2.5mm earthing conductor from TN-S supply

I found a nice easy second job for my Elecsa assessment. A mate wants an outside light fitting in his porch. He had a new consumer unit fitted last year, all circuits are RCBO protected, and he had a load of other work done at the same time. He's also had some other minor work done since then by a different company. So I expected the existing installation to be in reasonably good shape.

I went round to have a look at his porch and while I was there I had a quick look in the cupboard under the stairs just to check that it all looked ok. My first impression was that the MET looked a bit empty (see attached photo). It was a bit dark and to start with I thought there was no earthing conductor but then, once I got hold of a torch, I found that there actually is an earthing conductor but it is very thin and very black. In fact I think that it's the black core out of a bit of 2.5mm T&E!



Now I'm expecting to find stuff like this all the time, and worse, but what surprises me is that he has an EIC for the consumer unit change, upgrading the main bonding and the other work he had done at the same time, and a minor works certificate (from a big national company) for some other work, and neither of them even mentions the fact the the earthing conductor is far too small. In fact they have ticked a box on the minor work certificate to say that the earthing conductor is not undersized. Is this odd or am I just being naive?

So do I have to fix this problem before I can sign off the minor work? I know that normally you don't need to upgrade earthing and bonding for minor works but does that assume that it at least stood some chance of conforming to the regs at some point?

Obviously I would like to fix this, but I think that maybe it's the suppliers responsibility, is that correct? What's the best thing to do - make a note on the cert and contact the supplier to get them to sort it, or just fit a new 16mm conductor?

He had his meter changed recently as well and they ignored the earthing conductor. He also thinks that the guy who fitted the new consumer unit also fitted the MET in the picture and that the 2.5mm cable used to go to the old fuse box, which means that he reconnected it to the MET along with his new main bonding.

Looks easy to fix to me but since everyone else seems to be avoiding it I'm wondering if I'm missing something?

looks like an easy job to replace that earthing conductor. as it's TN-S, you size it according to the adiabatic equation. measure Ze and PEFC, then S= sq.rt.( Isq x t)/k. this will give you a minimum size for the conductor. or, if lazy, just bang a bit of 16mm in.

where is the 6mm earth going, have you checked??? you sure thats not the old and not being used, im only guessing here on what i cAN SEE

what value do you use for t ??? 5 secs???

Excelent, thanks!

I think that I'll have a go at the adiabatic equation, just for the practice, but probably bang a bit of 16mm in anyway just to be sure.

well when i do it, it comes out at needing 6mm earth, but your ze will change this completely, so if doing new install do you need to know the ze reading???? any info??? put me right

look on page 244 of brb. you will see that a BS1361 fuse will blow at 0.1 secs (@1100A for a 80A fuse), that is t. your value for k is 143. using those figures then S = 2.43, so 2.5mm is adequate for a PEFC of 1100A.

edit: but look at table 54.8. supply conductors <35mm . main earthing conductor min 10mm

i=p/u said:

where is the 6mm earth going, have you checked??? you sure thats not the old and not being used, im only guessing here on what i cAN SEE

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The two green/yellow cables you see in the picture are 10mm, one is the main bonding to the gas and the other connects to the consumer unit.

The main bonding from the water (also 10mm) goes into the consumer unit - not sure if this is strictly allowed, I thought that you could only have one MET, either a terminal block like the one in the picture or the earthing bar in the consumer unit, and the means of earthing and all main bonding had to come back to this point, but I may be mistaken on this? That's what I would have done anyway it I was installing new main bonding, but I guess its a minor point.

well there is two B symbols in cu for i assume this....

if this is new MET how they forget to label it?????.

leonard said:

if this is new MET how they forget to label it?????.

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probably because they didn't realiise it would be photographed and put on this forum.

I hope nobody here recognises it!

I'm sure the label was there, maybe the guy who fitted the new meter nicked it

Id also suggest replacing those old pvc insulated tails.

Chr!s said:

Id also suggest replacing those old pvc insulated tails.

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Those belong to the distributor.