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What Is An Acceptable Earth Resistance Value, 5 Ohm Or 25 Ohm?

Mohammad asks,
In the USA NEC it is stated that the maximum earthing resistance for a house or commercial building shall not exceed 25 ohms but I think even 25 ohms is on the high side. Can you quote some ANSI STANDARD or BRITISH STANDARD (Please quote standard number and clause number) where this value is indicated as 3 ohms or less? Thank you.

Thank you for your question. It is our pleasure to help.

You are correct that the NEC states that if you use one earth rod that the resistance to earth must be 25 ohms.  If it is higher, then you must place a second earth rod.  There is no requirement to retest the resistance.  In practice, for a residential structure, most electricians place two earth rods and call it done.  This does not create the best earth, 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-earth, 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 earthing of Industrial and Commercial Power Systems (The Green Book) Power stations and substations

“For satisfactory lightning protection, substation earthing 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-earth less than 5 ohms for most installations.  For substations or sites with sensitive electronics, we recommend a design goal of a resistance-to-earth of less than 1 ohm.

Best regards,
The Engineering Team at E&S earthing Solutions

Photo credit: commons.wikimedia.org

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