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 3.8.1 Substation Grounding. In the past, substation grounding has been taken for granted and
has even been ignored. The large growth of the electrical system and the number of
interconnections with the power suppliers increase the ground fault current and the probability of
electric shock when substation equipment is not grounded.
Utilization of sound engineering principles is required for acceptable substation grounding
providing a safe and shockproof grid around the substation. The old practice of installing just a
few conductors for a substation grid around the substation transformer is no longer acceptable.
The four steps presented on the following pages (soil resistivity test, maximum ground fault
current, design of grid, and measurement of soil resistivity) are all necessary functions to ensure
safety around the substation.
3.8.1.1 Soil Resistivity Test. Preliminary designs of substations should include an
investigation of the general soil type to aid in foundations design. Any such investigation will
indicate a rough approximation of the soil resistivity similar to the typical values shown in
Table 3-3. Testing soil samples in the lab, the three pin method, and the four pin method are
three procedures used to determine soil resistivity in the substation lot.
3.8.1.2 Maximum Ground Fault Current. The value of maximum ground fault current
must be calculated for use in the substation ground design. An allowance for future growth
should be made, and an adjustment should also be made for the effect of direct current offset and
attenuation of alternating current and direct current transient components of fault current.
3.8.1.3 Grid Design. The preliminary design of the station grounding should consist of
placing and noting of all the required grounding. The following is a list of the grounding
requirements normally found in substations.
(a) Power transformers require two grounds, each capable of carrying full fault current.
(b) Lightning arresters require one ground connected from the grid directly to the
lightning arrester base. The ground must be capable of carrying the discharge.
(c) Dead-end towers normally require one ground per each leg connected to the
structure.
(d) The following items or structures require a minimum of one ground per each leg:
o
Switch Stand.
o
Bus Support Structures.
o
Potential and Current Transformers.
o
Coupling Capacitors.
o
All Miscellaneous Structures.
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