AEMC 4620 User Manual page 24

FPN: The paralleling efficiency of rods longer than 8 feet is improved by
spacing greater than 6 feet apart.
The National Electrical Code (NEC ) states that the resistance to ground
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shall not exceed 25Ω. This is an upper limit and guideline, since much
lower resistance is required in many instances.
"How low in resistance should a ground be?"
An arbitrary answer to this in ohms is difficult. The lower the ground resis-
tance, the safer, and for positive protection of personnel and equipment, it
is worth the effort to aim for less than one ohm. It is generally impractical
to reach such a low resistance along a distribution system or a transmis-
sion line or in small substations. In some regions, resistances of 5Ω or less
may be obtained without much trouble. In others, it may be difficult to bring
resistance of driven grounds below 100Ω.
Accepted industry standards stipulate that transmission substations
should be designed not to exceed one ohm resistance. In distribution sub-
stations, the maximum recommended resistance is 5Ω. In most cases, the
buried grid system of any substation will provide the desired resistance.
In light industrial or in telecommunications central offices, 5Ω is often the
accepted value. For lightning protection, the arresters should be coupled
with a maximum ground resistance of 1Ω.
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
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these situations develop, several methods of lowering the ground resis-
tance 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
instances unreliable.
Auxiliary ground electrodes will be required in high voltage transmission
lines, where maximum resistance of 15 ohms is recommended, and in
distribution lines, where maximum resistance of 25 ohms is preferred. All
electrical systems constructed in accordance with the National Electrical
Code , should not exceed 25 ohms.
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Digital Ground Resistance Tester Model 4620 and 4630