Heat damaged MET - DNO responsibility?

[ccam108]

I was at my mother-in-law's house earlier to fix a faulty wall light and while I was there I noticed some severe heat damage to the main earthing conductor:



The fault (whatever it was) was bad enough to char the wooden board everything is mounted on. Also, inside the CU I could see one earth that also showed signs of heat damage (belonging to a 1.5 or 2.5 T&E). Clearly something bad has happened and this needs fixed as a matter of urgency. There is still continuity from the incomer sheath to the earth bar in the CU, but I only had a basic meter with me and couldn't measure the resistance.

My worry is that the MET has probably been damaged and will need replaced along with the cable. Is this the supplier's responsibility? Disconnecting and replacing it myself would mean disconnecting the earth for the other supply which disappears beneath the floor, potentially feeding a neighbour.

 

[bernardgreen]

This happens when the "Earth" supplied by the DNO is not at local ground potential and something such as a metallic water supply pipe is bonded to the MET.

The metal water pipe has a very low impedance to Ground so a high current flows from Ground to the Earth via the MET.

One reason the "Earth" supplied by the DNO is not at Ground potential is when copper thieves have stolen Neutral conductors and / or bonds at the substation.

There are other causes but almost all of them relate to a fault on the Earth supplied by the DNO's incoming supply.

You were a bit lucky in that the MET with the old and corroded stranded copper "Earth" wire appears of have had a poor connection which limited the current and also created a visible hot spot.

 

[ccam108]

Thanks for the explanation Bernard.

The supply appears to be a TN-S, not TN-C-S, as the DNO's earth comes from the (lead?) sheath of the supply cable. Surely a stolen neutral wouldn't affect this? This earth enters the bottom of the MET (small black cable) and the bare stranded earth at the top belongs to the two tails that disappear beneath the floor, probably to a neighbour's house.

I will check the bonding to the gas and water supplies, but if that was the issue, why would the heat-damaged earth in the CU belong to a T&E cable?

 

[bernardgreen]

why would the heat-damaged earth in the CU belong to a T&E cable?

Do you know which circuit this damaged earth belonged to ?

If the bare braided wire from the MET is the earth to the other property then it maybe a live earth fault in that property has occured without tripping their over current device ( fuse wire or MCB ).

The poor ( resistive ) joint in the MET would have limited the fault current to a low ( but dangerous ) level that was too low to operate the over current protection but high enough to cook the MET.

You need to ask the DNO as a matter of urgency to inspect and improve the earthing they are providing.

 

[ccam108]

I couldn't see what circuit it belonged to and I didn't have my tools with me so I wasn't able to start doing any real investigation.

I will get in contact with the DNO today and I'll be sure to follow up with the outcome once I've found out what happened.

 

[securespark]

That unprotected rubber cable going under the floor looks gash...

 

[ban-all-sheds]

The supply appears to be a TN-S, not TN-C-S, as the DNO's earth comes from the (lead?) sheath of the supply cable.

Does it?

We can't see a clamp, but we can see what look just like strands from a concentric or waveform cable going to the MET, and if we zoom in on photo #2 we can see what might be armour cut back just before the cable enters the cutout.

But it does look like TN-S.

 

[ban-all-sheds]

That unprotected rubber cable going under the floor looks gash...

Is it usual to supply a neighbouring property with unprotected tails like that? Wouldn't we expect to see another "DNO supply cable" looped out of the cutout here:

?

 

[ccam108]

The tails from next door appear to go into a conduit along with the multi-strand earth from the MET. Looks rough, but if the DNO's happy with it then fair enough.

On your close-up, you can see a ring where the earth has been soldered about an inch or so before the supply cable enters the cutout. This is the black-insulated wire in the bottom of the MET.

Anyway, someone came out from SP Energy Networks today, replaced the MET and did something to re-arrange the supplies to this house and next door. I haven't seen the finished job yet, but he even mounted the MET higher up the board so he could re-terminate the existing earth wire to the CU.

As for the cause, I discovered that the electric shower had been wired with neutral and earth swapped. Combine a regular ~10kW load over 8 years with a loose old MET and no RCD protection and you end up in the situation shown here.

I'll pop over again later to see the completed work and get some photos.

 

[bernardgreen]

electric shower had been wired with neutral and earth swapped.

Nasty, as you say an RCD would have detected that error. That is the sort of thing a rank amateur bodger does. How many other errors are there in the wiring ?

Might be a good idea to have an RCD installed as soon as possible just in case there are any other errors waiting to bite.

 

[ccam108]

Yes, she questioned whether it was time for a full rewire a few days ago. Everything looks to be around 30 years old by my guess so I think the wiring should be ok for the most part, but there is no RCD protection and the wiring around the CU looks a bit sloppy, so I think a new RCD board and full EICR is definitely a good idea.

As for something a rank amateur would do, how about a large bathroom installer company?

 

[bernardgreen]

As for something a rank amateur would do, how about a large bathroom installer company?

Even though it is 8 years old I would still consider contacting the company to tell tham that one of their employees ( or sub contractors ) made a serious mistake in installation and obviously failed to test his ( or her ) work. If that person is still employed they could be re-assessed for the role they have.

If that MET joint had failed then the entire "earth" system in the house would have become live. The only immediate indication of a fault would be the shower failing to work. But anyone touching an "earthed" but now live appliance would have received a severe, possibly fatal shock had they touched anything in contact with true ground. An outside tap if "bonded" to the "earth" would have been lethal.

 

[ban-all-sheds]

After 8 years there is zero chance of proving, or even getting the company to accept, that they were responsible.

 

[bernardgreen]

After 8 years there is zero chance of proving, or even getting the company to accept, that they were responsible.

Obviously they will not admit or accept any liability now but it might trigger an internal evaluation of the quality of the people they employ.

 

[AndyPRK]

Like they will know who did the install.

How did you discover the incorrect wiring and where was it ?