TN-S with a tenby clamped lead sheath for earth

[slippyr4]

Hi All,

My parents have just bought a house, and on my first visit, I had a look in the meter cupboard to get an idea of the state of the wiring. There are some things in there which are criminally bad, and need very urgent remedial action (such as some 10mm T+E for a shower connected directly to a henley block (completely unfused).

However one thing that raised my eyebrows which i'd like an opinion on is the state of the main earth:

The supply appears to be TN-S with a lead sheathed supply cable. There is evidence of the remains of an earth soldered on to the sheath, but that's broken off and has been replaced with a bit of 10mm into a standard earth clamp as would be used for bonding services.

Is that acceptable? My hunch is not.

If that's the case, and therefore the main supply earth has failed, I presume this is a job for the DNO? What, practically would they do about it?

I suspect their preferred approach would be to convert to PME, but the rest of the bonding doesn't look up to spec at the moment (the fusebox is connected with some 10mm but all the other earth wires (which I presume bond various services - i've not followed them yet) are 6mm which is inadequate for PME.

Any thoughts?

 

[Taylortwocities]

I'll wait for Westie or other DNO-centric folk to come along for a definitive response but my understanding is that the DNO normally is responsible for providing the earth for TN-S and TNC-S services.

Something in the back of a brain cell says that they are known to be reluctant to 'own' this once the service has been established, so you may need to argue the point. Or call them and say there's no earth connection.

As for conversion to TNC-S. This would be very unlikely as there will be no multiple earth points established on a TN-S route. Migration from TT to TNC-S does occur, but I would expect that you'll always have TN-S.

 

[RF Lighting]

First thing to do is an earth fault loop impedance test of the incoming supply. If the reading is in spec then I'd be happy to leave it be.

My DNO were still fitting BS951 clamps to PILC cables until about 10 years ago.

 

[westie101]

As far as we are concerned in this DNo if there is an earth we are obliged to maintain it (though maybe not to the size in BS7671)

On older lead sheaths it would have been in the form of a sweated earth, but various clamps have been used over the years, subject to them not damaging the lead sheath and providing sufficient continuity.

However if you think of it the present clamp is deemed fine for water pipes (copper or lead) and gas pipes, so why would it be inadequate on the lead sheath of a power cable?

If you are concerned contact your DNO to see of they will fit a different type of arrangement (we use a roll spring)

And as others have said, it would be unlikely to be converted to PME (TNCS) which doesn't really give any advantages

 

[ban-all-sheds]

However if you think of it the present clamp is deemed fine for water pipes (copper or lead) and gas pipes, so why would it be inadequate on the lead sheath of a power cable?

Because it can crush it?

 

[slippyr4]

Thanks guys. I will investigate more thoroughly next time I am in the property, and do an EFLI test.

Actually I wonder if its a TT with a "bonus" earth clamp - there are more earth cables in the earth block than i can account for, so maybe one goes to an earth rod that I havent found yet. The whole installation has an RCCB so is not beyond the realm of possibility.

 

[JohnW2]

Actually I wonder if its a TT with a "bonus" earth clamp - there are more earth cables in the earth block than i can account for, so maybe one goes to an earth rod that I havent found yet. The whole installation has an RCCB so is not beyond the realm of possibility.

One interesting thing about that possibility is that, if you found an earth rod and hence regarded it as a TT installation, then (assuming it comes from underground) the lead sheath of the incoming supply would presumably constitute an extraneous-conductive-part which would have to be main bonded to the MET - so there would still be an issue of having a satisfactory connection to the lead!

Kind Regards, John

 

[ericmark]

An electrician working on my parents house found a earth stake and seemed to assume the house was TT. He actually connected the stake to the new consumer unit he was fitting.

However the earth stake was for party line telephones and nothing to do with the power supply. It just had never been removed when the post office fitted a dedicated phone line.

 

[Taylortwocities]

An electrician working on my parents house found a earth stake and seemed to assume the house was TT. He actually connected the stake to the new consumer unit he was fitting.

I wonder what his earth impedance reading was like, those old GPO earths were only diddly little things….

 

[plugwash]

However if you think of it the present clamp is deemed fine for water pipes (copper or lead) and gas pipes, so why would it be inadequate on the lead sheath of a power cable?

My understanding was that standard earthing clamps are not considered suitable for lead pipes.