Q & A of the Day – What reading should I be getting when using earth rods on a construction site?

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Question: What reading should I be looking for when using earth rods on a construction site for a 1,600A mains? We are obviously using an RCD backup.

Some people say that it should 100 ohms; others say 200 ohms. Some even say 1 ohm – so what is your opinion?

Answer: In a TT installation, the value of earth resistance required for the main earth electrode depends on the required value for the primary protective device of the installation.

In BS 7671, in the case of where an RCD is fitted, reference should be made to Table 41.5 for a non-delayed RCD, and Table 3A for a Type S time delayed RCD.

BS 7430:2011, section 9.1 states that ‘this connection should have a resistance not greater than that required to operate the safety device’.

BS 7671, Table 41.5, Note 2 recommends that it should be as low as practical and a value above 200 ohms may not be stable.

Regulation 542.2.4 states that the earth electrode shall be such that soil drying and freezing will not increase its resistance above the required value.

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