Upgrading meter tails and main earth for a C/U change

[The General]

I've been out to look at a C/Unit replacement, and was wondering if anyone could enlighten me as to exactly what will/won't be acceptable because (as always) the client wants the cheapest job with the minimum of disruption...

Firstly, there's nothing majorly wrong with the old board - 16th Edition with 2 non-RCD protected breakers and 5 on the RCD - but he was told by his builder that "he needed a new box" so that's what he wants. I'm happy to oblige, and intend to put a new box with RCBO's in as he was fed up with everything tripping when one of his lights went. I did explain that it wasn't strictly necessary, but he liked the idea of it.

So, anyway, I start to look a bit further.u
Quick check of IR across the whole board doesn't pick up any problems, 10mm water & gas bonds are already in place, it's all looking good, except that the old meter tails are 16mm and the main earth is only 10mm.

The problem is that the meter box/distributor cut-out is outside, whilst the C/U is on the first floor, at the opposite side of the house. I estimate the tails are about 15 metres long! They run surface mounted on the outside of the house before popping through the wall and running clipped direct (behind some tasteful tongue&groove wall panelling) It's not impossible to replace them, but I'm not imagining it's going to be easy either, and it's going to require some destruction of the house to make it happen.


The question therefore is what options might be available (if any) to avoid upgrading the tails and main earth bond.


A mate proposed that with an upfront 100mA time-delay RCD (directly after the meter box) I could protect the tails and justify retaining 16mm rather than upgrading to 25mm. Would that be acceptable?

How about the main earth bond - any justification that could allow a 10mm cable to remain?

I'm still relatively new to this, so apologies if I'm asking a daft question, but I want to get it right.

Many thanks,

 

[trev]

There's nothing wrong with 16mm tails, why do you think they need to be replaced, same question applies to your bonding conductors. With the length of the tails you should be thinking of protection for them, what does it say in the regs about this?

 

[dave274]

Can't you just fill the existing board with rcbo's ( if you can find them to fit )

 

[trev]

Can't you just fill the existing board with rcbo's ( if you can find them to fit )

For hells sake why man! it's a 16th CU! Why do unnecessary work for a customer? Just for a few quid? No thanks I'd rather keep my integrity.

 

[dave274]

The customer has asked for rcd protection. If he's made the customer aware that the board is ok but the customer still wants it upgrading then what does he do? Refuse?

 

[Robson689]

He also said that the customer has cost at mind so the builder was probably scaremongering and the customer may need reassurance.

 

[trev]

Comment withdrawn

 

[trev]

The OP should also know the answers to the questions I put directly to him.

 

[dave274]

I agree 100% robson

 

[FAA Greenie]

What building work is being carried out? Does it affect the unprotected circuits?

What rating is the cut out? If its a 60A 16mm is fine - if it's 100A - what is the max demand?

Main earth - if you are keeping the 16mm then a 10mm is 'probably' going to be ok - you will maybe need to look at the adibiatic to be sure. Make sure there are no signs of heating/damage/corrosion etc.

If all is ok you may want to cover yourself by stating in the cert that all relevents calcs etc were carried out and the tails/earth were adequate at the time of install.

 

[power ranger]

General

Whats the earthing arrangement? If TNCS then i would have thought your assessor will be looking for 16mm earthing conductor in case of a lost neutral on the suppliers network.

Normally the DNO (WPD by me) require tails over 2 meters should be protected by a switch fuse so just install a 60amp sf and that should be fine for that length of tails taking into account volt drop. However more info on the max demand and what the new building work is going to include would help to make a final decision.

As youre not installing new tails then I'd say no need for upfront 100ma time delay rcd. If you were installing new tails and these were chased into the wall <50mm and not swa then you would need the upfront rcd protection.

Suppose it depends really on your assessor interpretation of these things and how you can back up your methods of working.

Oh and try to convince your customer that your the electrician and not the builder so you will suggest the best course of action on the electrical side of things.

 

[SPARTYKUS]

Putting meter tails on a 100mA time delayed RCD does not make it comply.

cables buried in walls as mentioned always need 30mA RCD protection....but its better than before

 

[The General]

Thanks to all for your comments/suggestions

Apologies, I thought my OP mentioned it was a PME supply, but I hadn't.

Building work was finished about 6mths ago, not entirely sure what was done though. Nothing further happening, and max load not increasing from current demand.

Trev asked why I thought the tails and earth needed upgrading? Because that was what I was taught and have been told by others electricians - if you change the board, you need to make sure that tails are 25mm and earth is 16mm to meet 17th Ed requirements. If that's not the case, then fine - lesson learnt!

 

[Engineer54]

Thanks to all for your comments/suggestions

Apologies, I thought my OP mentioned it was a PME supply, but I hadn't.

Building work was finished about 6mths ago, not entirely sure what was done though. Nothing further happening, and max load not increasing from current demand.

Trev asked why I thought the tails and earth needed upgrading? Because that was what I was taught and have been told by others electricians - if you change the board, you need to make sure that tails are 25mm and earth is 16mm to meet 17th Ed requirements. If that's not the case, then fine - lesson learnt!

You only need 25mm tails in the case of a 100A fuse being fitted at the service cut-out. Otherwise it's a pretty pointless exercise and cost!! What should be in place, is a switch fuse (not just an isolator) to protect those long tails you have. I think most get confused, because of the drawing in the OSG, that shows 25mm tails. It's then taken as gospel that all tails have to be 25mm!! ...lol!!

 

[telectrix]

exactly as eng54 says. fit a 60A SW.Fuse then 16mm tails will be fine.

 

[D Skelton]

If TNCS then i would have thought your assessor will be looking for 16mm earthing conductor in case of a lost neutral on the suppliers network.

And why would the method of supply dictate the size of the main earthing conductor???

Apologies, I thought my OP mentioned it was a PME supply, but I hadn't.

It is of no consequence what the method of supply is, your main bonding conductors are 10mm so all you need is an adiabatic to confirm that the csa of the main earthing conductor is adequate.

Because that was what I was taught and have been told by others electricians - if you change the board, you need to make sure that tails are 25mm and earth is 16mm to meet 17th Ed requirements. If that's not the case, then fine - lesson learnt!

Forgive me if I sound a bit harsh here but you would only have been taught this on a Electrical Trainee course, hence my hatred of the people who peddle these useless courses to the masses as all they do is preach myth and here say. I can assure you that no electrician would have told you that this is the case because real electricians can differenciate between the OSG and the BGB.

If I'm wrong, forgive me, but if I'm right and you're a Electrical Trainee yourself I would heavily recommend you spend the next few weeks taking your BGB to bed and spending a couple of hours a night on some bed time reading. Personally, I believe that a front to back knowledge of the BGB combined with the understanding of the type of circuits you are working on is all you need to be a good electrician.

 

[power ranger]

Putting meter tails on a 100mA time delayed RCD does not make it comply.

cables buried in walls as mentioned always need 30mA RCD protection....but its better than before

Thanks spartykus,
I never picked that one up last (well it was late on a friday night!)

Yes 30ma rcd for buried cables in plaster etc.

 

[trev]

Thanks to all for your comments/suggestions

Apologies, I thought my OP mentioned it was a PME supply, but I hadn't.

Building work was finished about 6mths ago, not entirely sure what was done though. Nothing further happening, and max load not increasing from current demand.

Trev asked why I thought the tails and earth needed upgrading? Because that was what I was taught and have been told by others electricians - if you change the board, you need to make sure that tails are 25mm and earth is 16mm to meet 17th Ed requirements. If that's not the case, then fine - lesson learnt!

Wasn't having a pop at you mate (after I reread your OP ) merely trying to get you to think about the situation as it's better for you than having the answer handed to you on a plate. See E54's post re the need for 25mm tails v 16mm ones.

 

[The General]

Thanks to both of you - that explains a lot of the misunderstanding!
Cheers for the advice.

 

[telectrix]

and just to clarify. if the existing meter tails are buried<50mm, you do not have to fit RCD to protect them. you only need to do this if you fit new tails. that's because regs. are not retrospective.