What Is An Acceptable Earth Ground Resistance Value?

What Is An Acceptable Earth Ground Resistance Value? - E&S Grounding Ask the Experts
Mohammad asked:
What Is An Acceptable Earth Ground Resistance Value, 5 Ohm Or 25 Ohm?

Our Response:
The NEC states that if you use one ground rod that the resistance to ground must be 25 ohms.  If it is higher, then you must place a second ground rod.  There is no requirement to retest the resistance.  In practice, for a residential structure, most electricians place two ground rods and call it done.  This does not create the best ground, but it meets the bare minimum of the code.  I do not know of an equivalent requirement in the British Standards.

There are other standards that recommend a specific resistance-to-ground, depending on the type of system you are working with.  Here are two such examples:

ANSI/BICSI 002: Data Center Design and Implementation Best Practices

Recommends 5 Ohms Maximum, but recommends 3 Ohms for Class F2 & F3 Data Centers, and 1 Ohm for Class F4 Data Centers

IEEE 142: Recommended Practice for Grounding of Industrial and Commercial Power Systems (The Green Book)

3.3.4.6 Power stations and substations

“For satisfactory lightning protection, substation grounding network resistance must not exceed 5 ohm; for large stations lower values are more desirable.”

4.1.3 Recommended acceptable values

“Resistances in the 1 ohm to 5 ohm range are generally found suitable for industrial plant substations and buildings and large

commercial installations.”

Generally, we recommend a design goal of a resistance-to-ground less than 5 ohms for most installations.  For substations or sites with sensitive electronics, we recommend a design goal of a resistance-to-ground of less than 1 ohm.

The Engineering Experts at E&S Grounding Solutions

nec, resistance, Grounding & Earthing Applications, ohms, power stations, substations

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