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Summary:
"What
is the 'grounding path' and how good is it?"
Looking
at the codes and standards governing electrical systems.
With
trilingual summary
By
Richard L. Nailen, EA Engineering Editor
For safety reasons, grounding of electrical power systems is
common practice throughout the world. What links the system to the
earth itself is the grounding electrode, in its simplest form a metal
rod driven into the earth. The resistance of the grounding path
through that rod is meant to be low enough to allow sufficient ground
fault or induced surge current to operate protective devices on the
system, and to keep voltage drop throughout the path low enough for
personnel safety.
Many studies have been made of methods for maintaining low ground
electrode resistance. What's most important is the interface between
rod and earth, and the resistivity of the soil itself--particularly
that immediately surrounding the rod.
Soil chemistry and moisture content, temperature, and the depth of
rod penetration into the soil all influence the grounding path
resistance. Some electrodes contain chemical salts, or are equipped
with water supply systems, to keep resistance low. In large
industrial installations, encasing electrodes in concrete has been
beneficial, leading to use of building reinforcing steel as a system
ground.
In recent years, considerable controversy has involved electrode
materials. In corrosive soils, ordinary steel ground rods may be
quickly destroyed. Stainless steel, once thought to be immune from
corrosion, has sometimes proven to be otherwise.
Ground path resistance is tested by applying a measured voltage
between the electrode and a separate probe in the soil some distance
away. Current flowing between those two points causes a voltage drop
along the soil path, which is measured between that probe and a
second one. Resistance is then calculated by Ohm's Law. In evaluating
soils themselves as suitable electrode locations, soil resistivity is
measured in a similar way using a four-point Kelvin bridge
connection.
A self-contained clamp-on tester is useful for measuring
resistance of a single electrode path when the system contains a
number of other low-resistance grounding electrodes as well. It
injects an alternating current into the electrode under test, through
transformer action, and automatically calculates the impedance of the
electrode circuit from the voltage and current involved.
Copyright 2002, Barks Publications, Inc., Chicago.
Reproduction by any means prohibited.
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