TN-S Earth clamp

ben · 2020-05-24T19:49:58+00:00

Had a proper look at the earthing on the new place today. The supply earth is provided by a clamp onto the sheath. It's not, so far as I can, a tightenable clamp. It has a 10mm cable in which looks like it only just fits, can't see how I'd get a 16 in there. (tails are - at the moment - 25mm as far as a Henley, then down to 16mm)I noticed that the clamp rotates freely around the sheath. Is that to be expected? I will get round to measuring the earth soon when the replacement test lead turns up.

May 24, 2020 Last reply: 6 years ago 32 Replies

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Robin 6 years ago

All I meant was that I agree with those who argue a screwed connection in a main bonding conductor brings it within 514.13.1((ii) ("a point of connection of every bonding conductor to an extraneous-conductive-part") and so requires the warning notice. It can be argued the other way but I was persuaded by the fact that the join must be accessible (else you wouldn't use a screwed connection, would you?) so it makes sense to require a warning notice there as at connections to pipes etc.

What happens in practice is of course another matter. But I've seen labels used where the bonding used a combination of cable and pipe.

I don't know just what your chunk of brass was for but 514.13.1((iii) provides that no label is needed for a MET by switchgear.

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ARW 6 years ago

Just call it an earth marshalling bar?

Stickers that say "earth warning do not remove" are cheap

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Robin 6 years ago

I'll be guided by you!

I've seen those labels on EMBs a few times (once inside a CU) but never really understood why they are used instead of BS 951 labels or how they finesse 514.13.1.

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Dave Plowman (News) 6 years ago

That sounds like what existed here on an old installation. Where the incoming lead water main and the sheath to the mains riser provided the rudimentary earth.

If you can use the mains riser for earth it needs the correct clamp - but that's really a job for the supply company to make sure it is up to it.

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charles 6 years ago

The electrical installaton here, probably 1920s, got its earth from the incoming cable sheath. However, after a few days of being fed from a generator, tests on reconnection showed an inadequate earth from that cable sheath. The supply company changed things to PME.

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ARW 6 years ago

Personally I do not think it matters that much.

But some people like to see safety stickers and they get paid to look for non problems.

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ben 6 years ago

Leads finally turned up. Ze is 0.2. I moved their clamp gently, and established that it's a) stiffer than I remembered from Monday, and b) didn't affect the measurement. BTW it's not a 951 clamp.

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Robin 6 years ago

Fair point. I'll put me clipboard away.

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Roger Hayter 6 years ago

It's slighly interesting. The person who wired my house originally took equipotential bonding remarkably seriously and throughout the house (at least in all rooms with any CH pipes or extraneous metal work which I think was everywhere except the conservatory) he ran a network of

10mm insulated earth wire in parallel with every bit of the socket wiring plus to every pipe, even ones connected by copper back to the boiler. If I felt like burning the insulation off I'd have about 10kg of copper I've ripped out doing other jobs. A consequence was an earth terminal block in almost every room and the electrician used one of them which was actually directly connected to the supply earth block, rather than pulling a new wire in from the supply board to do the one bit of bonding the original owner forgot, which was the oil feed into the house

- one of very few that was actually required. Though since it goes to the outside oil tank with no earthing to speak of it therefore creates a new PME hazard. In the context a label saying this connection is actually wanted might be sensible, though it would have to say why in the context of dozens of other pointless earth labels.

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tabbypurr 6 years ago

When you've finally ripped it all out, then the regs will require equipotential bonding in all rooms.

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Robin 6 years ago

On 29/05/2020 12:38, Roger Hayter wrote: <snip>

Others may know what was behind that. To me it sounds like someone (a) installing high integrity earthing without understanding what was and wasn't required or (b) with an awful lot of 10mm green/yellow single to bill to an unsuspecting client. Any idea when it was done?

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