Main earth size on TN-S

zvspark · Jan 20, 2023

Hi guys.
I was on EICR yesterday, where i found that the main earthing conductor is with very small csa- no more than 2.5cm2.
Also they run the wire directly behind the CU, where it is twisted with 10mm2 and continues to the MET inside the board.
Ze at the board is 0.28 ohms and seems to be ok, but the size worries me.
How would you code that and whose responsibility is to replace that wire?
Thanks.

westward10 · Jan 20, 2023

If you say it is about 2.5 it is probably a couple of strands of imperial 0.064 or 0.052.

pc1966 · Jan 20, 2023

If, as it appears to be, a TN earthing arrangement then it would be C2 for "Cross-sectional area of the earthing conductor does not satisfy adiabatic requirements (that is, does not comply with Regulation 543.1.1)" from latest BPG#4

The connection to the incoming cable would be the DNO's responsibility, but there is also the chance they decline to provide an earth and you have to TT the installation as a result. That would obviously have major implications for the extent of RCD protection.

davesparks · Jan 20, 2023

pc1966 said:
If, as it appears to be, a TN earthing arrangement then it would be C2 for "Cross-sectional area of the earthing conductor does not satisfy adiabatic requirements (that is, does not comply with Regulation 543.1.1)" from latest BPG#4

Not until you have actually done the calculation though!

pc1966 · Jan 20, 2023

davesparks said:
Not until you have actually done the calculation though!

True...but min CPC for even the low-end assumption 63A fuse at that sort of Zs is 4mm and its not looking to meet that!

Edits:
A quick check is to look in the OSG Table B3/B4/B5 and see if it meets/exceeds the acceptable CPC for a given supply fuse and Zs.

Also not clear if PME condition exist or not, if so then it is going to fail on the 10mm minimum (reg 542.3.1 that cites 544.1.1 for that)

Also 543.1.1 also has a minimum of 2.5mm and I strongly suspect it fails on that as well, but come callipers and calculation would be needed to verify it.

cliffed · Jan 21, 2023

davesparks said:
Not until you have actually done the calculation though!

Not sure if you can calculate for the Main Earth Conductor

davesparks · Jan 21, 2023

cliffed said:
Not sure if you can calculate for the Main Earth Conductor

Yes of course you can, how else would you establish what size is the minimum requirement to be safe?

pc1966 · Jan 21, 2023

cliffed said:
Not sure if you can calculate for the Main Earth Conductor

My first visual check was it looked like two strands of 30A fuse wire!

My own starting point is you should be able to clear the DNO fuse worst case, hence the starting point of looking at the OSG tables for the fuse CPC versus Zs values that meet the adiabatic. But ultimately if should be at least 6mm on old systems and really ought to be 16mm these days based on the 16mm/25mm sort of supply cable.

Even if you were looking at final circuit clearing, it would have to be something like the CPC size of the heaviest circuit to be acceptable, so again typically 2.5mm to 6mm or so on that optimistic case.