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Topic Title: Twin & Earth to CU
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Created On: 20 October 2017 01:19 PM
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 20 October 2017 01:19 PM
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Montyjon

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Joined: 24 March 2016

I've noticed that my house meter tails go into 2-pole fused switch then from the switch it is taken to the CU with 16mm T&E (approx. 2m away) is this acceptable as the CPC would only be 6mm?

Could I run an additional length of 10mm earth cable to the earth connector at the meter?

Supply type is TN-S
 20 October 2017 01:47 PM
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dustydazzler

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My old flat had 10mm t&e straight from the meter pocked up through the cavity about 6 feet and into the fuse box.
All the flats were just the same in the block , 100a fuses as well.
Flats were built in the mid 80s where 'anything goes' was the order of the day
 20 October 2017 06:09 PM
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geoffsd

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Originally posted by: Montyjon

I've noticed that my house meter tails go into 2-pole fused switch then from the switch it is taken to the CU with 16mm T&E (approx. 2m away) is this acceptable as the CPC would only be 6mm

acceptable, yes.

Could I run an additional length of 10mm earth cable to the earth connector at the meter?

Meter ???
Where is the Main Earthing Terminal?

Supply type is TN-S

IF it still is then it could be alright as is.
However everything should be treated as PME these days and so you would need 16 sq.mm. earthing conductor supply to MET.
 21 October 2017 10:27 AM
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Montyjon

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Main earthing terminal is next to the supply cable connected to a metal terminal block taken from the outer armour of supply.

Also the connection from the armour to the MET is a dreadful looking peace of braded steel which looks rather thin!
 21 October 2017 10:39 AM
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dustydazzler

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A braided strap is fairly common
 21 October 2017 10:41 AM
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Zoomup

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Originally posted by: Montyjon

Main earthing terminal is next to the supply cable connected to a metal terminal block taken from the outer armour of supply.



Also the connection from the armour to the MET is a dreadful looking peace of braded steel which looks rather thin!


With a clamp connected to the lead sheath?

Z.
 21 October 2017 11:50 AM
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Montyjon

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With a clamp connected to the lead sheath?



Z.



No it looks like its soldered to the sheath
 22 October 2017 05:21 PM
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AJJewsbury

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is this acceptable as the CPC would only be 6mm?

Where does your main bonding connect? CU earth bar or at the supply end of the T&E? (i.e. is the T&E providing just a c.p.c. to the CU or is providing main bonding too). If the main bonding is connected to the CU, 6mm2 is probably (almost certainly) sub-standard these days (especially considering your supply is very likely to be PME in reality if from a public supply, regardless of how it is presented at the cut-out). 6mm2 has been acceptable in the past for main bonding, and may well be OK in practice if no thermal damage is evident, but isn't ideal even then.

Is the CU metal or plastic? If it's plastic and things are arranged so that there's no practical chance of a L-PE fault before the outgoing protective devices (like we'd typically claim for a TT install) then as long as the T&E's c.p.c. isn't smaller than any of the outgoing c.p.c.s (and as long as it's not being relied on for main bonding) it should be fine. Ditto for a metal CU if a 'Class II' kit has been used.

If there is a chance of L-PE faults before the outgoing devices, the you'd have to do a little calculation to prove that 6mm2 was OK, depending on the loop impedance and rating/type of the fuse - but chances are it would pass.

- Andy.
 04 November 2017 08:52 AM
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Montyjon

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Originally posted by: AJJewsbury

is this acceptable as the CPC would only be 6mm?


Where does your main bonding connect? CU earth bar or at the supply end of the T&E? (i.e. is the T&E providing just a c.p.c. to the CU or is providing main bonding too). If the main bonding is connected to the CU, 6mm2 is probably (almost certainly) sub-standard these days (especially considering your supply is very likely to be PME in reality if from a public supply, regardless of how it is presented at the cut-out). 6mm2 has been acceptable in the past for main bonding, and may well be OK in practice if no thermal damage is evident, but isn't ideal even then.



Is the CU metal or plastic? If it's plastic and things are arranged so that there's no practical chance of a L-PE fault before the outgoing protective devices (like we'd typically claim for a TT install) then as long as the T&E's c.p.c. isn't smaller than any of the outgoing c.p.c.s (and as long as it's not being relied on for main bonding) it should be fine. Ditto for a metal CU if a 'Class II' kit has been used.



If there is a chance of L-PE faults before the outgoing devices, the you'd have to do a little calculation to prove that 6mm2 was OK, depending on the loop impedance and rating/type of the fuse - but chances are it would pass.



- Andy.

The main bonding is connected to the CU, the only connection to the MET is via the T&E.

Would it be acceptable to run another earth from the CU directly to the MET?
 04 November 2017 10:03 AM
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AJJewsbury

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Would it be acceptable to run another earth from the CU directly to the MET?

Entirely acceptable & commonly done - best kept as close as practical to the T&E/tails.
- Andy.
 04 November 2017 03:10 PM
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geoffsd

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Originally posted by: Montyjon

The main bonding is connected to the CU, the only connection to the MET is via the T&E.

The MET, therefore, is the CU earth bar, not the block.
 05 November 2017 09:21 PM
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Montyjon

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Originally posted by: AJJewsbury

Would it be acceptable to run another earth from the CU directly to the MET?


Entirely acceptable & commonly done - best kept as close as practical to the T&E/tails.

- Andy.


Would 10mm2 be acceptable?
 05 November 2017 09:41 PM
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sparkingchip

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My understanding is that if it is TNCS earthing you have to run a 16mm main earth conductor in addition to 6mm in the T&E, as we should treat TNS as TNCS then we have to do the same. However with TT we can just use the 6mm, if it is going to the right place to connect to the earth electrode.

Andy B.
 06 November 2017 12:20 PM
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AJJewsbury

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Would 10mm2 be acceptable?

Depends on the rating & type of the fuse in the switchfuse & earth loop impedance (but chances are it would be OK) - this is in effect a submain c.p.c. also serving as a bonding conductor rather than an earthing conductor as such.

To avoid the calculation and still be sure to be safe you'd need a min 16mm2 overall (presuming it's not more than a 100A supply & 25mm2 tails) - in this case you're probably safe to add the c.s.a. of the T&E's c.p.c. to your additional conductor to get 16mm2 - so if the T&E contains 6mm2 another 10mm2 should do. (That approach is usually frowned upon where the main cable contains steel armour or is in steel conduit as the steel can influence the current sharing between the two protective conductors) but with plain PVC insulated cables and all in close proximity I think it should be OK.

For "least astonishment" for those that follow, a 16mm2 would be immediately recognised as fine.

- Andy.
 06 November 2017 03:05 PM
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Montyjon

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fuse is 80a which seems to large for 16mm T&E?

also I've just had the meter changed and nothing was said about the lack of earth, also there was an absence of any earth sleeve it was wrapped in yellow & green insulation tape!
 06 November 2017 03:13 PM
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dustydazzler

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As I pointed out earlier i lived in an 1980s built flat with 10mm twin and earth from the meter up to the flat on a 100amp supply.
Nothing new hear what so ever

The green tape wrapped round the earth wire is rough , not to mention what a waste of perfectly good tape. I would have left it bare
 07 November 2017 11:48 AM
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AJJewsbury

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fuse is 80a which seems to large for 16mm T&E?

Depends on how the T&E is installed, but probably OK. 16mm2 is good to 85A if clipped direct.

- Andy.
 09 November 2017 10:19 AM
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AJJewsbury

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Originally posted by: geoffsd

Originally posted by: Montyjon



The main bonding is connected to the CU, the only connection to the MET is via the T&E.


The MET, therefore, is the CU earth bar, not the block.


That's got me thinking... my first reaction was to agree, but now I'm not so sure. I think the MET isn't necessarily defined by where main bonding is physically connected. Bare with me...

Say we had an earth block connecting the supplier's earth facility to one CU which had say the water bond connected to its earth bar, and that CU then fed a remote CU (with a suitably large protective conductor) and that remote CU had the gas bond connected to its earth bar. Clearly the remote CU's earth bar isn't "the MET" - so the gas bond and remote CU's c.p.c.s are connected "collectively" (i.e. sharing a connection via a single common protective conductor) to the MET.

But using the same logic the same could be said of the earth connection to the first CU - which would make the earth block the MET.

If the earth block also connected a 3rd bond (say to structural steel) I think we'd all agree that it would then be the MET - even though the arrangement from there to the main CU is identical.

So what I think I'm concluding is that the MET needs to be the 'root of the tree' of the protective conductors - (i.e. the only connection 'upstream' of the MET is the installation's earthing conductor) but where there are several terminals in series between the means of earth and the first branch, it's arbitrary which is deemed to be the "MET".

I could even imagine a designer, say in an installation with no mains bonds (all plastic plumbing say), designating an earth block with no branch connections at all as being the MET - just to provide a facility for future connections (especially for things like data or telecoms functional earthing which may well be added without wanting to shut down the entire installation to re-configure the earthing arrangement).

- Andy.
 09 November 2017 12:49 PM
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geoffsd

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Originally posted by: AJJewsbury

That's got me thinking... my first reaction was to agree, but now I'm not so sure. I think the MET isn't necessarily defined by where main bonding is physically connected.

It is in the definitions.

Are we agreed that there can be only one MET and one Earthing Conductor?


I can see the logic in what you say, but -

Firstly, I don't see how a block which only serves as a joint in THE Earthing Conductor, while all the bonds are connected at the CU, can be deemed as THE MET.



In your example, the gas bond connected to the second CU could be said to be using the CPC from the first as a continuation to connect it to the first's earth bar, THE MET.
This CPC would have to be large enough to use as a MPB conductor.

I suppose it comes down to the sizes of the conductors which lead back to the MET at the first CU.



As for the structural steel being later connected to the block, that is fine electrically but I don't know what to say about that specifically, other than would that then mean the block is now the MET and the bonds are connected to it through the first CU earthing conductor which would become (just) a CPC?



It would, because of your examples, seem to be academic which is the MET - as everything is connected to everything else.
Does it just relate to the size of the conductors? If the block had all the bonds connected to it then it would be THE MET and the CPCs to the CUs could possibly be quite small - i.e. would not have to be 16 sq.mm. for PME.
 09 November 2017 01:06 PM
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mapj1

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"Are we agreed that there can be only one MET and one Earthing Conductor? "

In a large installation, not always.
Especially with sub-mains to out buildings that also have multiple services in pipes to be bonded, or in situations like a block of flats with a CU each.

There is one or none suppliers earth terminal, but then it gets messy, unless you think the submain CPC is part of the bonding to the outbuilding, and not a CPC. Of course in terms of diverted current it could flow either way in such a case, or even up one pipe and down another, without troubling the company earth at all.
Then there are a few odd cases when there appear to be more than one company earth.

-------------------------
regards Mike
IET » Wiring and the regulations » Twin & Earth to CU

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