MIL-HDBK-1004/2A
5.6.2
Existing Construction. When electrical upgrading of existing
construction is involved, take into account any extra space and costs required
to meet current NFPA 70 code requirements. Working space and access
requirements have become more stringent as successive codes are issued and
many existing vaults or substation installations may not meet current
criteria. If the design cannot meet current NFPA 70 requirements, then a
waiver must be obtained from the proper authority.
5.7
Grounding. Provide a grounding electrode system for each substation
and connect equipment and system grounds so as to provide personnel and
equipment protection and grounding continuity.
5.7.1
Grounding Electrode Systems. Provide electrode systems made of
either the girdle or grid type using horizontal conductors running between
vertical ground rods.
5.7.1.1 Girdle Type. Use girdle type systems for pad-mounted com-partmental-
type transformers and secondary unit substations (see Figure 4).
5.7.1.2 Grid Type. For the much larger voltage gradients at substations for
utility-Navy interconnections use the grid-type, buried grounding network.
Mesh spacings of 10 to 12 feet (3 to 3.5 m) are commonly used and normally
such spacings can control surface voltage gradients even though the ground
resistance may be relatively high.
5.7.1.3 Special Techniques. See recommendations of IEEE 80, Guide for Safety
in Substation Grounding. Where the local utility or activity indicates the
special grounding techniques are necessary because of poor soil conductivity,
their recommendations should be followed. Refer to IEEE 142,
Recommended Practice for Grounding of Industrial and Commercial Power Systems
for a discussion of soil resistivity. Refer to NFGS-16302 for maximum
allowable ground resistance values.
5.7.2
Equipment Grounding. Ground metallic enclosures, including cases of
primary and secondary switchgear and transformers, to protect operating
personnel.
System Grounding. Unless functional requirements prohibit grounding,
5.7.3
provide all transformers with wye-connected neutral connections grounded
independently at each voltage level, that is, at the transformer secondary.
5.7.3.1 Neutral Grounding. Normally, provide solid grounding, since this is
the least expensive method of limiting transient overvoltages while obtaining
sufficient ground fault current for selective tripping. Provide impedance
grounding only when required to limit ground fault currents to acceptable
values or where required by code, such as for portable substations or to match
existing system design. Refer to MIL-HDBK-1004/1 for additional requirements.
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