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Result of earth loop impedance test - as I have an RCD, is it safe?

Posted by John Davies, on
Hi. A Corgi gas boiler engineer refused to service my boiler because he plugged a device into a couple of sockets (A Martindale E-Ze Check Extra) and said my system failed an earth loop impedance test. He let me photograph the reading which showed that the wiring was correct, the voltage was OK, and the loop test showed an impedance of 5 to 10 ohms. I did a bit of research online which said that such a reading is safe if I have an RCD fitted, which I do. In the light of this is the system safe and should I be able to ask the engineer to return to complete my boiler service? Thanks, John D , UK.
David West

David West

5 to 10 ohms is not a very accurate reading for a Ze test. Depending on what earthing arrangement you have, if it is TNCS then a maximum Ze should be 0.35 ohms, TNS is 0.8 ohms or TT systems where you have an earth rod in the ground with earth wire attached will be up to 200 ohms. So if your system is TNS or TNCS 5 - 10 ohms is very high irrespective of whether it is RCD protected or not. I would get a professional electrician to check it out properly with a view to contacting your supplier to make sure you have the correct earthing arrangement.

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