Main earth after cut-out replacement

MKhan7249 · Aug 15, 2022

Hi all,

After months of waiting, the fused neutral cut-out at my parents house has been replaced but I am concerned about the main earth.

I believe it to be a TN-S as the main earth is attached to the back of the main incoming cable. In December, I got the CU replaced and isolator fitted and was advised by an electrician that cut-out replacements would alter the earthing system to TN-C-S or PME.

This morning, the DNO dug up the pavement (assuming they cut the cable) and reconnected after the replacement. However, the main earth was not touched at all. It is 4mm2 cable that goes behind the incoming cable is just stuck to the wall with no earth clamp.

The DNO measured the earth impedance using the live and neutral and quoted it was 0.3 with the maximum allowed is 0.8

I've attached photos. In the photos, you see two earths, the larger earth leads outside to the gas pipe, the smaller earth goes behind the incoming cable. Also, notice that the fuse carrier is not sealed.

Any advice much appreciated.

timhoward · Aug 15, 2022

MKhan7249 said:
I believe it to be a TN-S as the main earth is attached to the back of the main incoming cable.

Correct

MKhan7249 said:
In December, I got the CU replaced and isolator fitted and was advised by an electrician that cut-out replacements would alter the earthing system to TN-C-S or PME.

That would have been a fair guess, but it was a wrong guess as things turned out. No big deal.

MKhan7249 said:
This morning, the DNO dug up the pavement (assuming they cut the cable) and reconnected after the replacement. However, the main earth was not touched at all. It is 4mm2 cable that goes behind the incoming cable is just stuck to the wall with no earth clamp.

I think it is actually brazed to the lead cable. Rather difficult to say from photo.

MKhan7249 said:
The DNO measured the earth impedance using the live and neutral and quoted it was 0.3 with the maximum allowed is 0.8

I'm assuming/hoping they also had the test device connected to the 4mm earth wire with it disconnected from the metal MET block.
If so that's a good result and proves a decent connection.

As far as I can see, whether it's a 60 or 80A fuse in the cut-out, the regs are satisfied (adiabatic equiation) with a 4mm earth.
Worst case (lower limit) fault current of 766A . The max prospective fault current will be higher as the neutral is bigger, lower impedance = higher current = faster trip time.

For 766A fault current, 60A disconnection time is 0.2s. 80 disconnection time is 0.7s.
I get between 3mm (so 4mm) for 60A and 4mm for 80A.
At this point I'm not concerned.

If you have concerns it's a quick job to have a sparks verify the Ze (earth impendence).

davesparks · Aug 15, 2022

Which DNO did that? Was it UKPN by any chance?

MKhan7249 · Aug 15, 2022

davesparks said:
Which DNO did that? Was it UKPN by any chance?

Correct.

MKhan7249 · Aug 15, 2022

timhoward said:
Correct

That would have been a fair guess, but it was a wrong guess as things turned out. No big deal.

I think it is actually brazed to the lead cable. Rather difficult to say from photo.

I'm assuming/hoping they also had the test device connected to the 4mm earth wire with it disconnected from the metal MET block.
If so that's a good result and proves a decent connection.

As far as I can see, whether it's a 60 or 80A fuse in the cut-out, the regs are satisfied (adiabatic equiation) with a 4mm earth.
Worst case (lower limit) fault current of 766A . The max prospective fault current will be higher as the neutral is bigger, lower impedance = higher current = faster trip time.

For 766A fault current, 60A disconnection time is 0.2s. 80 disconnection time is 0.7s.
I get between 3mm (so 4mm) for 60A and 4mm for 80A.
At this point I'm not concerned.

If you have concerns it's a quick job to have a sparks verify the Ze (earth impendence).

Would they still have maintained the TN-S supply after reconnecting?

By reconnecting I mean cutting and rejoining.

timhoward · Aug 15, 2022

MKhan7249 said:
Would they still have maintained the TN-S supply after reconnecting?

It looks like they did, and they told you an earth impedance and the max limit for TN-S, so the laws of probability say yes.

default · Nov 14, 2022

Cant do anything about the size of the main earth the DNO supply. We're only concerned about the consumer side not the supply side. If the supply company have tested their side and say that's the reading their getting then we have to go with that If your worried get it tested but its probably fine.