AEMC 4620 User Manual page 29

"How low in resistance should a ground be?"
An arbitrary answer to this in ohms is difficult. The lower the ground resistance,
the electrical system and equipment attached to it are safer. It is worth the effort
to aim for less than one ohm for positive protection of personnel and equipment.
It is generally impractical to reach such a low resistance along a distribution
system or a transmission line or in small substations. In some regions,
resistances of 5 Ω or less may be obtained without much trouble. In others, it
may be difficult to bring resistance of driven grounds below 100 Ω.
Accepted industry standards stipulate that transmission substations should be
designed not to exceed one ohm resistance. In distribution substations, the
maximum recommended resistance is 5 Ω. In most cases, the buried grid system
of any substation will provide the desired resistance.
In light industrial or in telecommunications central offices, 5 Ω is often the
accepted value. For lightning protection, the arresters should be coupled with a
maximum ground resistance of 1 Ω.
These parameters can usually be met with the proper application of basic
grounding theory. There will always exist circumstances which will make it difficult
to obtain the ground resistance required by the NEC . When these situations
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develop, several methods of lowering the ground resistance can be employed.
These include parallel rod systems, deep driven rod systems utilizing sectional
rods and chemical treatment of the soil. Additional methods, discussed in other
published data, are buried plates, buried conductors (counterpoise), electrically
connected building steel, and electrically connected concrete reinforced steel.
Electrically connecting to existing water and gas distribution systems was often
considered to yield low ground resistance; however, recent design changes
utilizing non-metallic pipes and insulating joints have made this method of
obtaining a low-resistance ground questionable and, in many cases, unreliable.
Auxiliary ground electrodes are required in high-voltage transmission lines, where
a maximum resistance of 15 Ω is recommended, and in distribution lines, where
maximum resistance of 25 Ω is preferred. All electrical systems constructed in
accordance with the National Electrical Code should not exceed 25 Ω.
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The measurement of ground resistances may only be accomplished with
specially-designed test equipment. Most instruments use the Fall of Potential
principle of alternating current (AC) circulating between an auxiliary electrode
and the ground electrode under test. The reading will be given in ohms and
represents the resistance of the ground electrode to the surrounding earth.
AEMC Instruments has also introduced clamp-on ground resistance testers that
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make ground resistance testing easier since the system does not have to be
disconnected or de-energized and no probes need to be driven.
The National Electrical Code and NEC are registered trademarks of the
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National Fire Protection Association.
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Digital Ground Resistance Tester Models 4620 & 4630