Published: 30 January 2015
Category: Q&A
Rob asks this question, which concerns an electrical installation re-wire and unusual earth rod readings. It has been answered by the NICEIC (Theme - Fire, Safety & Security):
This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.
Question: I’m currently working on an electrical installation re-wire job, and there’s a 2.5mm main earth on a TN-C (this was originally a DIY job carried out about 30 years ago), so I called the distributor who said: “There’s no protective multiple earth....get your sparky to sort it”!
The earth rod reads 1200 ohms as the ground is very gravelly, so – after calling upon technical helpline advice, I put in a second earth rod. This has brought the reading down to 475 ohms....and the third earth rod read 160 ohms. I was happy, as it was on the north side of the house and never dries out completely.
I was expecting the (R1+R2) plus Ze to be higher than usual but the loop test on the main ring (for example) was 0.34.
The rewire is now complete and all tests were well within limits, but I am confused as to where the 160 ohms from the earth rods went. Where did it go?
Answer: Parallel earth paths may exist on the installation. Water and gas utilities pipework may be in contact with earth.
You must be able to solely rely upon your earth electrode as the protective main earth for the installation. If the Ze is 160 ohms and the circuits are protected by a 30mA RCD, then the earth fault loop impedance will satisfy the requirements of BS 7671: 2008 for the Automatic Disconnection of Supply (ADS).
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