Is a borrowed earth allowed?

a-z electrics · Oct 13, 2018

Done an EICR yday on an old house and all ok apart from 2 issues..
1) 32A cooker circuit had high resistance pec continuity (over2kohms) ALTHO it did buzz out on my voltage probes. Hence not able to get a Zs reading and no rcd tripping. The dual rcd consumer unit was fitted 10 yrs ago and those test readings all available and fine. Kitchen was refitted 3 yrs ago and no docs available so they extended original cooker cable prob from the switch which is now tiled over somewhere.
.. I've put in an earth wire link from an immediately adjacent rfc socket into the cooker switch and this solves this problem.
Question .. is this ok as a permanent solution if i label up what ive done?
2) lowish IR readings 1.75M L-E and N-E on light circuits and one rfc but possibly in hindsight due to some bulbs, fan and appliances still plugged in. Could this account for it?

Midwest · Oct 20, 2018

Hope it wasn't one of these;

[ElectriciansForums.net] Is a borrowed earth allowed?

Pete999 · Oct 20, 2018

a-z electrics said:
But it was MF..

What type of connector was used?

mrloy99 · Oct 20, 2018

There are other concerns too..maybe there could be another radial in the kitchen with a 'lost ' earth and the previous spark has already borrowed the cooker earth......So new cooker circuit in /old cooker circuit out might leave another circuit unprotected...Full test might identify that..I suppise...I dont really see a problem with just installing a new separate earth..Why does it need to be in the same cable.

Midwest · Oct 20, 2018

mrloy99 said:
Why does it need to be in the same cable.

We were arguing that last weekend. You'll have to read the posts yourself.

Deleted member 9648 · Oct 20, 2018

a-z electrics said:
Funnily enough, I made a similar mistake a couple months ago.. I put in a socket spur from a socket on the rfc which was being covered over.. after plasterer had been I retested and found there wad no earth at new socket! I had to cut out an inspection hole and found the new cpc had actually snapped off inside the terminal for some reason (possibly a hurried stripping operation).. so it DOES happen!

GASP!

a-z electrics · Oct 20, 2018

SparkyChick said:
Just out of interest... what sort of joint was it?

Push in Wagos .. but I have a feeling the cpc wire was nicked whilst stripping, hence snapped off whilst being pushed into the box..

alasdairp · Oct 20, 2018

Sorry this is late, I only usually look once a week when the summary comes around. I have carried quite a few 'serious' Grounding / Earthing related works in the past. In one case, in a very large shopping and business complex in the upper Midlands, with its own substation in the basement, I determined (and they eventually admitted after legal pressure) that they had failed !!!!! to reconnect the Earth to the whole building after they had tested and commissioned the substation! So much for the "professionals" in the DNO's.

Another case was a multiply-supplied (with originally 6 separate meters) industrial/scientific starter-unit complex (with a different DNO) that eventually had one pharmaceutical owner and had really high EMF magnetic fields in their labs (like 50 microteslas) from stray currents which were affecting the biological research results. I determined what the problem was. It was in the local distribution network and not their own supply cables. The only way to sort it was to remove all but one of the network protective Earths and rely on that and the buildings steel structures into the ground (like super Earth Rods). That sorted the problem and the DNO agreed with me that it was the only practical solution. So we did that BUT also affixed signed warning notices at all the incoming supply points to say what we had done in some detail and why.

I agree with SparkyChic that ideally the CPC should be of adequate size, but, if as you say, it is a dual RCD protected installation then all any CPC needs to carry will be 30 mA (or possibly 100 mA). This is not yet reflected in BS7571 - even in the latest 18th Edition.

I think, if you leave it that way, you should stick a notice near the CU saying that is what you have done. It should be safe, but it should not be un-noted. I would have done what you did.

alasdairp · Oct 20, 2018

alasdairp said:
Sorry this is late, I only usually look once a week when the summary comes around. I have carried quite a few 'serious' Grounding / Earthing related works in the past. In one case, in a very large shopping and business complex in the upper Midlands, with its own substation in the basement, I determined (and they eventually admitted after legal pressure) that they had failed !!!!! to reconnect the Earth to the whole building after they had tested and commissioned the substation! So much for the "professionals" in the DNO's.

Another case was a multiply-supplied (with originally 6 separate meters) industrial/scientific starter-unit complex (with a different DNO) that eventually had one pharmaceutical owner and had really high EMF magnetic fields in their labs (like 50 microteslas) from stray currents which were affecting the biological research results. I determined what the problem was. It was in the local distribution network and not their own supply cables. The only way to sort it was to remove all but one of the network protective Earths and rely on that and the buildings steel structures into the ground (like super Earth Rods). That sorted the problem and the DNO agreed with me that it was the only practical solution. So we did that BUT also affixed signed warning notices at all the incoming supply points to say what we had done in some detail and why.

I agree with SparkyChic that ideally the CPC should be of adequate size, but, if as you say, it is a dual RCD protected installation then all any CPC needs to carry will be 30 mA (or possibly 100 mA). This is not yet reflected in BS7571 - even in the latest 18th Edition.

I think, if you leave it that way, you should stick a notice near the CU saying that is what you have done. It should be safe, but it should not be un-noted. I would have done what you did.

The secod one was was due to PME problems in the area.

SparkyChick · Oct 20, 2018

a-z electrics said:
Push in Wagos .. but I have a feeling the cpc wire was nicked whilst stripping, hence snapped off whilst being pushed into the box..

Were the Wago's in a Wagobox by any chance?

buzzlightyear · Oct 20, 2018

SparkyChick said:
Just out of interest... what sort of joint was it?

a smoking one why

gerry whitwham · Oct 21, 2018

I would say that borrowed earths are not good practice ……..one alternative may have been to run a separate 4 0r 6mm cpc back to the consumer unit.

Intoelectrics · Oct 21, 2018

gerry whitwham said:
I would say that borrowed earths are not good practice ……..one alternative may have been to run a separate 4 0r 6mm cpc back to the consumer unit.

I don't think anybody is arguing that it is good practice. The op was about whether it was an acceptable alternative in the circumstances he/she faced. If you are going to the trouble of running a new single cpc back to the CU, I suspect in this case you may as well run in a new twin & earth.