Ground Resistance Values - AEMC 4500 User Manual

Ground Resistance Values

NEC ® 250-84 (1987): Resistance of man-made electrodes:
"A single electrode consisting of a rod, pipe, or plate which does
not have a resistance to ground of 25 ohms or less shall be
augmented by one additional of any of the types specified in
section 250-81 or 250-83. Where multiple rod, pipe or plate
electrodes are installed to meet the requirements of this sec-
tion, they shall be not less than 6 ft (1.83 m) apart."
The National Electrical Code
ground 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 resistance, 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 transmission 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
substations, 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 these situations develop, several methods of lowering the
ground resistance can be employed. These include parallel rod sys-
tems, deep driven rod systems utilizing sectional rods and chemical
Digital Ground Resistance Tester Model 4500
® (NEC ® ) states that the resistance to
25