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_______________
INTERNATIONAL TELECOMMUNICATION UNION
CCITT K.11
THE INTERNATIONAL
TELEGRAPH AND TELEPHONE
CONSULTATIVE COMMITTEE
PROTECTION AGAINST INTERFERENCE
PRINCIPLES OF PROTECTION
AGAINST OVERVOLTAGES
AND OVERCURRENTS
Recommendation K.11
Geneva, 1991
Printed in Switzerland
FOREWORD
The CCITT (the International Telegraph and Telephone Consultative Committee) is a permanent organ of the
International Telecommunication Union (ITU). CCITT is responsible for studying technical, operating and tariff
questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide
basis.
The Plenary Assembly of CCITT which meets every four years, establishes the topics for
study and approves Recommendations prepared by its Study Groups. The approval of Recommenda-
tions by the members of CCITT between Plenary Assemblies is covered by the procedure laid down in
CCITT Resolution No. 2 (Melbourne, 1988).
Recommendation K.11 was prepared by Study Group V and was approved under the Resolution
No. 2 procedure on the 18th of March 1991.
___________________
CCITT NOTE
In this Recommendation, the expression "Administration" is used for conciseness to indicate both a telecom-
munication Administration and a recognized private operating agency.
aITU1991
All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, elec-
tronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU.
PAGE BLANCHE
Recommendation K.11
Recommendation K.11
PRINCIPLES OF
PROTECTION AGAINST OVERVOLTAGES AND OVERCURRENTS
(Geneva, 1972; modified at Malaga-Torremolinos, 1984 and at Melbourne, 1988, revised in 1990)
Introduction
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.
The object of the present Recommendation is to set out principles which enable the fre-
quency 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).
This Recommendation deals principally with the local exchange, local loop plant and sub-
scribers' equipment, but its contents may have wider application.
Note û 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.
1 General considerations
1.1 Origin of dangerous
overvoltages and overcurrents
1.1.1 Direct lightning strikes
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.
1.1.2 Lightning strikes nearby
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.
1.1.3 Induction from fault currents in power lines including electric traction systems
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 (occasion-
ally even longer) according to the fault clearing system used on the power line.
1.1.4 Contacts with power lines
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 coun-
tries 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 arrange-
ments usually ensure that the voltage is removed in a short time if a fault occurs. The overvolt-
age may cause excessive currents to flow along the line to the exchange earth causing damage to
equipment and danger to staff.
1.1.5 Rise of earth potential
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:
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.
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.
1.2 Methods of protection
1.2.1 Some of the protective measures for lines which are described in _ 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.
1.2.2 Other protective measures which may be applied to specific parts of the system as indi-
cated in __ 2, 3 and 4 fall broadly into 2 classes:
û 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);
û the use of equipment with suitable dielectric strength, current carrying capacity and impedance so
that it can withstand the conditions applied to it.
1.3 Types of
protective devices
1.3.1 Air-gap protectors with carbon or metallic electrodes
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.
1.3.2 Gas discharge tubes
Usually connected between each wire of a line and earth or as 3-electrode 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.
Detailed requirements for gas discharge tubes appear in Recommendation K.12.
1.3.3 Semi-conductor protective devices
Developments in the technology of these devices have produced some units which may be used as
primary protectors and others which should be used as secondary protectors only.
Primary and secondary protection should always be coordinated correctly.
a) Primary protection devices
A widespread trial of these devices is taking place in which the units are being used to replace carbon
electrode protectors and gas discharge tubes. The outcome of the trials may depend on both technical and commer-
cial factors.
Typically, the benefits which may be provided over carbon electrodes may be the elimination of circuit
noise and lower operating levels, while the disadvantages may be higher costs and self-capacitance with lower cur-
rent handling capability.
In the case of gas discharge tubes, benefits may arise from the elimination of spark discharges while
the higher self-capacitance and the lower current handling capability may be considered to be disadvantageous.
The trials are continuing and further developments of this technology may arise.
b) Secondary protection devices
Depending on their location in a circuit, they may be designed to have overvoltage operation levels as
low as 1V. They are precise and fast-acting, but may be damaged by excessive currents unless coordinated correctly.
1.3.4 Fuses
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-acting fuses have a uniform wire which
melts quickly when a large current flows, and a spring-loaded fusible element which melts gradu-
ally 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.
1.3.5 Heat coils
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 cur-
rents of, typically, 500 mA flow for some 200 s.
1.3.6 Self-restoring current-limiting devices
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.
1.3.7 Fusible links
Fuseless over-voltage protector assemblies installed on telecommunication lines can be pro-
tected against the risk of overheating in the event of a prolonged contact between the telecommuni-
cation line and a power distribution line by means of a fusible link.
A fusible link usually consists of insulated conductors in series with the telecommunica-
tion line and located between the exposure to the power line and the protector assembly. The con-
ductors are usually at least two wire gauges smaller than the conductors terminated on the
protector assembly and are of a suitable length to avoid a sustained arc if the power system does
not de-energize promptly and the conductors fuse. If the fusible link or part of it is installed in a
building or other location where a fire hazard might occur, it is enclosed within a cable sheath,
splice enclosure, or other suitable enclosure to contain any arcing that may result if the conduc-
tors fuse.
1.4 Residual effects
The essential purpose of protective measures is to ensure that the major part of the electri-
cal 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 suppress-
ing ideally all voltages or currents connected with disturbances, for the following reasons:
1.4.1 Residual overvoltages
Account should be taken of:
a) voltages which are unaffected by the protective device because they are below its operating level;
b) transients which pass before the device operates;
c) residuals which are sustained after the device operates;
d) transients produced by the operation of the device.
1.4.2 Transverse voltages
Protective devices on the two wires of a pair may not operate simultaneously and so a trans-
verse pulse may be produced. Under certain conditions, particularly if the equipment to be pro-
tected 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.
1.4.3 Effect on normal circuit operation û Coordinated design
Sufficient separation should be allowed between the operating voltage of the protective
devices and the highest voltage occurring on the line during normal operation.
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.
1.4.4 Modifying effects
A protective device may safeguard one part of a line at the expense of another, e.g. if a main dis-
tribution 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.
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.
1.4.5 Coordination of primary and secondary protection
For the protection of sensitive equipment it is sometimes necessary to use more than one protec-
tive device, e.g. a fast-operating, low-current device such as a semiconductor and a slower-oper-
ating, high-current device such as a gas-discharge tube. In such cases steps must be taken to
ensure that in the event of a sustained overvoltage, the low-current device does not prevent the
operation of the high-current device since, if this happens, the smaller device may be damaged, or
the interconnecting wiring may conduct excessive current.
1.4.6 Temperature rise
Protective components should be designed and positioned in such a way that the rise in tempera-
ture which occurs when they operate is unlikely to cause damage to property or danger to people.
1.4.7 Circuit availability
The circuit being protected may be temporarily or permanently put out of service when a protec-
tive device operates.
1.4.8 Fault liability
The use of protective devices may cause maintenance problems due to unreliability. They may
also prevent some line and equipment testing procedures.
1.5 Assessment of risk
1.5.1 The performance of a telecommunications system with respect to overvoltages depends on:
û the environment, i.e. the magnitude and probability of overvoltages occurring in the line network
associated with the system;
û the construction methods used in the line network, see _ 2;
û the resistibility of equipment in the system to overvoltages;
û the provision of protective devices;
û the quality of the earth system provided for operation of the protective devices.
1.5.2 The environment
In assessing the environment, consideration should be given to the factors mentioned in _ 1.1.
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.
û 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 "unexposed". Recom-
mendations 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.
û All other environments are classified as "exposed" but this, of course, covers a wide range of condi-
tions including exceptionally exposed situations where a satisfactory service can only be achieved by the use of all
available protective measures.
In the case of induced voltages and rise of earth potential the overvoltages can be calcu-
lated as indicated in [2] which also recommends the maximum values which may be permitted under
various conditions.
1.5.3 Fault records
The risk of overvoltages and overcurrents can only be properly assessed in the light of expe-
rience. It is recommended that fault statistics be kept in a form which is convenient for that pur-
pose. 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.
1.6 Decision on protection
1.6.1 In considering the degree to which a telecommunications network should withstand overvolt-
ages, two classes of failure may be recognized:
û Minor failures affecting only small parts of the system. These may be allowed to occur at a level
acceptable to the Administration.
û Major breakdowns, fires, exchange failures, etc., which must, so far as possible, be avoided com-
pletely.
Examples of conditions which may be permitted to cause minor failures but not major break-
downs are given in Recommendation K.20. It is desirable also that failure of a single protective
device should not cause a major breakdown.
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.
1.6.3 It should be noted that over-protection, 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.
To avoid disturbances in telecommunication circuits caused by activated protective
devices, the striking voltage values and the numbers of arrestors should be considered.
1.6.4 In the light of the above considerations and the assessment of risks in accordance with _
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.
The responsibility for making this decision and for ensuring the provision of any protec-
tive devices needed to coordinate lines and equipment should be clearly laid down.
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 con-
straints 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 proce-
dures such as the production of local standards. Recommendations K.20 and K.21 give guidance for
the preparation of these standards.
2
Protection of lines
2.1 Protective measures external to the conductors themselves
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.
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.
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 consider-
ations arise but it is often possible to benefit from jointly using trenches, poles and ducts, pro-
viding 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.
2.2
Special cables
Special cables of high dielectric strength may be used where high overvoltages are likely to
occur.
Standard plastic insulated and sheathed cables have a higher dielectric strength than
paper insulated, lead-sheathed 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 expo-
sure to lightning due to high keraunic level and low soil conductivity.
Other examples of the use of special cables are:
û cables with metal sheaths which provide a good reduction factor to screen circuits within the cable;
û cables which carry circuits to exposed radio towers and which must be able to carry lightning dis-
charge currents without damage;
û all-dielectric (i.e. non-metallic) optical fibre cables to effect isolation between conductive lengths of
cable.
2.3 Use of protective devices
The use of protective devices may be desirable in the following circumstances:
2.3.1 They may be more economical than the special construction described in __ 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 expen-
sive, usually incur no continuing costs.
2.3.2 Cables with extra thick insulation may themselves be undamaged by overvoltages or over-
currents 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.
2.3.3 Induced overvoltages from power or traction line faults may still exceed levels permitted
by the Directives even after all practicable avoidance measures have been followed.
2.4 Installation of protective devices
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 connec-
tion.
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 inter-
vals and at both ends of the affected length of line, or as near to this as practicable.
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 sub-
scribers' terminals reduce the risk of damage to lines but their main function is to protect compo-
nents having lower dielectric strength than the cables. See Recommendations K.20 and K.21.
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.
2.5 Planning of works
The general considerations of __ 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 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.
2.6 Recommended policy
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.
3
Protection of exchange and transmission equipment
3.1 Need for protection external to the equipment
Operating organizations should take account of the possible need to fit protection external to
the equipment, bearing in mind the following considerations:
3.1.1 A telecommunication line will give some protection to equipment under certain conditions, e.g.:
û a conductor may melt and disconnect an excessive current;
û conductor insulation may break down and reduce an overvoltage;
û air-gaps in connection devices may break down and reduce overvoltages.
3.1.2 The increased robustness of plastic insulated cables has the effect of increasing the lev-
els 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.
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 _ 1.3 between the cable conductors and the equipment to which they are con-
nected, preferably on the MDF. This will prevent cables from the MDF to equipment from having to
carry heavy overcurrents.
The protective devices are fitted to the line side of the MDF to avoid the need to carry dis-
charge 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.
3.1.3 In less exposed locations it may be that disturbances (voltages and currents) have statis-
tical characteristics of level and frequency so low that in practice the risks do not exceed those
resulting from the residual effects indicated in _ 1.4 for exposed regions. Protective devices then
serve no purpose and are an unnecessary expense.
3.2 Need for equipment to have a minimum level of electrical robustness
In locations where lines are exposed and protective devices are provided, the residual effects
considered in _1 can cause overvoltages and overcurrents to appear in the equipment. In less
exposed environments the disturbances described in _ 3.1.3 can cause similar effects. It is neces-
sary 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.
3.3 Effect of switching conditions
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.
The effectiveness of the protection provided can be influenced by the reduction in the prob-
ability 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.
4
Protection of subscribers' terminal equipment
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.
4.1
Degree of exposure
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 _ 2.1.
On the other hand, lines to installations remote from built-up areas can be very exposed on
account of their length, the absence of a protective environment, overhead construction at the sub-
scriber'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.
4.2 Dielectric strength
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.
4.3 Use of protectors
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 con-
necting protective devices of the types described in _ 1.3 between the line conductors and the earth
terminal.
The terminal equipment dielectric strength should be chosen taking account of the break-
down voltage of the protective device and the impedance of the protector-line to earth connection.
4.4 Common bonding
At installations of subscriber terminal equipment a low resistance earth for overvoltage
protectors may be unavailable, or the costs of procuring a suitable low-resistance 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 electric-
ity system.
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 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.
4.5 National regulations
Many countries have national standards covering the protection of users of telecommunica-
tions equipment not only from the risks associated with connection to the electricity mains but
also from conditions which may appear on the telephone line.
4.6 High cost of maintenance of subscribers' installations
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. More-
over, insufficient protection is the cause of repeated interruptions of service which are particu-
larly damaging to the quality of service and the satisfaction of the customer. This justifies the
granting of special attention to protection measures.
References
[1] CCITT Manual The protection of telecommunication lines and equipment against lightning discharges, ITU,
Geneva 1974, 1978.
[2] CCITT Directives concerning the protection of telecommunications lines against harmful effects from electric
power and electrified railway lines, ITU, Geneva, 1988.