Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This Question of the Day, concerning an earth electrode rod connected to a TT earthing system, is answered by the NICEIC:
Answer: There is no requirement in BS 7671:2008 to achieve a resistance of below 21 ohms.
Below Table 41.5 of the Regulations (see page 50), there is a Note 2, which requires the resistance of the electrode to be below 200 ohms. Anything above that reading is classed as being unstable - it should be a low as possible in all cases.
The 21 ohms is the maximum resistance stated by the distributor on their electrode back at the supply transformer.
Further guidance is given in the On Site Guide on the bottom of page 11.
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