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Topic Title: 6mm Earth & Bonding
Topic Summary: Conflicting advice is being given on the required cable size for earth and bonding on my domestic installation.
Created On: 10 October 2018 07:21 PM
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 10 October 2018 07:21 PM
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DPOC

Posts: 7
Joined: 28 September 2018

Hello all, I hope that you can give me some advice on the regulations regarding upgrading my consumer unit to the latest standard. Sorry but with the background it becomes a long question.

I am not an electrician and therefore do not have direct access to BS 7671. I am being given conflicting advice by two electricians that I have consulted. I would therefore like to get a definitive answer.

My house is a 1970's build and has a Wylex CU with wired fuses. It has a 60Amp main fuse and 16mm CU tails. The earth and bonding are in 6mm cables from the mains supply. This system has worked perfectly satisfactorily to date.

I am now thinking of upgrading the CU to the latest standard to benefit from the added protection provided by RCB's / RCBO's. One electrician says that I must increase cable sizes to 25mm tails and 10mm earthing/bonding. The other says this is not necessary on an upgrade.

I do not want to mess up the house internally or externally so I am hoping that the second electrician is correct. I will do nothing if I have to change the tails and earth/bonding cables to fit a new CU.

Can someone quote the specific BS regulations that force a change of cable sizes or alternatively the regulation that allows use of the existing cables for an upgrade.
 10 October 2018 08:23 PM
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geoffsd

Posts: 2181
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Originally posted by: DPOC
This system has worked perfectly satisfactorily to date.

Maybe nothing has gone wrong - yet.

One electrician says that I must increase cable sizes to 25mm tails and 10mm earthing/bonding. The other says this is not necessary on an upgrade.

Tails are sized the same as any other cable - able to handle the current (main fuse rating) taking into account the installation method.
Bonding should be 10sq.mm. now because the supply company do not maintain their cables satisfactorily so we must treat everything as PME.
If you definitely have TN-S still or TT(earth rod) then perhaps not.

I do not want to mess up the house internally or externally so I am hoping that the second electrician is correct. I will do nothing if I have to change the tails and earth/bonding cables to fit a new CU.

Ok.

Can someone quote the specific BS regulations that force a change of cable sizes or alternatively the regulation that allows use of the existing cables for an upgrade.

For the bonding 544.1.1
 10 October 2018 08:33 PM
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sparkiemike

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Firstly it depends on the earthing arrangement. Do you know what it is.

Secondly if the electrician is using the tables in regulation 543.1.4 then your earthing conductor will need to be 16mm2. However if the electrician is calculating using regulation 543.1.3 then 6mm2 could be adequate, they would need to do the calculation.

Earthing arrangements could mean a bonding (and earthing) conductor of 10mm2

If your main fuse is indeed 60A then your 16mm tails will be OK, provided they show no signs of overheating.

Any new work will need to comply with current regulations. If any tails, earthing and bonding conductors are undersized they will need to be replaced. It is quite common to replace the tails and earthing conductor on CU upgrade. It makes a for neater install and at a glance you know it complies.

Edited: 10 October 2018 at 10:47 PM by sparkiemike
 10 October 2018 09:48 PM
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sparkingchip

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Originally posted by: DPOC

Can someone quote the specific BS regulations that force a change of cable sizes or alternatively the regulation that allows use of the existing cables for an upgrade.


No.
 10 October 2018 10:27 PM
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IronFreely

Posts: 359
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Much depends on the Earthing system you have, if it's PME you should have the Earthing and bonding upgraded regardless of your desire to "not mess up the house" I dare say it does all work but everything is fine until it's not and you'll be glad you had the upgrade the day it's not, Earthing and Bonding usually only relevant under fault conditions and don't provide any day to day functionality so you'd never know it was undersized until the day it goes wrong, and the results could be quite alarming if it was undersized and it did go wrong....
If you have TT Earthing then 6mm may be sufficient, there is an equation but as you've already said you're not an electrician unless you're a mathematician with access to the crucial data in BS7671 it's likely to be beyond you... Saying that there is a school of thought which says that if you do follow the rule of thumb which is 16mm Main Earthing and 10mm bonding then it's definitely compliant and future proof and easy for any visiting engineer to confirm this with one look.
Technology has moved on a lot since rewirable fuses and it's definitely best to have RCD/RCBO protection and in terms of peace of mind I know I'd rather have the work done even if it is a bit disruptive to get the new bonding in. Personally I'll never live in a house that literally relies on combustion to disconnect the supply in the event of a fault. No one can make you do it.

If you really do have a 60A supply then 16mm tails are sufficient provided there is no sign of degradation or over heating and so long as the DNO/Suppliers don't have a specific requirement to have 25mm... There may be practical reasons for replacing them in a CU upgrade, for example they may be to short to work with as new consumer units are a bit bigger than those old rewirable ones, personally if I have to replace them I usually just use 25mm because I always have plenty in the van and it's future proof. There is no specific regulation that I can think of that specifies that old tails can or can't be used beyond them being correctly sized and in good condition.

My advice is ask the two Sparks if they've confirmed the required size of Earthing and Bonding by calculation and for them to confirm your style of Earthing, once we know that for sure we can give you much better advice on how to continue...
 10 October 2018 10:45 PM
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sparkingchip

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Are the incoming water and gas pipes plastic?

If you asked me for calculations with a quote for a consumer unit change I'd tell you that is a chargeable service.

Andy B.
 10 October 2018 11:51 PM
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sparkingchip

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Originally posted by: DPOC

I do not want to mess up the house internally or externally so I am hoping that the second electrician is correct. I will do nothing if I have to change the tails and earth/bonding cables to fit a new CU


I thought you have a background in electrical engineering and electronics, so doing nothing seems a somewhat surprising outcome to resolving any issues.

Without a site survey it's impossible to accurately assess the installation in your home and recommend the upgrading it sounds like it urgently requires. The regulations don't actually tell you how to wire a house, only what should be achieved.

You could have a £60 consumer unit installed and it may be just what is required or it could be a big mistake.

There are lots of questions to ask, like what washing machine do you have, as that will affect your choice of consumer unit and the protective devices within in. My last job today was to replace RCDs at a house, because the people have not been able to use their washing machine since they bought the house and moved in four weeks ago, the lady has been taking her washing to the school she works at and doing it in the school washing machine.

You have two quotes that differ, ask for another one off someone else and it could be different again, particularly if I did it.

I will say that I'd probably leave the tails, but not without checking what loads there are in the house, I might possibly leave the main protective conductors as well, but again not without checking them.

In the past I have given people quotes and said to them that they will certainly be able to get cheaper quotes, but not to accept them if they aren't the same specification as mine, as they will be silly to do so.

Often they don't listen, but that's their problem not mine, putting price before safety and security of supply. It does wind me up though when people are penny pinching when organising electrical work for elderly parents who are dependent on stair lifts and the like, but won't pay to have the consumer unit made accessible.

Pay to get the job done right and maybe it will last another forty years, like the last one did.
 11 October 2018 12:22 AM
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mapj1

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As others have said without looking at it, a resounding 'maybe'. The 16mm tails, if in good shape mechanically will be fine on a 60A fuse. (who checked it really is 60A and not 100A though, or is it just written on in crayon?)
The main earth at 6mm well maybe enough, note though you need no earth at all, until there is a fault, so the fact it is OK may mean it is really 'untested' rather than 'working well'.
this article may help explain the various types of earthing in the UK, one of which I hope you recognise as what you have.

-------------------------
regards Mike
 11 October 2018 08:41 PM
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UKPN

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Your second electrician is correct in saying that an upgrade is not necessary. 16mmsq meter tails meets the 60amp main fuse and 6mmsq meets the PME approvals of the day. The 6mmsq minimum applies only to electrical connections required by the approval. It does not apply to bonding leads or other earth continuity conductors otherwise required by the IEE requirements of the era. Any such leads or conductors should be to the size specified by those "regulations"

Regards, UKPN.
 11 October 2018 08:50 PM
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IronFreely

Posts: 359
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Originally posted by: UKPN

Your second electrician is correct in saying that an upgrade is not necessary. 16mmsq meter tails meets the 60amp main fuse and 6mmsq meets the PME approvals of the day. The 6mmsq minimum applies only to electrical connections required by the approval. It does not apply to bonding leads or other earth continuity conductors otherwise required by the IEE requirements of the era. Any such leads or conductors should be to the size specified by those "regulations".


Try telling that to my assessor, it might be ok for you network guys but when I change a consumer unit I must comply with current BS7671 requirements for Earthing and bonding or I won't continue to be affiliated with a Competant Person Scheme. Also I don't know about your local area but if I call my DNO they say that all their PME installations require 16/10 respectively.
 11 October 2018 09:45 PM
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geoffsd

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Originally posted by: UKPN

The 6mmsq minimum applies only to electrical connections required by the approval.

What does that mean? What is the approval?
 11 October 2018 11:07 PM
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Alcomax

Posts: 355
Joined: 12 November 2009

Originally posted by: geoffsd

Originally posted by: UKPN






The 6mmsq minimum applies only to electrical connections required by the approval.


What does that mean? What is the approval?


Never heard of "approval" but familiar with " pre 1988 pme". Thing is, the network is not frozen in time and , though approved pre 1988, is 6mm really still valid? Surely only if you can guarantee local network is the same as pre 1988. Anyhow pre DNO / supplier split about that time, it was common for the local board to insist any 6mm main earth's or bonds be upgraded to new PME sizes if they were called to do any reconnection on older supplies and even the most minimal of works.
 14 October 2018 06:53 PM
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DPOC

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Thank you all for your responses and sorry for this delayed reply - busy with other activities and I have had difficulties in accessing this website and posting to this forum - probably down to me! Sorry also that this post is a bit long.

Your various responses seem to be a 50:50 situation with no clear cut answer to my original question. This is a similar situation to the two electricians that I consulted. Nevertheless I did get some useful information from your responses and was able to decide the way ahead for me.
Note: I am satisfied that both of my electricians were competent guys but, like the respondents, they just held different views on this topic - see later.

On the various questions you posed I can answer some but not all:
- Thanks for the earthing article but I still do not know the earthing arrangement. I am confident it is not TT because the earth goes back to the mains source cable and is hidden under a DNO sealed unit - it could be TN-S or TN-C-S (PME). There is a small electrical sub-station about 100 meters away probably supplying my house and many others (100's) on the estate. Is this a clue? I checked with the DNO and was surprised to learn they do not keep any records of the earthing arrangements. They said it is down to the local electrician to determine. It does puzzle me as to how this can be done, but perhaps there are some very sophisticated tools around these days.
- I do know that it is a 60 A main fuse - confirmed when an isolator was fitted. I suspect that our electricity usage has gone down over recent years (kids gone, LED's going in everywhere, gas CH etc) so 60A should be OK for now.
- The existing tails, earthing and bonding look pristine - no signs of overheating.
- Both electricians planned to do some measurements and calculations as part of the CU upgrade task to confirm that all was OK.
- I was surprised at the suggestion that supply companies do not maintain their cables! I hope this is not true for my area. However, it is interesting to note that PME is the worst case for earthing.
- I agree that RCD/RCBO is a better option these days, that is why I am going over to them. Fuses are very simple, reliable and have fewer nuisance occurrences, not a hazard in themselves but they offer little other protection.
- The DNO in my area do not have any requirements for tail sizes, they say it is down to the customers electrician.
- The incoming water and gas pipes are metal and clearly bonded/labelled with 6mm cables.
- I am willing to pay "to get the job done right". The "job" in this case is to 'improve' the electrical system but not to make it fully compliant with the latest requirements if that requires messing up the house. It is a step by step approach - RCD/RCBO being an improvement on wired fuses. The two quotes I received were not so different to sway a decision. A fully compliant and aesthetically pleasing upgrade would I suspect run into thousands (concrete floors chased, new carpets, tiles, etc). This can wait until the house gets a major upgrade when all of these types of activities can be done.
- The view of assessors is a potential problem area that I had not considered. Clearly they have a role in ensuring that any work completed is up to standard but they should not be adding their personal preferences when doing assessments only compliance with regulations. I understand how it might be simpler for electricians to do work in a way to keep their assessors happy. This may not be what the customer actually wants from them.

After considering all your responses I finally concluded there is no 'requirement' to change the cables of this existing installation provided the earthing measurements and calculations are satisfactory. On that basis I will get the fuse board upgraded to an all RCBO CU system with the existing earthing and bonding cable sizes (by electrician 2, who stated he was familiar with upgrades in this area). An increase in cable sizes can wait until a future upgrade of the house overall.

On subsequently speaking to electrician 1 he commented that the job could be done using the existing cables but for many of the reasons given above and also that he did not like to put caveats on his certificates he did not want to do the job that way. That is fair enough, I just wish it was presented that way initially.

Have I made the right decision? Time will tell.

Thank you all for your help.
 14 October 2018 08:00 PM
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geoffsd

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Are the water and gas supplies next/near each other?

If they are, then you could connect both 6sq.mm. cables to the water (making a 12sq.mm. bonding conductor) and then connect the gas to the water with a 10sq.mm. cable.
 14 October 2018 08:10 PM
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chrispearson

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Originally posted by: DPOC

- Thanks for the earthing article but I still do not know the earthing arrangement. I am confident it is not TT because the earth goes back to the mains source cable and is hidden under a DNO sealed unit - it could be TN-S or TN-C-S (PME). There is a small electrical sub-station about 100 meters away probably supplying my house and many others (100's) on the estate. Is this a clue? I checked with the DNO and was surprised to learn they do not keep any records of the earthing arrangements. They said it is down to the local electrician to determine. It does puzzle me as to how this can be done, but perhaps there are some very sophisticated tools around these days.


If you are (like me) in leafy suburbia, a TT installation is unlikely. Given the date of construction in the 1970s, TN-S is possible; however, it may have been converted to TN-C-S (mine has). So whilst 6 sqmm earthing and bonding may have complied with the situation in the 1970 and the regulations in force then, it may not now. If you can upgrade the cables without digging into the fabric of the house, then I would firmly recommend 16 sq mm earthing cable and 10 sqmm main protective bonding.

I find it bizarre that DNOs do not maintain a database of earthing arrangements. Instead they seem prepared to send a man round. The nearest approach is that the DNOs must maintain up to date maps of their underground network i.a.w. R.15 of ESQCR 2002 which you may obtain for a "reasonable fee" (it used to be free of charge). The safest approach is to assume TN-C-S (or PME).
 14 October 2018 08:16 PM
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Farmboy

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Originally posted by: geoffsd

Are the water and gas supplies next/near each other?



If they are, then you could connect both 6sq.mm. cables to the water (making a 12sq.mm. bonding conductor) and then connect the gas to the water with a 10sq.mm. cable.


Does the MPB conductor have to be a continuous conductor when connecting the two ex-c-parts?
 14 October 2018 08:51 PM
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IronFreely

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From your description of of the Earthing arrangement I think that you have a TNCS (PME) Earthing arrangement, the sealed unit you mentioned sounds like the main fuse head where the TNCS bond between neutral and Earths usually made. If it is TNCS then 16mm Earthing and 10mm bonding is imperative and there is no room in the requirements of BS7671 to not upgrade to the correct size when upgrading a consumer unit.
It's a simple fact that if a new unit is installed then Earthing and bonding MUST be compliant with current BS7671 requirements, there is no middle ground here and it is irresponsible of any electrician to upgrade your consumer unit without ensuring upgrades to current requirements....
If you do have TNS the Earthing and bonding arrangements may sufficiently sized already, this can and must be calculated before upgrading a consumer unit. From your discription I think you have TNCS as with TNS you can usually see a braded wire attached to the outer lead sheath of the main incoming supply just below your sealed fuse head, it's usually wrapped up in black tape, with TNCS the earth wire tends to go directly inside that sealed fuse head. See if you can get us a picture and we will advise further.
I personally would be very dubious of any spark who would be prepaired to install a new consumer unit and not upgrade bonding to 10mm when PME (TNCS) conditions apply, I'd also be dubious if with TNS they hadn't either demonstrated they had correctly calculated the required size or suggested upgrading to the 16/10 format which is a rule of thumb that we can use that guarantees compliance for domestic properties with a supply up to 100A.
One possibility for you may be to calculate if the bonding you have is suitably sized for a TT Installation, you could install a TT rod and have the DNO remove the PME connection which could possibly allow you to be compliant with current requirements and go ahead with your consumer unit upgrade. It would be prudent to install and test the rod first to be sure it will be suitable.

I agree that in most situations RCBOs are a very good choice of protection, especially if you did have a TT rod installed.
 14 October 2018 08:51 PM
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geoffsd

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Originally posted by: Farmboy

Does the MPB conductor have to be a continuous conductor when connecting the two ex-c-parts?

No.

You may even use the water pipe as the MPB for the gas - but not the other way round.
 14 October 2018 09:15 PM
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Farmboy

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Originally posted by: geoffsd

Originally posted by: Farmboy



Does the MPB conductor have to be a continuous conductor when connecting the two ex-c-parts?


No.



You may even use the water pipe as the MPB for the gas - but not the other way round.

I ask because with a non-continuous conductor, it's conceivable the first connection in the run could be disconnected, thereby leaving the second connection disconnected from the MPB, but not if it was continuous.
 14 October 2018 09:21 PM
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perspicacious

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You may even use the water pipe as the MPB for the gas[/I

?

Regards

BOD
IET » Wiring and the regulations » 6mm Earth & Bonding

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