses have a uniform wire which melts quickly
when a large current flows, and a spring\(hyloaded fusible element which
melts gradually and
disconnects when lower currents flow for a prolonged time. High level currents
of 2\ A and prolonged currents of 250\ mA are typical operating levels.
Fuses
should not sustain an arc after operation. Fuses do not give protection
against lightning surges and in districts where such surges are common,
fuses of a high rating (up to\ 20\ A) may be necessary to avoid trouble
from fuse failures. Such fuses may not give adequate protection against
power line contacts. Fuses can also be a source of noise and disconnection
faults.
.RT
.sp 1P
.LP
1.3.5
\fIHeat coils\fR
.sp 9p
.RT
.PP
Fitted in series with each wire of a line, heat coils either
disconnect the line, earth it, or do both, with the earth extended to line.
Heat coils have some fusible component and operate when currents of,
typically,\ 500\ mA flow for some 200\ s.
.RT
.sp 1P
.LP
1.3.6
\fISelf\(hyrestoring current\(hylimiting devices\fR
.sp 9p
.RT
.PP
Fuses and heat coils have the disadvantage that they permanently
interrupt a circuit when operated and it is then necessary to replace them
manually. Certain variable impedance devices are available which, when
heated by overload currents, increase their electrical resistance to a
very high
value. The device will return to a normal low electrical resistance when the
overload current is removed. Attention is drawn to the response time and
voltage handling capabilities of these items.
.RT
.sp 1P
.LP
1.4
\fIResidual effects\fR
.sp 9p
.RT
.PP
The essential purpose of protective measures is to ensure that the major
part of the electrical energy arising from a disturbance is not
dissipated in a vulnerable part of the installation and does not reach
personnel. However, no device exists which has characteristics for suppressing
ideally all voltages or currents connected with disturbances, for the following
reasons:
.RT
.sp 1P
.LP
1.4.1
\fIResidual overvoltages\fR
.sp 9p
.RT
.PP
Account should be taken of:
.RT
.LP
a)
voltages which are unaffected by the protective device
because they are below its operating level;
.LP
b)
transients which pass before the device operates;
.LP
c)
residuals which are sustained after the device operates;
.LP
d)
transients produced by the operation of the device.
.sp 1P
.LP
1.4.2
\fITransverse voltages\fR
.sp 9p
.RT
.PP
Protective devices on the two wires of a pair may not operate
simultaneously and so a transverse pulse may be produced. Under certain
conditions, particularly if the equipment to be protected has a low impedance,
operation of one protective device may prevent the operation of the other
one and a transverse voltage may remain as long as the longitudinal voltages
are on the line.
.RT
.sp 1P
.LP
1.4.3
\fIEffect on normal circuit operation\ \(em\ coordinated design\fR
.sp 9p
.RT
.PP
Sufficient separation should be allowed between the operating
voltage of the protective devices and the highest voltage occurring on
the line during normal operation.
.PP
Likewise the normal characteristics (internal impedances) of the
protective elements must be compatible with the normal functioning of the
installations, which must take account of their possible presence.
.bp
.RT
.sp 1P
.LP
1.4.4
\fIModifying effects\fR
.sp 9p
.RT
.PP
A protective device may safeguard one part of a line at the expense of
another, e.g.\ if a main distribution frame (MDF) fuse operates due to
a
power line contact, the voltage on the line may rise to full power line
voltage when the fuse disconnects the telecommunication's earth.
.PP
Likewise the operation of a protector may greatly reduce the
equivalent internal impedance of a circuit relative to equipment connected
to it, thus permitting the circulation of currents which may cause damage.
.RT
.sp 1P
.LP
1.4.5
\fICoordination of primary and secondary protection\fR
.sp 9p
.RT
.PP
For the protection of sensitive equipment it is sometimes necessary to
use more than one protective device, e.g.\ a fast\(hyoperating, low\(hycurrent
device such as a semiconductor and a slower\(hyoperating, high\(hycurrent
device
such as a gas\(hydischarge tube. In such cases steps must be taken to ensure
that in the event of a sustained overvoltage, the low\(hycurrent device
does not
prevent the operation of the high\(hycurrent device since, if this happens, the
smaller device may be damaged, or the interconnecting wiring may conduct
excessive current.
.RT
.sp 1P
.LP
1.4.6
\fITemperature rise\fR
.sp 9p
.RT
.PP
Protective components should be designed and positioned in such a way that
the rise in temperature which occurs when they operate is unlikely to cause
damage to property or danger to people.
.RT
.sp 1P
.LP
1.4.7
\fICircuit availability\fR
.sp 9p
.RT
.PP
The circuit being protected may be temporarily or permanently put out of
service when a protective device operates.
.RT
.sp 1P
.LP
1.4.8
\fIFault liability\fR
.sp 9p
.RT
.PP
The use of protective devices may cause maintenance problems due to unreliability.
They may also prevent some line and equipment testing
procedures.
.RT
.sp 1P
.LP
1.5
\fIAssessment of risk\fR
.sp 9p
.RT
.PP
1.5.1
The performance of a telecommunications system with respect to overvoltages
depends on:
.sp 9p
.RT
.LP
\(em
the environment, i.e.\ the magnitude and probability of
overvoltages occurring in the line network associated with the system;
.LP
\(em
the construction methods used in the line network, see
\(sc\ 2;
.LP
\(em
the resistibility of equipment in the system to
overvoltages;
.LP
\(em
the provision of protective devices;
.LP
\(em
the quality of the earth system provided for operation of the protective
devices.
.sp 1P
.LP
1.5.2
\fIThe environment\fR
.sp 9p
.RT
.PP
In assessing the environment, consideration should be given to the factors
mentioned in \(sc\ 1.1.
.PP
The severity of overvoltages due to lightning varies widely in
different localities. A high
keraunic level
and a high
soil
resistivity
increase the risk of direct and nearby lightning strokes and, since lightning
is the cause of a large proportion of power system faults,
induction and rise of earth potential effects are also increased. On the
other hand buried metal plant such as water pipes, armoured cables,\ etc.,
screens
telephone cables and greatly reduces overvoltages due to lightning or
induction.
.RT
.LP
\(em
In city centres and in regions of low keraunic activity
experience shows that overvoltages rarely exceed the residual voltages of
protective devices and such environments may be classified as \*Qunexposed\*U.
Recommendations\ K.20 and\ K.21 specify the tests to be applied to equipment
for use in unexposed environments without protection and these tests give
an
indication of the most severe environment which can be regarded as unexposed.
.bp
.LP
\(em
All other environments are classified as \*Qexposed\*U but this, of course,
covers a wide range of conditions including exceptionally exposed
situations where a satisfactory service can only be achieved by the use
of all available protective measures.
.PP
In the case of induced voltages and rise of earth potential the
overvoltages can be calculated as indicted in [2] which also recommends the
maximum values which may be permitted under various conditions.
.sp 1P
.LP
1.5.3
\fIFault records\fR
.sp 9p
.RT
.PP
The risk of overvoltages and overcurrents can only be properly
assessed in the light of experience. It is recommended that fault statistics
be kept in a form which is convenient for that purpose. Faults due to overvoltages
or overcurrents and faults due to failures of protective components should
be separated from each other and from other component faults.
.RT
.sp 1P
.LP
1.6
\fIDecision on protection\fR
.sp 9p
.RT
.PP
1.6.1
In considering the degree to which a telecommunications network
should withstand overvoltages, two classes of failure may be recognized:
.LP
\(em
minor failures affecting only small parts of the system.
These may be allowed to occur at a level acceptable to the Administration;
.LP
\(em
major breakdowns, fires, exchange failures, etc., which must, so far
as possible, be avoided completely.
.PP
Examples of conditions which may be permitted to cause minor
failures but not major breakdowns are given in Recommendation\ K.20. It is
desirable also that failure of a single protective device should not cause a
major breakdown.
.PP
1.6.2
Particular attention should be given to overvoltage and
overcurrent protection for new types of exchange or subscribers' equipment
to ensure that the benefits of its improved facilities are not lost due
to
unacceptable failures arising from exposure to overvoltages or overcurrents.
Such equipment may be inherently sensitive to these conditions and damage or
malfunction may affect large parts of a system.
.PP
1.6.3
It should be noted that over\(hyprotection, by the provision of
unnecessary protective devices, is not only uneconomic but may actually
worsen system performance since the devices themselves may have some liability
to
cause failures.
.PP
To avoid disturbances in telecommunication circuits caused by
activated protective devices, the striking voltage values and the numbers of
arrestors should be considered.
.PP
1.6.4
In the light of the above considerations and the assessment of
risks in accordance with \(sc\ 1.5, a decision should be made on the protection
to be provided in all parts of the system. Account should be taken of commercial
considerations such as the cost of protective measures, the cost of repairs,
relations with customers and the probable frequency of faults due to
overvoltage and overcurrent relative to the fault rate due to other causes.
.PP
The responsibility for making this decision and for ensuring the provision
of any protective devices needed to coordinate lines and equipment
should be clearly laid down.
.PP
It is necessary for manufacturers of equipment to know from the
operating Administration the conditions the equipment will need to resist
and for line engineers to know the resistibility of the equipment which
will be
connected to the lines. The line engineer should also define the constraints
which equipment connected to the line will encounter, depending on the
standards of line protection provided. Where parts of the network, such as
subscribers' apparatus, lines and switching centres may be under different
ownership, this coordination may require formal procedures such as the
production of local standards. Recommendations\ K.20 and\ K.21 give guidance
for the preparation of these standards.
.bp
.RT
.sp 2P
.LP
\fB2\fR \fBProtection of lines\fR
.sp 1P
.RT
.sp 1P
.LP
2.1
\fIProtective measures external to the conductors themselves\fR
.sp 9p
.RT
.PP
2.1.1
Telecommunication lines may be shielded from lightning to some
extent by adjacent earthed metal structures, e.g.\ power lines or electric
railway systems. Efficient metallic screens either in the form of cable
sheaths, cable ducts or lightning guard wires, reduce the effects of lightning
surges and power line induction. In areas with a high risk of lightning
strikes special cables with multiple screens and high strength insulation
are often
used. Bonding all metal work gives useful protection.
.PP
2.1.2
Induction from power lines may be minimized by coordinating
the construction practices for the power and telecommunication lines. The
level of induction may be reduced at its source by the installation of
earth wires
and current limiters in the power system.
.PP
2.1.3
The likelihood of contacts occurring between power and
telecommunications lines is reduced if agreed standards of construction,
separation and insulation are followed. Economic considerations arise but
it is often possible to benefit from jointly using trenches, poles and
.LP
ducts, providing suitable safe practices are adopted. (See Recommendations\
K.5 and\ K.6.) It is particularly important to avoid contacts with high
voltage
power lines by a high standard of construction since, if such contacts
occur, it may be very difficult to avoid serious consequences.
.sp 1P
.LP
2.2
\fISpecial cables\fR
.sp 9p
.RT
.PP
Special cables of high dielectric strength may be used where high overvoltages
are likely to occur.
.PP
Standard plastic insulated and sheathed cables have a higher
dielectric strength than paper insulated, lead\(hysheathed cables and are
suitable for most situations where cables with extra thick insulation were
formerly
used. The use of cables with strengthened insulation may be justified in
situations where there is exceptional proximity or length of parallelism to
power lines, high rise of earth potential in the immediate neighbourhood of
power stations or extreme exposure to lightning due to high
keraunic
level
and low
soil conductivity
.
.PP
Other examples of the use of special cables are:
.RT
.LP
\(em
cables with metal sheaths which provide a good reduction
factor to screen circuits within the cable;
.LP
\(em
cables which carry circuits to exposed radio towers and which
must be able to carry lightning discharge currents without
damage;
.LP
\(em
all\(hydielectric (i.e.\ non\(hymetallic) optical fibre cables to
effect isolation between conductive lengths of cable.
.sp 1P
.LP
2.3
\fIUse of protective devices\fR
.sp 9p
.RT
.PP
The use of protective devices may be desirable in the following
circumstances:
.RT
.PP
2.3.1
They may be more economical than the special construction
described in \(sc\(sc\ 2.1 and 2.2. In this connection the cost of maintenance
should not be overlooked since protective devices inevitably incur some
maintenance
expenditure whereas special cables, screening,\ etc., though initially
expensive, usually incur no continuing costs.
.PP
2.3.2
Cables with extra thick insulation may themselves be undamaged
by overvoltages or overcurrents but they can nevertheless conduct such
conditions to other more vulnerable parts of the network. Extra protection
is then required for the more vulnerable cables and is particularly important
if these are large underground cables which are expensive to repair and
affect
service to many customers.
.PP
2.3.3
Induced overvoltages from power or traction line faults may
still exceed levels permitted by the \fIDirectives\fR even after all practicable
avoidance measures have been followed.
.sp 1P
.LP
2.4
\fIInstallation of\fR
\fIprotective devices\fR
.sp 9p
.RT
.PP
2.4.1
To protect conductor insulation it is beneficial to bond all
metal sheaths, screens, etc., together, and to connect overvoltage protectors
between the conductors and this bonded metal which should be connected
to
earth. This technique is particularly useful in districts of high
soil
resistivity
as it avoids the need for expensive electrode systems for the protector
earth connection.
.bp
.PP
2.4.2
Where protectors are used to reduce high voltages appearing in
telecommunication lines due to induction from power line fault currents,
they should be fitted to all wires at suitable intervals and at both ends
of the
affected length of line, or as near to this as practicable.
.PP
2.4.3
To protect underground cables against lightning surges
protective devices may be placed at the points of connection to overhead
lines. The protective devices fitted at the MDF and at subscribers' terminals
reduce the risk of damage to lines but their main function is to protect
components
having lower dielectric strength than the cables. See Recommendations\ K.20
and\ K.21.
.PP
2.4.4
Connections for lines and earth to overvoltage protectors used
against lightning should be as short as possible to minimize surge voltage
levels between lines and the equipotential bond point.
.sp 1P
.LP
2.5
\fIPlanning of works\fR
.sp 9p
.RT
.PP
The general considerations of \(sc\(sc\ 1.5 and 1.6 apply to the
protection of lines. To the greatest extent possible it is recommended
that the protective measures applied to the line should be decided at the
outset of a
measures applied to the line should be decided at the outset of a project
and should depend on the environment. It may be difficult and expensive
to achieve a satisfactory standard of reliability from a line provided
initially with
insufficient protection.
.RT
.sp 1P
.LP
2.6
\fIRecommended policy\fR
.sp 9p
.RT
.PP
Where lines in a telecommunications network are exposed to frequent or
severe disturbances from power line faults or lightning, the voltage of
these lines relative to local earth potential should be limited either by
connecting protective devices between the line conductors and earth or
by using appropriate construction methods for the line.
.RT
.sp 2P
.LP
\fB3\fR \fBProtection of exchange and transmission equipment\fR
.sp 1P
.RT
.sp 1P
.LP
3.1
\fINeed for protection external to the equipment\fR
.sp 9p
.RT
.PP
Operating organizations should take account of the possible need to fit
protection external to the equipment, bearing in mind the following
considerations:
.RT
.PP
3.1.1
A telecommunication line will give some protection to equipment
under certain conditions, e.g.:
.LP
\(em
a conductor may melt and disconnect an excessive current;
.LP
\(em
conductor insulation may break down and reduce an
overvoltage;
.LP
\(em
air\(hygaps in connection devices may break down and reduce
overvoltages.
.PP
3.1.2
The increased robustness of plastic insulated cables has the
effect of increasing the levels of overvoltages and overcurrents which can
circulate in the lines and be applied to equipment. By contrast the use of
miniature electronic components in exchange and transmission equipment
tends to increase its vulnerability to electrical disturbances.
.PP
For these reasons, in districts exposed to frequent and serious
disturbances (lightning, power lines, soil of low conductivity), it is
usually necessary to interpose protective devices of the types described
in \(sc\ 1.3
between the cable conductors and the equipment to which they are connected,
preferably on the MDF. This will prevent cables from the MDF to
equipment from having to carry heavy overcurrents.
.PP
The protective devices are fitted to the line side of the MDF to avoid
the need to carry discharge currents in the MDF jumper field and to expose
as little of the MDF wiring and terminal strips as possible to mains voltage
in
the event that a mains voltage line contact causes a series protective
device to disconnect the line.
.RT
.PP
3.1.3
In less exposed locations it may be that disturbances (voltages
and currents) have statistical characteristics of level and frequency so low
that in practice the risks do not exceed those resulting from the residual
effects indicated in \(sc\ 1.4 for exposed regions. Protective devices
then serve no purpose and are an unnecessary expense.
.sp 1P
.LP
3.2
\fINeed for equipment to have a minimum level of electrical\fR
\fIrobustness\fR
.sp 9p
.RT
.PP
In locations where lines are exposed and protective devices are
provided, the residual effects considered in \(sc\ 1 can cause overvoltages and
overcurrents to appear in the equipment. In less exposed environments the
disturbances described in \(sc\ 3.1.3 can cause similar effects. It is
necessary
for equipment to be designed to withstand these conditions and detailed
recommendations on the resistibility which equipment should possess are
given in Recommendation K.20.
.bp
.RT
.sp 1P
.LP
3.3
\fIEffect of switching conditions\fR
.sp 9p
.RT
.PP
Since the configuration and interconnection of equipment connected to a
given line is required to vary during the successive stages of connecting
a call, it is important not to limit the study of protection solely to
individual line equipments. Much equipment is common to all lines and can be
exposed to disturbances when connected to a particular line.
.PP
The effectiveness of the protection provided can be influenced by the reduction
in the probability of exposure if the effective duration of the
connection to lines is short. On the other hand common equipment should be
better protected since its failure risks more serious degradation in the
performance of the exchange or the district.
.RT
.sp 2P
.LP
\fB4\fR \fBProtection of subscribers' terminal equipment\fR
.sp 1P
.RT
.PP
The protection methods already set out for exchange equipment can often
be usefully applied to subscribers' equipment. Detailed tests to
determine the resistibility of subscriber equipment are given in
Recommendation\ K.21. It is also appropriate to consider the specific aspects
described below.
.RT
.sp 1P
.LP
4.1
\fIDegree of exposure\fR
.sp 9p
.RT
.PP
Lines to installations near exchanges in urban or industrial zones are
usually little exposed to surges on account of the screening effect of
numerous nearby metallic structures as described in \(sc\ 2.1.
.PP
On the other hand, lines to installations remote from built\(hyup areas
can be very exposed on account of their length, the absence of a protective
environment, overhead construction at the subscriber's end and the high
resistivity of the soil. The mechanical robustness of the overhead cables at
the subscriber's end makes the effect of surges all the more serious since
the line itself can carry higher voltages and currents.
.RT
.sp 1P
.LP
4.2
\fIDielectric strength\fR
.sp 9p
.RT
.PP
It is desirable to have a high dielectric strength for the
insulation between the conducting parts connected to the lines and all parts
accessible to the user.
.RT
.sp 1P
.LP
4.3
\fIUse of protectors\fR
.sp 9p
.RT
.PP
Where telephone lines are exposed to frequent and severe
disturbances from power line faults or lightning, the voltage of the lines
relative to local earth potential should be limited by connecting protective
devices of the types described in \(sc\ 1.3 between the line conductors and
the earth terminal.
.PP
The terminal equipment dielectric strength should be chosen taking
account of the breakdown voltage of the protective device and the impedance
of the protector\(hyline to earth connection.
.RT
.sp 1P
.LP
4.4\fR \fICommon bonding\fR
.sp 9p
.RT
.PP
At installations of subscriber terminal equipment a low resistance earth
for overvoltage protectors may be unavailable, or the costs of procuring
a suitable low\(hyresistance earth may be excessive compared to other installation
costs. Furthermore, the terminal equipment may be located adjacent to earthed
systems, such as water pipes, or may receive power from an electricity
system.
.PP
To minimize both equipment damage and exposure of the subscriber to
high voltages, even if the earth resistance is not sufficiently low, all
earthed systems, signalling earths and the power neutral should be bonded
.PP
together either directly or by means of a spark gap. Although this bonding
may be expensive it allows the difficulty of providing a low resistance
earth to be resolved and is a technique widely used. In some countries
connection to the
electricity system neutral is governed by national regulations, so that
agreement with the electrical Authority should be obtained.
.RT
.sp 1P
.LP
4.5
\fINational regulations\fR
.sp 9p
.RT
.PP
Many countries have national standards covering the protection of users
of telecommunications equipment not only from the risks associated with
connection to the electricity mains but also from conditions which may
appear on the telephone line.
.bp
.RT
.sp 1P
.LP
4.6
\fIHigh cost of maintenance of subscribers' installations\fR
.sp 9p
.RT
.PP
The cost of repairs at exposed terminal installations may be high by reason
of the distance from the maintenance centre, transport delays and,
possibly, the seriousness of the damage. Moreover, insufficient protection
is the cause of repeated interruptions of service which are particularly
damaging to the quality of service and the satisfaction of the customer.
This justifies the granting of special attention to protection measures.
.RT
.sp 2P
.LP
\fBReferences\fR
.sp 1P
.RT
.LP
[1]
CCITT manual \fIThe protection of telecommunication lines and equipment\fR
\fIagainst lightning discharges\fR , ITU, Geneva 1974, 1978.
.LP
[2]
CCITT \fIDirectives concerning the protection of telecommunications\fR
\fIlines against harmful effects from electric power and electrified railway\fR
\fIlines\fR , ITU, Geneva,\ 1988.
.sp 2P
.LP
\fBRecommendation\ K.12\fR
.RT
.sp 2P
.ce 1000
\fBCHARACTERISTICS\ OF\ \fR \fBGAS\ DISCHARGE\ TUBES\ FOR\ THE\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.12''
.OF '''Volume\ IX\ \(em\ Rec.\ K.12 %'
.ce 0
.sp 1P
.ce 1000
\fBPROTECTION\ OF\ TELECOMMUNICATIONS\ INSTALLATIONS\fR
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1972, modified at Malaga\(hyTorremolinos, 1984\fR |
\fIand at Melbourne, 1988)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.LP
\fBIntroduction\fR
.sp 1P
.RT
.PP
This Recommendation gives the basic requirements to be met by gas discharge
tubes for the protection of exchange equipment, subscribers' lines
and subscribers' equipment from over\(hyvoltages. It is intended to be used for
the harmonization of existing or future specifications issued by gas discharge
tube manufacturers, telecommunication equipment manufacturers, or
Administrations.
.PP
Only the minimum requirements are specified for essential
characteristics. As some users may be exposed to different environments
or have different operating conditions, service objectives or economic
constraints,
.PP
these requirements may be modified or further requirements may be added to
adapt them to local conditions.
.PP
This Recommendation gives guidance on the use of gas discharge tubes to
limit over\(hyvoltages on telecommunications lines.
.RT
.sp 2P
.LP
\fB1\fR \fBScope\fR
.sp 1P
.RT
.PP
This Recommendation:
.RT
.LP
a)
gives the characteristics of gas discharge tubes used in
accordance with CCITT Recommendation\ K.11 for protection of
exchange equipment, subscribers' lines and subscribers' equipment
against over\(hyvoltages,
.LP
b)
deals with gas discharge tubes having 2 or 3 electrodes,
.LP
c)
does not deal with mountings and their effect on tube
characteristics. Characteristics given apply to gas discharge
tubes by themselves mounted only in the ways described for the
tests,
.LP
d)
does not deal with mechanical dimensions,
.LP
e)
does not deal with quality assurance requirements,
.LP
f
)
does not deal with gas discharge tubes which are
connected in series with voltage\(hydependent resistors in order to limit
follow\(hyon currents in electrical power systems,
.LP
g)
may not be sufficient for gas discharge tubes used on high frequency
or multi\(hychannel systems.
.sp 2P
.LP
\fB2\fR \fBDefinitions\fR
.sp 1P
.RT
.PP
Appendix I gives definitions of a number of terms used in
connection with gas discharge tubes. It includes some terms not used in this
Recommendation.
.bp
.RT
.sp 2P
.LP
\fB3\fR \fBEnvironmental conditions\fR
.sp 1P
.RT
.PP
Gas discharge tubes shall be capable of withstanding during storage the
following conditions without damage:
.RT
.LP
\(em\ Temperature:
\(em40 to +90 | (deC;
.LP
\(em\ Relative\ humidity:
up to 95%.
.PP
See also \(sc\(sc 7.5 and 7.7.
.sp 2P
.LP
\fB4\fR \fBElectrical characteristics\fR
.sp 1P
.RT
.PP
Gas discharge tubes should have the following characteristics when tested
in accordance with \(sc\ 5.
.PP
Paragraphs 4.1 to 4.5 apply to new gas discharge tubes and also, where
quoted in \(sc\ 4.6, to tubes subjected to life tests.
.RT
.sp 1P
.LP
4.1
\fISpark\(hyover voltages\fR (see \(sc\(sc 5.1, 5.2 and
Figures\ 1/K.12, 2/K.12 and 3/K.12)
.sp 9p
.RT
.PP
4.1.1
Spark\(hyover voltages between the electrodes of a 2\(hyelectrode
tube or between either line electrode and the earth electrode of a 3\(hyelectrode
tube shall be within the limits in Table\ 1/K.12.
.LP
.sp 2
.ce
\fBH.T. [T1.12]\fR
.T&
lw(42p) | lw(42p) | lw(42p) | lw(54p) | lw(48p) .
.TE
.nr PS 9
.RT
.ad r
\fBTable 1/K.12 [T1.12], p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
.sp 2
4.1.2
For 3\(hyelectrode tubes, the spark\(hyover voltage between the line
electrodes shall not be less than the minimum d.c. spark\(hyover voltage in
Table\ 1/K.12.
.sp 1P
.LP
4.2
\fIHoldover voltages\fR (see \(sc 5.5 and Figures 4/K.12 and
5/K.12)
.sp 9p
.RT
.PP
All types of tube shall have a
current turn\(hyoff time
less than 150\ ms when subjected to one or more of the following tests
according to the projected use:
.RT
.PP
4.2.1
2\(hyelectrode tubes tested in a circuit equivalent to
Figure\ 4/K.12 where the test circuit components have the values in
Table\ 2/K.12.
.bp
.ce
\fBH.T. [T2.12]\fR
.T&
lw(72p) | lw(36p) | lw(36p) | lw(36p) .
.TE
.nr PS 9
.RT
.ad r
\fBTable 2/K.12 [T2.12], p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
4.2.2
3\(hyelectrode tubes tested in a circuit equivalent to Figure\ 5/K.12 where
components have the values in Table\ 3/K.12.
.ce
\fBH.T. [T3.12]\fR
.ce
TABLE\ 3/K.12
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(60p) | cw(54p) | cw(60p) | cw(54p) .
Component Test 1 Test 2 Test 3
_
.T&
cw(60p) | cw(54p) | cw(60p) | cw(54p) .
PS1 \ 52 V \ 80 V \ 135 V
_
.T&
cw(60p) | cw(54p) | cw(60p) | cw(54p) .
PS2 \ \ 0 V \ \ 0 V \ \ 52 V
_
.T&
cw(60p) | cw(54p) | cw(60p) | cw(54p) .
R3 260 \(*W 330 \(*W 1300 \(*W
_
.T&
cw(60p) | cw(54p) | cw(33p) | cw(27p) | cw(27p) | cw(27p) .
R2 \ua\d\u)\d 150 \(*W 272 \(*W | ub\d\u)\d 150 \(*W 272 \(*W | ub\d\u)\d
_
.T&
cw(60p) | cw(54p) | cw(33p) | cw(27p) | cw(27p) | cw(27p) .
C1 \ua\d\u)\d 100 nF 43 nF | ub\d\u)\d 100 nF 43 nF | ub\d\u)\d
_
.T&
cw(60p) | cw(54p) | cw(60p) | cw(54p) .
R4 | uc\d\u)\d 136 \(*W 136 \(*W \ 136 \(*W
_
.T&
cw(60p) | cw(54p) | cw(60p) | cw(54p) .
C2 | uc\d\u)\d \ 83 nF \ 83 nF {
\ \ 83 nF
\ua\d\u)\d
Components omitted in this test.
\ub\d\u)\d
Optional alternative.
\uc\d\u)\d
Optional.
}
_
.TE
.nr PS 9
.RT
.ad r
\fBTable 3/K.12 [T3.12], p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
4.3
\fIInsulation resistance\fR (see \(sc 5.3)
.sp 9p
.RT
.PP
Not less than 1000 Mohms initially.
.RT
.sp 1P
.LP
4.4
\fICapacitance\fR
.sp 9p
.RT
.PP
Not greater than 20 pF.
.RT
.sp 1P
.LP
4.5
\fIImpulse transverse voltage \(em 3\(hyelectrode tubes\fR (see \(sc\ 5.9
and Figure 6/K.12)
.sp 9p
.RT
.PP
The difference in time not to exceed 200\ ns.
.bp
.RT
.LP
.sp 1P
.LP
4.6
\fILife tests\fR (\(sc\(sc 5.6, 5.7 and 5.8)
.sp 9p
.RT
.PP
The currents specified in \(sc\ 4.6.1 for the appropriate nominal
current rating of the tube shall be applied. After each current application,
the gas discharge tube shall be capable of meeting the requirements of
\(sc\ 4.6.2. On completion of the number of current applications specified,
the tube shall be capable of meeting the requirements of \(sc\ 4.6.3.
.RT
.sp 1P
.LP
4.6.1
\fITest currents\fR
.sp 9p
.RT
.PP
Gas discharge tubes intended for use only on main distribution
frames or similar situations where connection to lines is via cable pairs,
shall be subjected to the currents of Columns\ 2 and\ 3 of Table\ 4/K.12. Gas
discharge tubes intended for applications where they are directly connected
to open wire lines will be designated EXT by the purchaser and shall be
subjected to the currents of Columns\ 2, 3 and\ 4 of Table\ 4/K.12.
.RT
.ce
\fBH.T. [T4.12]\fR
.T&
lw(24p) | lw(36p) | lw(48p) | lw(60p) | lw(60p) .
.TE
.nr PS 9
.RT
.ad r
\fBTable 4/K.12 [T4.12], p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
4.6.2
\fIRequirements during life test\fR \v'2p'
.sp 9p
.RT
.LP
Insulation resistance: not less than 10 Mohms.
.LP
D.c. and impulse spark\(hyover voltage: not more than the relevant
value in \(sc\ 4.1.
.sp 1P
.LP
4.6.3
\fIRequirements after completion of life test\fR \v'2p'
.sp 9p
.RT
.LP
Insulation resistance: not less than 100 Mohms (10 Mohms if
particularly specified by the purchaser).
.LP
D.c. and
impulse spark\(hyover voltage
: as in \(sc\ 4.1.
.LP
Holdover voltage: as in \(sc\ 4.2.
.sp 2P
.LP
\fB5\fR \fBTest methods\fR
.sp 1P
.RT
.sp 1P
.LP
5.1
\fID.c. spark\(hyover voltage\fR (see \(sc 4.1 and Figures 1/K.12
and 2/K.12)
.sp 9p
.RT
.PP
The gas discharge tube shall be placed in darkness for at least
24\ hours immediately prior to testing and tested in darkness with a voltage
which increases so slowly that the spark\(hyover voltage is independent of the
rate of rise of the applied voltage. Typically, a rate of rise of 100\ V/s is
used, but higher rates may be used if it can be shown that the spark\(hyover
.PP
voltage is not significantly changed thereby. The tolerances on the wave\(hyshape
of the rising test voltage are indicated in Figure\ 1/K.12. The voltage
is
measured across the open\(hycircuited terminals of the generator. \fIU\fR\d\fIm\fR\\d\fIa\fR\\d\fIx\fR\uof
Figure\ 1/K.12 is any voltage greater than the maximum permitted
d.c.\ spark\(hyover voltage of the gas discharge tube and less than three\
times the minimum permitted d.c. spark\(hyover voltage of the gas discharge
tube.
.bp
.PP
The test shall employ a suitable circuit such as that shown in
Figure\ 2/K.12. A minimum of 15\ minutes shall elapse between repetitions
of the test, with either polarity, on the same gas discharge tube.
.PP
Each pair of terminals of a 3\(hyelectrode gas discharge tube shall be
tested separately with the other terminal unterminated.
.PP
\fINote\fR \ \(em\ The use of Figure 1/K.12 may be explained as follows:
.PP
A single mask will do for all values of \fIU\fR\d\fIm\fR\\d\fIa\fR\\d\fIx\fR\uand
the
nominal rate of rise, provided that it is a suitable size for the display of
the waveform and that the scales of \fIU\fR and\ \fIT\fR of the waveform can be
adjusted. This follows because the Y\(hyaxis has arbitrary points marked\ 0
and\ \fIU\fR\d\fIm\fR\\d\fIa\fR\\d\fIx\fR\uwith 0.2\ \fIU\fR\d\fIm\fR\\d\fIa\fR\\d\fIx\fR\uat
the appropriate point between them while the X\(hyaxis has arbitrary
.PP
points marked 0 and \fIT\fR\d2\uwith \fIT\fR\d1\u(=\ 0.2\ \fIT\fR\d2\u),
0.9\ \fIT\fR\d1\u,
1.1\ \fIT\fR\d1\u, 0.9\ \fIT\fR\d2\u, 1.1\ \fIT\fR\d2\umarked at the appropriate
points. The X and\ Y zeros need not coincide and, in fact, need not be
shown at all.
.PP
To compare a waveform trace with the mask, it is necessary to know the
values of \fIU\fR\d\fIm\fR\\d\fIa\fR\\d\fIx\fR\uand the nominal rate of
rise for the waveform in
question. As an example, consider a waveform with \fIU\fR\d\fIm\fR\\d\fIa\fR\\d\fIx\fR\u\
=\ 750\ V and nominal rate of rise\ =\ 100\ V/sec.
.PP
Then 0.2 \fIU\fR\d\fIm\fR\\d\fIa\fR\\d\fIx\fR\u= 150 V, \fIT\fR\d2\u= 7.5
s, \fIT\fR\d1\u= 1.5 s.
.PP
Hold the mask against the trace and adjust the vertical scale so that the
150\ V calibration is against 0.2\ \fIU\fR\d\fIm\fR\\d\fIa\fR\\d\fIx\fR\uand
the 750\ V point
against \fIU\fR\d\fIm\fR\\d\fIa\fR\\d\fIx\fR\u. Adjust the horizontal scale
similarly for
1.5\ s\ =\ \fIT\fR\d1\uand 7.5\ s\ =\ \fIT\fR\d2\u. Slide the mask so that
the 150\ V point on the trace is within the bottom boundary of the test
window; the remainder of
the trace up to\ 750\ V must be within the test window.
.RT
.sp 1P
.LP
5.2
\fIImpulse spark\(hyover voltage\fR (\(sc 4.1 and Figures 1/K.12
and 3/K.12)
.sp 9p
.RT
.PP
The gas discharge tube shall be placed in darkness for at least
.PP
15\ minutes immediately prior to testing and tested in darkness. The voltage
waveform measured across the open circuit test terminals shall have a nominal
rate of rise selected from \(sc\ 4.1 and shall be within the enclosed limits
indicated in Figure\ 1/K.12. Figure\ 3/K.12 shows a suggested arrangement for
testing with a voltage impulse having a nominal rate of rise of 1.0\ kV/\(*ms.
.PP
A minimum of 15 minutes shall elapse between repetitions of the test, with
either polarity, on the same gas discharge tube.
.PP
Each pair of terminals of a 3\(hyelectrode gas discharge tube shall be
tested separately with the other terminal unterminated.
.RT
.sp 1P
.LP
5.3
\fIInsulation resistance\fR (\(sc\ 4.3)
.sp 9p
.RT
.PP
The insulation resistance shall be measured from each terminal to every
other terminal of the gas discharge tube. The measurement shall be made
at an applied potential of at least 100\ V and not more than 90% of the
minimum permitted d.c. spark\(hyover voltage. The measuring source shall
be limited to a short circuit current of less than 10\ mA. Terminals of
three\(hyelectrode gas
discharge tubes not involved in the measurement shall be left
unterminated.
.RT
.LP
.sp 1P
.LP
5.4
\fICapacitance\fR (\(sc 4.4)
.sp 9p
.RT
.PP
The capacitance shall be measured between each terminal and every other
terminal of the gas discharge tube. In measurements involving 3\(hyelectrode
gas discharge tubes, the terminal not being tested shall be connected to
a
ground plane in the measuring instrument.
.RT
.sp 2P
.LP
5.5
\fIHoldover test\fR (\(sc 4.2)
.sp 1P
.RT
.sp 1P
.LP
5.5.1
\fI2\(hyelectrode gas discharge tube\fR (Figure 4/K.12)
.sp 9p
.RT
.PP
Tests shall be conducted using the circuit of Figure 4/K.12. Values of
PS1, R2, R3 and\ C1 shall be selected for each test condition from
Table\ 2/K.12. The current from the surge generator shall have an impulse
waveform of 100\ A, 10/1000 or\ 10/700 measured through a short circuit
replacing the gas discharge tube under test. The polarity of the impulse
current through the gas discharge tube shall be the same as the current
from\ PS1. The time for current turn\(hyoff shall be measured for each
direction of current passage
through the gas discharge tube. Three impulses shall be applied at not
greater than 1\(hyminute intervals and the current turn\(hyoff time measured
for each
impulse.
.bp
.RT
.LP
.sp 1P
.LP
5.5.2
\fI3\(hyelectrode gas discharge tube\fR (Figure 5/K.12)
.sp 9p
.RT
.PP
Tests shall be conducted using the circuit of Figure 5/K.12. Values of
circuit components shall be selected from Table\ 3/K.12. The simultaneous
currents that are applied to the gaps of the gas discharge tube shall have
impulse waveforms of 100\ A, 10/1000 or\ 10/700 measured through a short
circuit replacing the gas discharge tube under test. The polarity of the
impulse
current through the gas discharge tube shall be the same as the current
from\ PS1 and\ PS2.
.PP
For each test condition, measurement of the time to current turn\(hyoff
shall be made for both polarities of the impulse current. Three impulses
in
each direction shall be applied at intervals not greater than 1\ minute
and the time to current turn\(hyoff measured for each impulse.
.RT
.sp 1P
.LP
5.6
\fIImpulse life\fR \(em \fIall types of gas discharge tube\fR (\(sc 4.6)
.sp 9p
.RT
.PP
Fresh gas discharge tubes shall be used and impulse currents shall be applied
as specified in Table\ 4/K.12, Column\ 3, for the relevant nominal
current of the tube. Half the specified number of tests shall be carried out
.PP
with one polarity followed by half with the opposite polarity. Alternatively,
half the tubes in a sample may be tested with one polarity and the other
half with the opposite polarity. The pulse repetition rate should be such
as to
prevent thermal accumulation in the gas discharge tube.
.PP
The voltage of the source shall exceed the maximum impulse spark\(hyover
voltage of the gas discharge tube by not less than 50\ per cent. The specified
impulse discharge current and waveform shall be measured with the gas discharge
tube replaced with a short circuit. For 3\(hyelectrode gas discharge tubes,
independent impulse currents each having the value specified in Table\
4/K.12, Column\ 3, shall be discharged simultaneously from each electrode
to the common electrode.
.PP
The gas discharge tube shall be tested after each passage of impulse
discharge current or at less frequent intervals if agreed between the supplier
and the purchaser to determine its ability to satisfy the requirements
of
\(sc\ 4.6.2.
.PP
On completion of the specified number of impulse currents the tube
shall be allowed to cool to ambient temperature and tested for compliance
with \(sc\ 4.6.3.
.RT
.sp 1P
.LP
5.7
\fIImpulse life \(em additional tests for tubes designated EXT\fR (\(sc\ 4.6)
.sp 9p
.RT
.PP
As in \(sc 5.6, but applying the conditions of Table 4/K.12,
column\ 4.
.RT
.sp 1P
.LP
5.8
\fIA.c. life\fR \(em \fIall types of tube\fR (\(sc\ 4.6)
.sp 9p
.RT
.PP
Fresh tubes shall be used and alternating currents applied as
specified in Table\ 4/K.12, Column\ 2, for the relevant nominal current of the
tube.
.PP
The time between applications should be such as to prevent thermal
accumulation in the tube. The rms a.c. voltage of the current source shall
exceed the maximum d.c. spark\(hyover voltage of the gas discharge tube by not
less than 50\ per cent.
.PP
The specified a.c. discharge current and duration shall be measured
with the gas discharge tube replaced with a short circuit. For 3\(hyelectrode
gas discharge tubes, a.c. discharge currents each having the value specified
in
Table\ 4/K.12 shall be discharged simultaneously from each electrode to the
common electrode.
.PP
The gas discharge tube shall be tested after each passage of a.c.
discharge current to determine its ability to satisfy the requirements of
\(sc\ 4.6.2.
.PP
On completion of the specified number of current applications, the
tube shall be allowed to cool to ambient temperature and tested for compliance
with \(sc\ 4.6.3.
.RT
.sp 1P
.LP
5.9
\fIImpulse transverse voltage\fR (\(sc 4.5 and Figure 6/K.12)
.sp 9p
.RT
.PP
The duration of the transverse voltage shall be measured while an impulse
voltage having a virtual steepness of impulse wavefront of 1\ kV/\(*ms
is applied simultaneously to both discharge gaps. Measurement may be made
with an arrangement as indicated in Figure\ 6/K.12. The difference in time
between the spark\(hyover of the first gap and that of the second is specified
in \(sc\ 4.5.
.bp
.RT
.sp 2P
.LP
\fB6\fR \fBRadiation\fR
.sp 1P
.RT
.PP
The emerging radiation from any radioactive matter used to
pre\(hyionize the discharge gaps must be within the limits specified as
admissible in the regulations concerning the protection from radiation
which are issued by the country of the manufacturer as well as of the user.
This provision applies both to individual and to a batch of gas discharge
tubes (for example, when
packed in a cardboard box for dispatch, storage,\ etc.).
.PP
The supplier of gas discharge tubes containing radioactive materials shall
provide recommendations, complying with the International Atomic Energy
Agency\ (IAEA) \*QRegulations for the safe transport of radio
active materials\*U and with all other relevant international requirements,
on the following
matters:
.RT
.LP
a)
maximum number of items per package,
.LP
b)
maximum quantity per shipment,
.LP
c)
maximum quantity which may be stored together,
.LP
d)
any other storage requirements,
.LP
e)
handling precautions and requirements,
.LP
f
)
disposal arrangements.
.sp 2P
.LP
\fB7\fR \fBEnvironmental tests\fR
.sp 1P
.RT
.sp 1P
.LP
7.1
\fIRobustness of terminations\fR
.sp 9p
.RT
.PP
The user shall specify a suitable test from International
Electrotechnical Commission\ (IEC) standard
68\(hy2\(hy21\ (1975) if
applicable.
.RT
.sp 1P
.LP
7.2
\fISolderability\fR
.sp 9p
.RT
.PP
Soldering terminations shall meet the requirements of IEC standard 68\(hy2\(hy20
(1979) Test Ta Method\ 1.
.RT
.LP
.sp 1P
.LP
7.3
\fIResistance to soldering heat\fR
.sp 9p
.RT
.PP
Gas discharge tubes with soldering terminations shall be capable of withstanding
IEC standard
68\(hy2\(hy20\ (1979) Test\ Tb Method\ 1B. After recovery, the gas discharge
tube shall be visually checked and show no signs of damage
and its d.c. spark\(hyover shall be within the limits for that tube.
.RT
.sp 1P
.LP
7.4
\fIVibration\fR
.sp 9p
.RT
.PP
A gas discharge tube shall be capable of withstanding IEC standard 68\(hy2\(hy6\
(1970) 10\(hy500\ Hz, 0.15\ mm displacement for 90\ minutes without damage.
The user may select a more severe test from the document. At the end of the
test, the tube shall show no signs of damage and shall meet the d.c.\ spark\(hyover
and insulation resistance requirements specified in \(sc\(sc\ 4.1 and\
4.3.
.RT
.sp 1P
.LP
7.5
\fIDamp heat cyclic\fR
.sp 9p
.RT
.PP
A gas discharge tube shall be capable of withstanding IEC standard 68\(hy2\(hy4
Test\ D Severity\ IV. At the end of the test, the tube shall meet the
insulation resistance requirement specified in \(sc\ 4.3.
.RT
.sp 1P
.LP
7.6
\fISealing\fR
.sp 9p
.RT
.PP
A gas discharge tube shall be capable of passing IEC standard
68\(hy2\(hy17\ (1978) Test\ Qk, severity 600\ hours, for fine leaks. Helium
shall be
used as the test gas. The fine leak rate shall be less than
10\uD\dlF261\u7\d\ bar | (mu | m\u3\d | (mu | \uD\dlF261\u1\d.
.PP
The tube shall then be capable of passing the coarse leak test\ Qc
Method\ 1.
.bp
.RT
.sp 1P
.LP
7.7
\fILow temperature\fR
.sp 9p
.RT
.PP
A gas discharge tube shall be capable of withstanding IEC standard 68\(hy2\(hy1\
Test\ Aa. \(em40 | (deC, duration 2\ hours, without damage. While at \(em40 | (deC
the tube must meet the d.c. and impulse spark\(hyover requirements of\
\(sc\ 4.1.
.RT
.sp 2P
.LP
\fB8\fR \fBIdentification\fR
.sp 1P
.RT
.sp 1P
.LP
8.1
\fIMarking\fR
.sp 9p
.RT
.PP
Legible and permanent marking shall be applied to the tube as
necessary to ensure that the purchaser can determine the following information
by inspection:
.RT
.LP
a)
manufacturer,
.LP
b)
year of manufacture,
.LP
c)
type.
.PP
The purchaser may specify the codes to be used for this
marking.
.sp 1P
.LP
8.2
\fIDocumentation\fR
.sp 9p
.RT
.PP
Documents shall be provided to the purchaser so that from the
information in \(sc\ 8.1 he can determine the following further
information:
.RT
.LP
a)
full characteristics as set out in this Recommendation,
.LP
b)
name of radioactive material used in the tube or statement that such
material has not been used.
.sp 2P
.LP
\fB9\fR \fBOrdering information\fR
.sp 1P
.RT
.PP
The following information should be supplied by the
purchaser:
.RT
.LP
a)
drawing giving all dimensions, finishes and termination
details (including numbers of electrodes and identifying
the earth electrode),
.LP
b)
nominal d.c. spark\(hyover voltage, chosen from \(sc\ 4.1.1,
.LP
c)
nominal current rating chosen from \(sc\ 4.6.1,
.LP
d)
the designation EXT if the tests of Table 4/K.12, column\ 4, are required,
.LP
e)
holdover voltage tests required in \(sc\ 4.2,
.LP
f
)
marking codes required for \(sc 8.1,
.LP
g)
robustness of terminations \(em test required for \(sc 7.1,
.LP
h)
destruction characteristic, if required, including failure mode (see\ Note),
.LP
i)
quality assurance requirements.
.PP
\fINote\fR \ \(em\ After passage of an alternating or impulse current of
value much higher than that shown in \(sc\ 4.6.1, the gas discharge tube may be
destroyed, i.e.\ its electrical characteristics may be greatly modified. Two
situations may occur:
.LP
1)
The gas discharge tube becomes in effect an insulator and presents a
higher dielectric strength than it had initially \(em\ that is to say, it
becomes open circuit.
.LP
2)
The gas discharge tube becomes of limited resistance \(em generally a low
value which does not allow normal operation of the line\ \(em that is to
say it becomes a short circuit. (This situation may be preferable
from the point of view of protection and maintenance.)
.LP
Test methods and the relations between the value and duration of the
destructive current are not detailed in this Recommendation nor is the
state of the element after destruction. Administrations should cover their
requirements in these respects in their own documentation.
.bp
.LP
.rs
.sp 30P
.ad r
\fBFigure 1/K.12, p.11\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 17P
.ad r
\fBFigure 2/K.12, p.12\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 21P
.ad r
\fBFigure 3/K.12, p.13\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 25P
.ad r
\fBFigure 4/K.12, p.14\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 31P
.ad r
\fBFigure 5/K.12, p.15\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 17P
.ad r
\fBFigure 6/K.12, p.16\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce 1000
APPENDIX\ I
.ce 0
.ce 1000
(to Recommendation K.12)
.sp 9p
.RT
.ce 0
.ce 1000
\fBDefinitions of terms associated with gas discharge tubes\fR
.sp 1P
.RT
.ce 0
.LP
I.1
\fBarc current\fR
.sp 1P
.RT
.PP
The current which flows after spark\(hyover when the circuit
impedance allows a current that exceeds the glow\(hyto\(hyarc transition
current.
.RT
.sp 1P
.LP
I.2
\fBarc voltage\fR
.sp 9p
.RT
.PP
The voltage appearing across the terminals of the gas discharge
tube during the passage of the arc current.
.RT
.sp 1P
.LP
I.3
\fBbreakdown\fR
.sp 9p
.RT
.PP
See \*Qspark\(hyover\*U.
.RT
.sp 1P
.LP
I.4
\fBcurrent turnoff time\fR
.sp 9p
.RT
.PP
The time required for the gas discharge tube to return itself to a nonconducting
state following a period of conduction.
.RT
.LP
.sp 1P
.LP
I.5
\fBdestruction characteristic\fR
.sp 9p
.RT
.PP
The relationship between the value of the discharge current and
the time of flow until the gas discharge tube is mechanically destroyed
(break, electrode short circuit). For periods of time between 1\ \(*ms
and some ms, it is based on impulse discharge currents, and for periods
of time of 0.1\ s and
greater, it is based on alternating discharge currents.
.RT
.sp 1P
.LP
I.6
\fBdischarge current\fR
.sp 9p
.RT
.PP
The current that passes through a gas discharge tube when
spark\(hyover occurs.
.RT
.sp 1P
.LP
I.7
\fBdischarge current, alternating\fR
.sp 9p
.RT
.PP
The r.m.s. value of an approximately sinusoidal alternating
current passing through the gas discharge tube.
.RT
.sp 1P
.LP
I.8
\fBdischarge current, impulse\fR
.sp 9p
.RT
.PP
\fB
The peak value of the impulse current passing through the gas
discharge tube.
.RT
.sp 1P
.LP
I.9
\fBdischarge voltage\fR
.sp 9p
.RT
.PP
The voltage that appears across the terminals of a gas discharge tube during
the passage of discharge current. Also referred to as \*Qresidual
voltage\*U.
.RT
.LP
.sp 1P
.LP
I.10
\fBdischarge voltage/current characteristic\fR
.sp 9p
.RT
.PP
The variation of crest values of discharge voltage with respect to discharge
current.
.RT
.sp 1P
.LP
I.11
\fBfollow current\fR
.sp 9p
.RT
.PP
The current from the connected power source that passes through a gas discharge
tube during and following the passage of discharge current.
.RT
.sp 1P
.LP
I.12
\fBgas discharge tube\fR
.sp 9p
.RT
.PP
A gap, or several gaps, in an enclosed discharge medium, other
than air at atmospheric pressure, designed to protect apparatus or personnel,
or both, from high transient voltages. Also referred to as \*Qgas tube
surge
arrester\*U.
.RT
.sp 1P
.LP
I.13
\fBglow current\fR
.sp 9p
.RT
.PP
The current which flows after spark\(hyover when circuit impedance
limits the discharge current to a value less than the glow\(hyto\(hyarc
transition
current.
.RT
.sp 1P
.LP
I.14
\fBglow\(hyto\(hyarc transition current\fR
.sp 9p
.RT
.PP
The current required for the gas discharge tube to pass from the glow mode
into the arc mode.
.bp
.RT
.sp 1P
.LP
I.15
\fBglow voltage\fR
.sp 9p
.RT
.PP
The voltage drop across the terminals of the gas discharge tube
during the passage of glow current.
.RT
.sp 1P
.LP
I.16
\fBholdover voltage\fR
.sp 9p
.RT
.PP
The maximum d.c. voltage across the terminals of a gas discharge tube under
which it may be expected to clear and to return to the high
impedance state after the passage of a surge, under specified circuit
conditions.
.RT
.sp 1P
.LP
I.17
\fBimpulse spark\(hyover voltage/time curve\fR
.sp 9p
.RT
.PP
The curve which relates the impulse spark\(hyover voltage to the time to
spark over.
.RT
.sp 1P
.LP
I.18
\fBimpulse waveform\fR
.sp 9p
.RT
.PP
An impulse waveform designated as \fIx\fR /\fIy\fR has a rise time of
\fIx\fR \ \(*ms and a decay time to half value of \fIy\fR \ \(*ms as standardized
in
IEC\ Publication\ 60.
.RT
.sp 1P
.LP
I.19
\fBnominal alternating discharge current\fR
.sp 9p
.RT
.PP
For currents with a frequency of 15 Hz to 62 Hz, the alternating discharge
current which the gas discharge tube is designed to carry for a
defined time.
.RT
.LP
.sp 1P
.LP
I.20
\fBnominal d.c. spark\(hyover voltage\fR
.sp 9p
.RT
.PP
The voltage specified by the manufacturer to designate the gas
discharge tube (type designation) and to indicate its application with
respect to the service conditions of the installation to be protected.
Tolerance limits of the d.c. spark\(hyover voltage are also referred to
the nominal d.c. spark\(hyover voltage.
.RT
.sp 1P
.LP
I.21
\fBnominal impulse discharge current\fR
.sp 9p
.RT
.PP
The peak value of the impulse current with a defined wave shape
with respect to time for which the gas discharge tube is rated.
.RT
.sp 1P
.LP
I.22
\fBresidual voltage\fR
.sp 9p
.RT
.PP
See \*Qdischarge voltage\*U.
.RT
.sp 1P
.LP
I.23
\fBspark\(hyover\fR
.sp 9p
.RT
.PP
An electrical breakdown of a discharge gap of a gas discharge
tube. Also referred to as \*Qbreakdown\*U.
.RT
.sp 1P
.LP
I.24
\fBspark\(hyover voltage\fR
.sp 9p
.RT
.PP
The voltage which causes spark\(hyover when applied across the
terminals of a gas discharge tube.
.RT
.LP
.sp 1P
.LP
I.25
\fBspark\(hyover voltage, a.c.\fR
.sp 9p
.RT
.PP
The minimum r.m.s. value of sinusoidal voltage at frequencies
between 15\ Hz and 62\ Hz that results in spark\(hyover.
.RT
.sp 1P
.LP
I.26
\fBspark\(hyover voltage, d.c.\fR
.sp 9p
.RT
.PP
The voltage at which the gas discharge tube sparks over with
slowly increasing d.c. voltage.
.RT
.sp 1P
.LP
I.27
\fBspark\(hyover voltage, impulse\fR
.sp 9p
.RT
.PP
The highest voltage which appears across the terminals of a gas
discharge tube in the period between the application of an impulse of given
waveshape and the time when current begins to flow.
.RT
.sp 1P
.LP
I.28
\fBtransverse voltage\fR
.sp 9p
.RT
.PP
For a gas discharge tube with several gaps, the difference of the discharge
voltages of the gaps assigned to the two conductors of a
telecommunications circuit during the passage of discharge
current.
.bp
.RT
.sp 2P
.LP
\fBRecommendation\ K.13\fR
.RT
.sp 2P
.ce 1000
\fBINDUCED\ VOLTAGES\ IN\ CABLES\ WITH\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.13''
.OF '''Volume\ IX\ \(em\ Rec.\ K.13 %'
.ce 0
.sp 1P
.ce 1000
\fBPLASTIC\(hyINSULATED\ CONDUCTORS\fR
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1972)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
According to [1], when a fault occurs on a power line near a
telecommunication cable having all its circuits terminated by transformers,
the permissible induced longitudinal voltage in the cable conductors should
not
exceed 60% of the voltage used to check the dielectric strength of the
cable, as required by individual specifications for checks of the breakdown
strength between the cable conductors and the sheath. This induced voltage
is generally 1200\ V r.m.s. value for paper\(hyinsulated conductors (60%
of 2000\ V). The
\fIDirectives\fR give no indication of the frequency of occurrence of such a
voltage or of its permissible duration. In order that such voltages do not
endanger line maintenance staff, the safety precautions for staff given
in\ [2] must be observed.
.sp 1P
.RT
.LP
.PP
Plastic\(hyinsulated cables can have a much higher
dielectric
strength
than paper\(hyinsulated cables. Moreover, this dielectric strength is retained
following the mechanical stresses that occur during the laying of the cable.
There should thus be no danger of breakdown of the insulation between the
conductors and the metal sheath when it is subjected to induced logitudinal
e.m.f. sufficiently below the breakdown voltage of the cable. A sufficient
safety margin is ensured if induced voltages are kept below 60% of the
voltage used for checking the dielectric strength of the cable as given
in the
individual specifications; this voltage is, of course, related to the
breakdown voltage
.
.PP
At very little extra expense, sleeves and joints can be made to have the
same dielectric strength as the insulation between the conductors and the
metallic sheath, although transformers and terminal equipments must be
suitably protected when their dielectric strength is not up to the conditions
concerned.
.PP
If the source of the induced longitudinal e.m.f. is a high\(hyreliability
power line, as defined in the \fIDirectives\fR , there is only a very small
probability that staff will be in contact with a line at the precise moment
.PP
when such a voltage of short duration occurs in the telecommunication cable.
Any danger to staff is very slight given due observation of the safety
precautions for maintenance staff working on telephone lines in which high
voltages may be induced by neighbouring electricity lines.
.PP
For a cable not having its circuits terminated by transformers the
above conditions also apply provided that surge voltages are prevented from
reaching the telecommunication equipment by the striking of the lightning
protectors installed at the ends of the circuits.
.RT
.sp 2P
.LP
\fIFor these reasons, the CCITT is unanimously of the opinion that:\fR
.sp 1P
.RT
.PP
\fB1\fR
It is possible to make telecommunication cables with
conductors that are insulated from each other and from the metallic sheath
by high breakdown strength plastics. For such cables, when there is a fault
on a neighbouring electricity line, the value of induced longitudinal e.m.f.
that
can be allowed is that which does not exceed 60% of the test voltage applied
between the conductors and the metallic sheath for checking the dielectric
strength (this test voltage, which is given in the individual cable
specifications, is related to the breakdown voltage) provided the following
conditions are observed:
.sp 9p
.RT
.LP
a)
circuits in such cables are terminated at their ends and at branching
points on transformers or are provided with
lightning
protectors
;
.LP
b)
equipment, joints and cableheads associated with such cables must have
a dielectric strength at least equal to that of the
insulation between the conductors and the metallic cable sheath
of the cable, given that the transformers mentioned in a) above
must be provided with lightning protectors when their dielectric
strength does not meet the required conditions;
.LP
c)
the power line causing the induction must meet the
conditions for high\(hyreliability power lines given in\ [1];
.LP
d)
staff working on telecommunication cables must take the
safety precautions specified in\ [2].
.PP
\fB2\fR
\fR When the circuits of such a cable are connected direct to the telecommunication
equipment, that is, when no transformers or lightning
protectors are inserted, and when the condition laid down in \(sc\ 1c)
above is\fI\fR fulfilled, the maximum permissible induced longitudinal
e.m.f. should be
650\ V.
.bp
.sp 9p
.RT
.sp 2P
.LP
\fBReferences\fR
.sp 1P
.RT
.LP
[1]
CCITT manual \fIDirectives concerning the protection of telecommunication\fR
\fIlines against harmful effects from electric power and electrified railway\fR
\fIlines\fR , Vol.\ VI, ITU, Geneva, 1988.
.LP
[2]
\fIIbid.\fR , Vol.\ VII.
.sp 2P
.LP
\fBRecommendation\ K.14\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBPROVISION\ OF\ A\ \fR \fBMETALLIC\ SCREEN\ IN\ PLASTIC\(hySHEATHED\
CABLES\fR
.EF '% Volume\ IX\ \(em\ Rec.\ K.14''
.OF '''Volume\ IX\ \(em\ Rec.\ K.14 %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1972; modified at Malaga\(hyTorremolinos, 1984)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.PP
A metal sheath provides a cable with
electrostatic
screening
and a degree of magnetic screening. A plastic sheath has no
intrinsic screening properties. Some plastic\(hysheathed cables, for example
those with paper\(hyinsulated cores, incorporate a metal screen as a water
barrier. Such a metal screen, which is usually in the form of a longitudinally
applied
.sp 1P
.RT
.LP
aluminium tape, provides the same screening properties as a nonferrous metal
sheath of the same longitudinal conductivity. The tape must, however, be
connected to the telephone exchange earth electrode systems at its ends
and/or to conveniently located earthing points, such as metal cable sheaths,
along its length. It is also important that at jointing points the tape
be extended
through by connections of very low resistance. Although the degree of screening
provided by the tape may be small at 50\ Hz, it can be considerable at
frequencies which give rise to noise interference. The presence of a screen
on a cable also reduces the induction arising from the high\(hyfrequency
components of transients caused by
power\(hyline
switching and also induced
transients from lightning strokes; such transient induced voltages are of
increasing importance with the increasing use of miniaturized telecommunication
equipment with very small thermal capacity.
.PP
On the basis of the above considerations and experience with the use of
plastic\(hysheathed cables,
.LP
.sp 2P
.LP
\fIthe CCITT recommends that the following provisions be observed:\fR
.sp 1P
.RT
.PP
\fB1\fR
Since plastic\(hysheathed subscriber distribution cables without a screen
give satisfaction for distribution from the exchange to subscribers, they
may be used in localities where there are no alternating current
electrified railways
. However, account must always be taken of the risk of noise interference
that may arise in the vicinity of electric railways,
especially those with
thyristor
controlled equipment in the
locomotives. Consideration should also be given to possible interference by
radio transmitters which operate in the same frequency range as the circuits
in the plastic\(hysheathed cable.
.sp 9p
.RT
.PP
\fB2\fR
Trunk and junction cables should contain a screen which can have the form
of an aluminium\(hytape
water barrier
. Cables provided with a screen having a conductance of the order of half
that of a cable having the same core diameter, but with a
lead sheath
, have given complete
satisfaction where there are no risks of severe magnetic induction.
.sp 9p
.RT
.PP
\fB3\fR
If a
plastic\(hysheathed cable
is provided with a screen of a conductance equivalent to that of a conventional
lead\(hysheathed cable, then in the presence of induction the plastic\(hysheathed
cable can be used in entirely the same circumstances as the lead\(hysheathed
cable.
.sp 9p
.RT
.LP
.PP
\fB4\fR
If the effect of the screen according to \(sc\(sc\ 2 and 3\ above is not
sufficient to limit the magnetic induction at mains frequencies, or to
these harmonics arising from neighbouring power lines or electric railways,
to permissible values the screening factor can be improved by increasing:
.sp 9p
.RT
.PP
4.1
the inductance of the
metal sheath
, if necessary, by a
lapping of steel tapes;
.PP
4.2
the conductance of the existing screen by additional metal tapes or wires
which are arranged below the screen.
.PP
An improved screening effect may also become necessary if there is the
risk of noise interference
in the vicinity of electric railways
equipped with thyristor controlled devices.
.bp
.PP
\fB5\fR
The screen must be connected to the earth electrode systems of the telecommunication
centres. In the case of subscribers' cables the remote end should be connected
to a suitable earth. It is also important for the
screen of the cable to be extended through at cable joints by means of
connections of very low resistance.
.sp 9p
.RT
.PP
\fB6\fR
In view of the increase in the number of electrical
installations and the level of harmonics resulting from new techniques,
it is to be expected that the effects of interference will become worse.
This being so, it may be extremely useful to improve the
screening effect
of
plastic\(hycovered cables as indicated above.
.sp 9p
.RT
.PP
\fB7\fR
If cables have to be laid in areas where there is a danger of atmospheric
discharges, attention is drawn to the importance of the metallic
screen and of its construction in the protection of cables against lightning
and also to the importance of the interconnections between the screen and
other structures. (See the manual cited in\ [1].)
.sp 9p
.RT
.sp 2P
.LP
\fB8\fR \fBScreening factor\fR
.sp 1P
.RT
.PP
The following considerations enable the screening factor at the
mains frequency to be determined fairly accurately for all types of cable
regardless of the outer plastic covering used. In particular, they show
how the screening factor to be used in practice may vary depending on the
conditions in which the cable is used.
.RT
.LP
.sp 1P
.LP
8.1
\fIGeneral\fR
.sp 9p
.RT
.PP
The screening effect produced by the metal screen of a cable mainly depends on:
.RT
.LP
\(em
the frequency of the induced e.m.f. The limitation of this
e.m.f. mains frequency (16 | /3\ Hz, 50\ Hz, 60\ Hz) is therefore a
determining factor in the choice of a cable from the standpoint
of safety of staff and installations. On the other hand, the
screening factor at higher frequencies should also be taken into
account in seeking to protect equipment against interference. A
substantial reduction of the induced e.m.f. at the mains
frequency may suffice for complete protection;
.LP
\(em
the level of induced e.m.f. per unit length in the case of
screens made by ferromagnetic material. The screening effect of
such a cable is optimum for a given value of induced e.m.f. per
unit length, so that a cable designed for the reduction of high
induced e.m.f. per unit length may be of no practical use for
protection against low induced e.m.f. per unit length. The
.LP
composition of the screen must be adapted to the level of the
induced e.m.f. per unit length;
.LP
\(em
the quality of its earthing. The screening effect is
determined by the value of the current circulating in the metal
screen. The resistance of the parts ensuring current flow
between screen and earth is therefore decisive. For cables with
an insulating plastic outer covering, if earth connections are
provided only at the ends, they must be of very low resistance:
the sheath should preferably be earthed at intervals along the
line. When the plastic outer covering is conductive, the sheath
is in practice continuously earthed;
.LP
\(em
the length of the induced section of the link to be
protected.
It is easier to improve the screening effect when this
section is long. The concept of length in this case relates
to the quality of earthing required.
.sp 1P
.LP
8.1.1
\fIThe screening factor\fR (for explanation of symbols, see
Appendix\ I)
.sp 9p
.RT
.PP
The following most frequently used screening factors are defined in the
\fIDirectives:\fR
.RT
.LP
\(em
Nominal screening factor, \fIk\fR\d\fIn\fR\u(see Figure\ 1/K.14). This
factor can easily be measured in a laboratory and is used to
qualify the efficiency of the screening effect.
.LP
.rs
.sp 9P
.ad r
\fBFigure 1/K.14, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
\(em
Screening factor related to distant earth,
\fIk\fR\d\fIf\fR\\d\fIf\fR\\d`\u(see\ Figure\ 2/K.14). This factor must
be taken into account in
ensuring protection against danger and interference, the
conductors of the subscriber pairs being connected at their
terminals to a neutral earth through certain parts of the
equipments, without transformers.
.LP
.rs
.sp 8P
.ad r
\fBFigure 2/K.14, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
\(em
Screening factor related to the sheath \fIk\fR\d\fIf\fR\\d\fIm\fR\u(see\
Figure\ 3/K.14). This factor must be taken into
consideration in cases where the only accessible earths are
those used for earthing the screen. This relates to cables
connecting telecommunication centres to one another, their
screens being connected to the earths of the centres.
.LP
.rs
.sp 9P
.ad r
\fBFigure 3/K.14, p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
The \fIDirectives\fR contain very detailed explanations and formulas for
the accurate calculation of these factors in a wide variety of situations.
On the other hand, these screening factors can be evaluated on the basis
of
simple expressions which often provide an adequate degree of accuracy. These
expressions differ according to whether the outer cable covering is insulative
or conductive and use the constants and variables listed in Appendix\ I.
.LP
.sp 1P
.LP
8.2
\fICables with insulating outer covering\fR
.sp 9p
.RT
.PP
The outer covering of the metallic cable sheath is made of an
insulating plastic material. To obtain a screening effect, this sheath
must be earthed at both ends and possibly at points in between.
.RT
.sp 1P
.LP
8.2.1
\fICalculation of the screening factor\fR
.sp 9p
.RT
.PP
The screening factor can then be calculated by means of the
expressions (see also the \fIDirectives\fR , Vol.\ II):
\v'6p'
.RT
.ce 1000
\fIk\fR \d\fIff\fR |
\u =
@ left | { fIZ\fR~$$Ei:\fIE\fR~:\fIi\fR~_~\fIL\fR~+~\fIW\fR~ | \fI~\dA\u\fR~+~\fIW\fR~ | \fI~\dB\u\fR } over { \fIZ\fR~$$Ei:\fIE\fR~:\fIe\fR~_~\fIL\fR~+~~~\fIZ~\ds\u\fR~\fIL\fR~+~\fIW\fR~ | \fI~\dA\u\fR~+~\fIW\fR~ | \fI~\dB\u\fR } right | @
.ce 0
.ad r
(8\(hy1)
\v'7p'
.ad b
.RT
.ce 1000
.sp 1
\fIk\fR \d\fIfm\fR \u =
@ left | { fIZ\fR~$$Ei:\fIE\fR~:\fIi\fR~_~\fIL\fR } over { \fIZ\fR~$$Ei:\fIE\fR~:\fIe\fR~_~\fIL\fR~+~\fIZ~\ds\u\fR~\fIL\fR~+~~\fIW\fR~ | \fI~\dA\u\fR~+~\fIW\fR~ | \fI~\dB\u\fR } right | @
.ce 0
.ad r
(8\(hy2)
\v'1P'
\v'1p'
.ad b
.RT
.LP
.sp 1
.PP
Strictly speaking, the use of these expressions presupposes that the sheath
is earthed only at the ends. It may be assumed, however, that in
fairly comparable situations only the earths near the ends have any influence
on the screening effect. The expression thus gives a good approximation
of the screening effect in the case of intermediate earths.
.PP
As a general consequence, earthing connections at intermediate points tend
to improve \fIk\fR\d\fIf\fR\\d\fIf\fR\\d`\u, but, on the other hand, make
\fIk\fR\d\fIf\fR\\d\fIm\fR\uworse.
.bp
.RT
.sp 1P
.LP
8.2.2
\fIInfluence of length\fR
.sp 9p
.RT
.PP
When the earths of a sheath required to obtain a screening factor \fIk\fR\d\fIf\fR\\d\fIf\fR\\d`\uclose
to nominal value \fIk\fR\d\fIn\fR\uhave a resistance value
which
makes earthing very difficult, the link may be considered to be \*Qshort\*U.
In the contrary case, it is regarded as \*Qlong\*U.
.PP
\fINote\fR \ \(em\ \*QLink\*U is held to mean the cable length actually
subjected
to induction.
.RT
.sp 1P
.LP
8.2.2.1
\fI\*QLong\*U links\fR
.sp 9p
.RT
.PP
Scrutiny of Equations (8\(hy1) and (8\(hy2) shows that for very long
links, screening factors \fIk\fR\d\fIf\fR\\d\fIf\fR\\d`\uand \fIk\fR\d\fIf\fR\\d\fIm\fR\uare
close to
\fIk\fR\d\fIn\fR\u. This is true of
lengths in excess of about
\v'6p'
.RT
.sp 1P
.ce 1000
10
@ { \fIW\fR~ | \fI~\dA\u\fR~+~\fIW\fR~ | \fI~\dB\u\fR } over { fIZ\fR~$$Ei:\fIE\fR~:\fIi\fR~_ } @
.ce 0
.sp 1P
.LP
.sp 1
.PP
In this case, a non\(hyarmoured cable (\fIZ\fR
\u\fIE\fR\d\fI\fI\d\fIe\fR\uclose to
\fIZ\fR
\u\fIE\fR\d\fI\fI\d\fIi\fR\u) may be used. Moreover, the longer the link, the
higher the resistance value of the sheath earthing may be.
.PP
This need not be taken into account in the choice of a cable, which
can be based on the curve of values of nominal screening factor \fIk\fR\d\fIn\fR\ufor
different values of induced e.m.f., since the efficiency obtained will
be very similar.
.RT
.sp 1P
.LP
8.2.2.2
\fI\*QShort\*U links\fR
.sp 9p
.RT
.PP
In this case, the value of \fIZ\fR
\u\fIE\fR\d\fI\fI\d\fIi\fR\u\fIL\fR is
approximately the same order of magnitude as the sum of the extreme terminal
earth values
\fIW\fR |
\fI
\dA\u\fR \ +
\fIW\fR |
\fI
\dB\u\fR . Screening factors
\fIk\fR\d\fIf\fR\\d\fIf\fR\\d`\u\fR and \fIk\fR\d\fIf\fR\\d\fIm\fR\umay
be calculated by means of Equations\ (8\(hy1) and\ (8\(hy2).
.PP
Armoured cables must be used to protect such links, and the screening effect
is then provided through the increase in the value of impedance
\fIZ\fR
\u\fIE\fR\d\fI\fI\d\fIe\fR\uobtained by using material with high magnetic
permeability for the outer part of the sheath.
.PP
To evaluate \fIk\fR\d\fIf\fR\\d\fIf\fR\\d`\uand \fIk\fR\d\fIf\fR\\d\fIm\fR\uby
means of
Equations\ (8\(hy1) and\ (8\(hy2), it is necessary to know the curve of
variations of \fIZ\fR
\u\fIE\fR\d\fI\fI\d\fIe\fR\uas a function of the current flowing through
the sheath (Figure\ 4/K.14).
.PP
The calculation then calls for some simple successive approximations for
evaluating \fIZ\fR \fI\fI\d\fIe\fR\u\fI\fI
\u\fIE\fR\dafter choosing a value of
\fIW\fR |
\fI
\dA\u\fR and
\fIW\fR |
\fI
\dB\u\fR corresponding to earths which may be expected to be feasible in
view of the
ground resistivity at the ends of the link.
.RT
.LP
.rs
.sp 19P
.ad r
\fBFigure 4/K.14, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
8.3
\fICables with conductive outer covering\fR
.sp 9p
.RT
.PP
The outer covering of the metallic cable sheath is made of a
conductive plastic material providing electrical contact between the sheath
and the earth surrounding the cable.
.PP
Intermediate connections of the sheath to the earth other than at the ends
will be unnecessary if the resistivity of the conductive material is close
to or better than that of the surrounding earth (values of about 50\ \(*W\
\(mu\ m are easily obtained).
.PP
The current flowing through the sheath varies along the link,
particularly near the terminals, and in the middle part remains at a value
very close to \fII\fR\d\fIM\fR\u\ =\ \fIe\fR /(\fIZ\fR
\u\fIE\fR\d\fI\fI\d\fIe\fR\u\ +\ \fIZ\fR\d\fIs\fR\u), corresponding to
the current which would circulate in the sheath if it were completely earthed
(earths with zero resistance value).
.PP
To calculate screening factor \fIk\fR\d\fIf\fR\\d\fIf\fR\\d`\u, we can
thus use an
equivalence consisting in replacing this cable by one with a sheath connected
to the earth at each end by zero resistance earths and of a length equal
to
that of the link\ \fIL\fR , shortened at each end by a length\ \fIl\fR
such that
| | fIP\fR \ | \fIl\fR \ =\ 1.
.PP
This means that the cable has a nominal screening factor on a shorter length
equal to \fIL\fR \ \(em\ 2\fIl\fR .
.PP
\fIk\fR\d\fIf\fR\\d\fIf\fR\\d`\ucan then be evaluated approximately by
means of the following expression:
\v'6p'
.RT
.LP
.ce 1000
\fIk\fR \d\fIff\fR |
\u = \fIk
\dn\u\fR
@ left ( 1~\(em~ { \fIl\fR } over { fIL\fR } right ) @ +
@ { \fIl\fR } over { fIL\fR } @
.ce 0
.ad r
(8\(hy3)
.ad b
.RT
.LP
.sp 1
In the same way, \fIk\fR\d\fIf\fR\\d\fIm\fR\ucan be expressed by:
\v'6p'
.sp 1P
.ce 1000
\fIk\fR \d\fIfm\fR \u = \fIk
\dn\u\fR
@ left ( 1~\(em~ { \fIl\fR } over { fIL\fR } right ) @
.ce 0
.sp 1P
.PP
.sp 1
Equation (8\(hy3) is not applicable in cases where the earthing of
the metallic sheath is really excellent. The link is then considered to be
\*Qlong\*U and \fIk\fR\d\fIf\fR\\d\fIf\fR\\d`\u\ =\ \fIk\fR\d\fIf\fR\\d\fIm\fR\u\
=\ \fIk\fR\d\fIn\fR\u.
.PP
The parameters required for the calculation are those of the cable
(\fIZ\fR
\u\fIE\fR\d\fI\fI\d\fIe\fR\u, \fIZ\fR
\u\fIE\fR\d\fI\fI\d\fIi\fR\u), the induced e.m.f. per unit
length and the admittance per unit length\ \fIY\fR of the sheath in relation
to the earth, which may be chosen according to ground resistivities between
1\ S and
10\ S (1\ S should be chosen if nothing is known about earthing quality).
.RT
.sp 1P
.LP
8.3.1
\fIInfluence of length\fR
.sp 9p
.RT
.PP
The remarks relating to cables with insulating covering are also
applicable in this case.
.RT
.sp 1P
.LP
8.3.2
\fI\*QLong\*U links\fR
.sp 9p
.RT
.PP
The screening factor is close to \fIk\fR\d\fIn\fR\u. The cable may or
may not be armoured, according to the results required.
.RT
.sp 1P
.LP
8.3.3
\fI\*QShort\*U links\fR
.sp 9p
.RT
.PP
Screening factor \fIk\fR\d\fIf\fR\\d`\umay be estimated by means of Equation\
(8\(hy3). The cable should be armoured in most cases.
.EF '% ''
.OF ''' %'
.RT
.sp 1P
.LP
8.4
\fIDetermination of cable parameters\fR
.sp 9p
.RT
.PP
If the nominal screening factor and impedance per unit length
.PP
\fIZ\fR
\u\fIE\fR\d\fI\fI\d\fIi\fR\ucan be measured by means of the
arrangement described in
the \fIDirectives\fR (Vol.\ IX), determination of impedance per unit
length \fIZ\fR
\u\fIE\fR\d\fI\fI\d\fIe\fR\u
can\ be based:
.RT
.LP
\(em
either on a calculation based on the phaser diagram, plotted
from the measured parameters \fII\fR , \fIU\fR\d\fIo\fR\\d\fIi\fR\uand\
\fIU\fR\d\fIo\fR\\d\fIe\fR\u;
.LP
\(em
or on the measurement of the voltage \fIU\fR\d\fIo\fR\\d\fIe\fR\uappearing
between the end of a conducting wire laid on the outside of the
sheath and reference point\ 3, the other end of the wire being
connected to the sheath (Figure\ 5/K.14).
.PP
For certain cables with screens consisting of several
non\(hyferromagnetic, highly\(hyconductive layers, these parameters can
be measured more approximately by a coaxial\(hytype measuring device.
.bp
.LP
.rs
.sp 25P
.ad r
\fBFigure 5/K.14, p.\fR
.sp 1P
.RT
.ad b
.RT
.ce 1000
APPENDIX\ I
.ce 0
.ce 1000
(to Recommendation K.14)
.sp 9p
.RT
.ce 0
.ce 1000
\fBLetter symbols used in Recommendation K.14\fR
.sp 1P
.RT
.ce 0
.LP
\fIZ\fR
\u\fIE\fR\d\fI\fI\d\fIi\fR\u |
Internal impedance per unit length with
external return. For power frequencies, this value is close to
resistance per unit length for direct current.
.sp 1P
.RT
.LP
\fIZ\fR
\u\fIE\fR\d\fI\fI\d\fIe\fR\u |
External impedance with external return per unit length.
.LP
\fIZ\fR\d\fIs\fR\u |
Ground return impedance per unit length.
.LP
\fIY\fR |
Admittance per unit length of the sheath\(hyearth circuit.
.LP
\fIP\fR |
Propagation constant of the sheath\(hyearth circuit.
.LP
\fIK\fR |
Characteristic impedance of the sheath\(hyearth circuit.
.LP
\fIW\fR |
\fI
\dA\u\fR ,
\fIW\fR |
\fI
\dB\u\fR |
Impedance value of earths at the
ends of the sheath.
.LP
\fIL\fR |
Length of link subject to induction.
.LP
\fIe\fR |
Induced e.m.f. per unit length.
.LP
\fIE\fR |
Total induced e.m.f.
.LP
\fII\fR |
Current flowing through the sheath.
\v'1P'
.sp 2P
.LP
\fBReference\fR
.sp 1P
.RT
.LP
[1]
CCITT manual \fIThe protection of telecommunication lines and\fR
\fIequipment against lightning discharges\fR , Chapter\ 4, \(sc\ 2.1, ITU,
Geneva,\ 1974, 1978.
.LP
.bp