MIL-HDBK-1004/2A
provide data on such treatment. Provide for easy maintenance and periodic
testing. Driving ground rods deeper using sectional rods may be more
effective than using multiple rods. In many cases, soil variations and
possible bedrock may make provision of additional rods less expensive.
2.8.4.2 Water Pipe Connections. Make no connection to any sprinkler piping
in accordance with NFPA 24, Installation of Private Fire Service Mains and
their Appurtenances. The electrical system may be grounded to a water supply
system except where nonmetallic pipes, cathodically protected metallic pipes,
or insulating couplings are incorporated in the water pipe system. Supplement
the water pipe connection by other grounding electrodes where required by NFPA
70.
2.8.4.3 Combination of Grounding Methods. Where the ground resistance in an
existing system is high, any of the aforementioned methods may be combined to
effect improvement.
2.8.4.4 Ground Connections. Keep wires running from protective devices (for
example, gaps, grading rings, expulsion or protection tubes, and surge
arresters) to ground as straight and short as possible. Where bends are
necessary, provide them of large radii to keep the surge impedance as low as
possible.
2.8.5
Overhead Ground Wires. Where overhead ground wires are used for
protection of electric lines, provide a ground connection from the overhead
ground wire to a wire loop or a ground plate at the base of the pole or to a
driven rod, depending on the existing soil conditions. Use of wire wraps or
pole butt plates is allowed by ANSI C2 only in areas of very low soil
resistivity. Ground the overhead ground wire at each pole.
2.8.6
Measurement of Ground Resistance.
Measure ground resistance by
using one of the following methods:
a) Three-Electrode Method. In the three-electrode method, two test
electrodes shall be used to measure resistance of the third electrode, the
ground point. A self-contained source of alternating current and a battery-
operated vibrator source providing direct reading are commercially available.
b) Fall-of-Potential Method. The fall-of-potential method involves
an ungrounded alternating current power source which circulates a measured
current to ground. Voltage readings taken of the connection to auxiliary
grounds allow use of Ohm's law to determine the ground resistance. Refer to
Fink and Beaty, Standard Handbook for Electrical Engineers.
Service Drop to Buildings. Local considerations and current
2.9
capacities dictate the type of service drop to buildings from overhead
distribution systems. Provide either underground service into the building
from a pole riser or self-supporting service cable strung from the pole to the
building (refer to ANSI C2).
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