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Grounding methods
EARTH (SOIL) GROUND AS AN ELECTRICAL CURRENT RETURN PATH
For a Holiday Detector to inspect a coated pipe properly an electrical ground
return path is needed. When a holiday in the coating is detected a spark occurs
forcing a low current to flow from the metal pipe back through the ground wire
of the detector. An alarm circuit inside the detector senses the low current and
a “JEEP” is heard.
The most optimum electrical ground return path is with the ground wire of
the Holiday Detector directly connected to the bear pipe end, “cutback”. This
method is called “Direct Grounding”. Using this method, the low current flows
through a wire of very low resistance back to the detector. In this case the
Earth is not part of the ground return path.
In many instances Direct Grounding is not possible. Two alternate methods
can be used, “Direct grounding using a secondary ground rod” or “Indirect
grounding using a trailing ground wire”. In both methods the Earth (soil) now
becomes part of the electrical ground return path allowing the low current to
flow back to the detector.
In all cases, for the safety of all personnel during the coating inspection, the
pipe should be grounded to Earth. Make an electrical connection to the bear
end of the pipe, “cutback”. Attach a wire from this connection to a ground rod
driven into the Earth. Depending on the moisture content of the soil the ground
rod should be driven down about 2 feet. Drive the rod deeper in dryer soils.
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