Q & A of the Day - Where have the 160 ohms from the earth rods gone?

Question: On a rewire job, there is a 2.5mm main earth on a TN-C (this was a DIY job carried out about 30 years ago), so I called the distributor who attended two weeks later and advised the following: 'No protective multiple earth....get your sparky to sort it'!

The earth rod reads 1200 ohms as the ground is very gravelly, so - upon technical helpline advice, I banged 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 north side of 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 BS7671:2008 for Automatic Disconnection of Supply (ADS).

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