MIL-HDBK-1004/2A
5.7.3.2 Ground Fault Protection. Ground-fault protection for each system
voltage level is independent of the protection at voltage levels for
transformers connected delta-wye or delta-delta. A circuit breaker having
ground fault protection can be set to operate for lower ground fault levels
and provides fast operation since no downstream coordination is required.
Fuses have fixed time-current characteristics. On secondary unit substations,
low-voltage circuit breakers with ground fault tripping should be used on
mains and on large or important feeder units as a minimum, if selectivity can
be achieved. On distribution substations, the additional cost for circuit
breakers must be justified on the basis that the improved selectivity is
required for reliability. Refer to IEEE 242, Recommended Practice for
Protection and Coordination of Industrial and Commercial Power Systems, for
ground fault protection application data.
5.7.4
Grounding Continuity. The most important consideration in grounding
is assuring continuity of the grounding system. Provide special danger points
such as metal-structure mounted switches and surge arresters of transmission
to distribution substations (see Figure 5) with dual ground paths. Ground
wires run with primary feeder cables may be required either because of
existing station practice or for other reasons.
5.7.4.1 Fault Current. NFPA 70 requires a low impedance ground path for
substations cannot be provided because of soil conditions, equipment
locations, or other factors, install a ground wire with the primary feeder
cable back to the distribution system substation.
5.7.4.2 Portable Substations. NFPA 70 requires that the exposed noncurrent-
carrying metal parts of portable substations must be connected with an
equipment grounding conductor run with the primary feeder back to the point
where the system neutral impedance is grounded. The reason for such a
requirement is that the mobility of the portable substation precludes a
predesigned grounding electrode system.
5.8
Safety Considerations. Provide for public safety and for protection
of operating personnel with respect to the factors described in paras. 5.8.1
through 5.8.5.
5.8.1
Fencing. Connect fencing, including gates, to substation grounding
(refer to DM-5.12, Fencing, Gates, and Guard Towers). For aesthetic reasons,
plastic-coated fabric may be used. Grounding this type of fabric requires
removing the coating to get a good ground. A touch-up plastic should recoat
any uncoated areas not covered by the ground connection. Application and
material should be as recommended by the fence manufacturer.
5.8.2
Metal Enclosures. For personnel safety, use metal or fiberglass
enclosures around all live parts.
5.8.3
Locking of Gates. Provide locks on gates.
Interlock switchgear
doors to prevent access to live parts.
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