Lion Share/lionsharecd.iso/ham/grounds.txt

Text File | 1992-07-24 | 8KB | 145 lines

                        PRINCIPLES OF GROUNDING
                          JD Delancy, K1ZAT/3


    The main purpose of equipment, facility, and system grounding is to
provide for the safety of personnel.  This is accomplished by insuring
that all equipment configurations, antenna, or support structures, as
well as all metal structures, motor, and generator frames, cable armor,
control equipment enclosures, conduits, and all portable electrical
equipment cabinets and housing are at ground potential thereby reducing
possibility of electrical shock of personnel coming in contact with
metal parts of the equipment and towers.

    The secondary function of all grounds is to improve the operation
and continuity of service of all equipment configurations.  Faulty
ground returns are detrimental to these functions and can result in
intermodulation effects and noise voltage build-up with their associated
service interruptions, false signals, equipment damage, or signal
distortion.

    Considering that the characteristics of the soil (earth) and the
weather vary greatly at the locations in which the installation are
planned, it is practically impossible to develop an earth grounding
system which can be utilized as a standard for all locations.  During
the planning stages of an extensive electrode system for substations or
other electrical power distribution system, consideration must be given
to the potential variations which can occur over the area of the ground
connections.

    A ground connection, regardless of its application, must meet
certain specifications.  The electrodes buried in the ground to form
an electrical connection to the earth must themselves be capable of
withstanding mechanical abrasion and have sufficient area in contact
with the soil so that the ground resistance is within the rated limits.
The resistance of this earth path must remain reasonable constant
throughout the seasons of the year and must be unaffected by unexpected
circulating currents resulting from the equipment configuration to which
the connection is made.  In short, ground connections should be durable,
have low D-C resistance, low A-C impedance, have adequate current
carrying capacity, and be of such a design that they can be readily
installed and maintained.

    Driven ground electrodes, more commonly referred to as ground rods
or pipes, are utilized where bedrock is beyond a depth of 10 feet.
Ground rods are commercially manufactured in 1/2", 5/8", 3/4" and one
inch diameters, and in lengths of 6', 8', 10', 12', and 16'.  The
National Electric Code (NEC) specifies that ground rods of steel or iron
shall be at least 5/8" in diameter and that rods of non-ferrous
materials shall not be less than 1/2" diameter.  Although galvanized
steel rods are used, the more commonly utilized material, copper clad
steel provides an excellent means of obtaining the lowest possible
resistance contact with the earth.

    The NEC requires that any water metering equipment be bypassed by a
jumper of a size not less than that required for the ground ing
conductor.     The ground conductor shall bypass the meter and service
unions.  The water piping system must be made electrically continuous by
bonding together all parts which may become disconnected.  As with other
ground connections, the resistance should be measured before deciding on
this type of ground installation.  It should be noted that where cast
iron screw type joints are utilized for joining together lengths of
pipe, they usually provide metallic connections of low resistivity.
However, if joints are made of "leadite" or similar types of cement, the
resistivity values of these connections may be several hundred ohms,
rendering the water system useless as a suitable ground system;
therefore, tests should be conducted to insure continuity of ground
circuits.

    Multiple driven electrodes will not always provide an adequate low
resistance to earth.  In such instances, it is generally possible to
reduce the resistivity of the soil immediately surrounding the driven
electrode by treating the soil with a substance which, when in solution,
is highly conductive.  There are several substances, however, the better
known, in order of preference are:

     a.  Magnesium sulphate (common name:  Epsom Salts)
     b.  Copper Sulphate (common name: Blue Vitriol)
     c.  Calcium Chloride
     d.  Sodium Chloride (common name: Common Salt)
     e.  Potassium Nitrate (common name: Saltpeter)

    Preference is given to use of magnesium sulphate, which is the most
common material used.  It combines low cost with high electrical
conductivity and low corrosive effect on a ground electrode or plate.
All electrodes used in the soil treatment should be of copperweld type.

    Large reductions in the ground contact resistance of the individual
ground electrodes may be expected after chemical treatment of the earth
where low resistances are difficult to obtain without chemical
treatment.  The initial effectiveness of chemical treatment is greatest
where the soil is somewhat porous because the solution permeates a
considerable volume of earth, and expanded ground contact thereby
increases the effectiveness of the electrode.  When soil of compact
texture is encountered, the chemical treatment is not as effective at
first because the solution tends to remain in its original location for
a longer period of time.  Chemical treatment limits the seasonal
variation of resistance and lowers the freezing point of the surrounding
soil.  Chemical treatment of the earth around a driven electrode
utilizing the Magnesium Sulphate and water solution is described as
follows:

    a.  A 4-foot length (approx) of 8 inch tile pipe is buried
in the ground approximately four inches from the ground
electrode, and filled to within one foot of the ground level with
the Magnesium Sulphate and water solution.  The 8-inch pipe
should have a wooden cover with holes, and be located at ground
level.

    b.  Forty to Ninety pounds of chemical will initially be
required, and will retain its effectiveness for two or three
years.  Each replenishment of chemical will extend its
effectiveness for a longer period, so future retreatment occurs
less and less frequently.

    The use of Common Salt or Saltpeter is not recommended as it will
require greater care to be given to protection against corrosion.
Additionally, any metal enclosure nearby and unrelated to grounding,
should also be treated to prevent damage by corrosion.  Therefore,
Common Salt or Saltpeter should be utilized only when absolutely
necessary to reduce the resistivity of the soil.

    When Common Salt must be utilized, the amount necessary to treat the
earth around a driven electrode depends upon the the available water
supply.  A decrease in resistivity of the earth can be achieved by
adding more water.  Additional water dissolves the salt and also aids in
carrying the salt solution throughout the conducting soil hemisphere.
Therefore, a minimum treatment of earth per ground electrode would
contain at least five pounds of salt and as much water as is required to
initially flood the area.  The rate at which chemical treatment will
lower the resistivity of the soil depends upon the rate at which the
solution will seep through the soil.  Commercial tests have shown that
an initial chemical treatment retains its effectiveness for at least one
year, however, porous soil and excessive rainfall or drainage would
reduce the period appreciably. In some cases, treatment has remained
effective for three to six years.

    If the station and equipment is located on a rocky mountain top,
this system could not be utilized since soil treatment would be
in-effective.


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