Tails and Main Earth

[C&GStudent]

Hi all, i am a C&G2330 student. I went to meet a registered spark today quoting to change my mums CU.

It is TNS with a 10mm main earth emerging from some insulation tape on the supply cable. The tails are also smaller than 25mm (not sure what size).

The spark said he can only upgrade the tails from the meter to CU, and it was the main suppliers responsibility to upgrade the main earth to 16mm and the tails from supply to meter to 25mm. Is this correct?

 

[JohnD]

the supplier's side of the meter is their responsibility, of course. They will probably not feel like upgrading their tails.

They are responsible for providing an earth, but that is probably a clamp or a sweated union under the insulation tape, so you can probably put a bigger cable on it. But if the incoming is old and crumbly, or covered in tar, cloth or rubber, it may be unpleasant or unsafe under the tape. You could write to the supplier and see if they will upgrade the earth - many will provide PME for a modest fee; and if you have an unsatisfactory pipe clamp round the sheath (not unusual) will provide something better.
your electrician will doubtless have a lot of local experience, and provided you have not upset him with your lack of trust should share it with you.

 

[Adam_151]

That is correct, he can't change the tails from head to meter himself, or sweat another cable onto the TNS sheath, it gets more complicated ith that the tails from meter to CU are not the DNOs responsibility, but only the DNO can make a connection into the meter...

10mm² earth should be sufficent when you use the adiabatic... table 54G over esitimates things... DNOs tails are not likely to cause a problem, short lengths in free air and they are not bount by BS7671 like we are, and sustained high current draws are not likely in domestic, you are not going to be drawing 80A for anything near an hour for example (what size fuse is in the head anyway? [not that your are supposed to pull it yourself....], if its 60A (or even posisbly 80A) there isn't going to be a problem with 16mm², best to try and upgrade them though)

 

[C&GStudent]

Thanks JohnD. I have tried not to upset him!

What would be the point of upgrading the tails then from meter to CU if the tails from supply to meter are smaller? It seems a bit pointless to me.

Would you recomend me having a look under the insulation tape to see if i can upgrade the main earth myself?

 

[C&GStudent]

Thanks adam as well. I will have to check the fuse size when i next go over. On a similar not to my previous post, would you recommend me upgrading the tails from fuse to meter myself? I know its a little naughty!

 

[JohnD]

An interesting question. I suppose because you are supposed to... and a metre or two of new tails is no great cost.

In your position I think I would unwind the tape and see what's there.

If it's a pipe clamp:

1) don't tighten the clamp yourself

2) ask your supplier to arrange for an improved earthing point.

If there are any signs of exposed conductors (except earth wires) or anything broken, stop unwinding and call your supplier.

Don't break into the meter to change the tails. You or your electrician can install a 100A DP breaker in the new tails to the CU, and ask the supplier to get their Meter company to fit your new tails to the meter (explaining that the old ones were undersized). There might be a small charge.

 

[Adam_151]

Why risk getting caught for 'abstactation of electricity' ? if the DNO is happy with them at 16mm² then I'm afraid I don't see the problem (though do try to get them to fit 25mm² if they can be persuaded), its not in the scope of BS7671 which has safety margins built in, and in a domestic house, natural diversity will in practice mean the cable won't overheat unless you draw canabbis in under large lamps in your attic...

 

[securespark]

Hey C & G student ..... you done a Ze yet?

 

[C&GStudent]

Yeh, i did a Ze, it was fine. Can't remember what it was but it was low. (i think about 0.18 ). The only reason i was concerned was the OSG stating tails should be a min of 25mm and main earth a min of 16mm?

However re-reading it also states if protected by distributers cut out, cross sectional areas must comply with requirements of the distributer. Therefore i suppose there is nothing to worry about with the tails.

If the Ze is good, the main earth can be left alone as well can it? I'm sure i read on a TN installation it must be a min of 16mm?