CU Change and Main Protective Bonding

[SJD]

I've seen various conflicting comments about upgrading main protective bonding conductors, and wonder if there is a consensus on this.

Typical scenario: Domestic CU change, PME supply, 16 or 25mm2 CSA tails, where the existing earthing and main protective bonding conductors do not meet current regulation - for example 10mm2 earthing conductor, 6mm2 bonding to gas and water.

Per the regs, the earthing conductor needs to be 16mm2, and typically this is fairly easy to upgrade if the DNO head is nearby, so just allow for this in the estimate and do it.

Per the regs, the main protective bonding conductors need to be 10mm2, but often this is not so easy to do. One service may enter nearby (e.g. Gas), while the other service (e.g. Water) may enter the opposite side of the house.

Guidance Note 8 (section 5.2.4) mentions for undersize main protective bonding, the designer can consider the adequacy and elect to retain the existing conductors, but then goes on to say for PME supplies a careful assessment is needed, and really the CSA should meet BS7671 table 54.8 i.e. 10mm2, and only don't upgrade if "comfortable with their adequacy", before finally adding that "consideration should be given to DNO requirements". Clear?

The ESC guide is just as helpful - "a 6mm2 bonding conductor could be deemed adequate ... if the conductors have been in place for a considerable time and show no signs of thermal damage".

My understanding is the 10mm2 bonding is needed to cope with a N fault in a PME supply, since the N current might travel back up the (16mm2) earthing conductor to the MET, and then via the (10mm2) main protective bonding conductors to the incoming services, and to earth via some other means. Please correct me if there is something more to this. How many incoming services there are to be bonded (and that they may be interconnected via pipework) could have an effect, though that is not considered in the regulations, as far as I am aware.

So assuming it is difficult to upgrade the main protective bonding to one service, and the customer would prefer it is not done, would you:

(a) insist they both have to be changed to 10mm2 anyway
(b) consider leaving the difficult one at 6mm2, but upgrade the other to 10mm2
(c) leave both main protective bonding cables at 6mm2
(d) something else - if so, what?

 

[Geoffsd]

Except for TNC-S, Adiabatic for earthing conductor and size bonding conductor appropriately.

TNC-S Table 54.8

 

[Engineer54]

You can where possible use a single 10mm bonding conductor to connect both incoming water and gas services!!

 

[SJD]

Except for TNC-S, Adiabatic for earthing conductor and size bonding conductor appropriately.

TNC-S Table 54.8

This is meant to be a TN-C-S example - I assume you are saying you always follow table 54.8, i.e. insist on upgrading to 10mm2 in the example.

 

[SJD]

You can where possible use a single 10mm bonding conductor to connect both incoming water and gas services!!

Yes, but not sure how that really helps - if water enters at the back of the house, gas in the meter cupboard at the front of the house (often see this round here).

 

[MDJ]

Love Geoffs post, it says it all IMO

 

[kingeri]

At the end of the day, it comes down to safety, so the customer should be willing to pay whatever it costs to get the bonding up to scratch. I know from experience that it's not always easy to convince them that a seemingly (to them) pointless wire needs to be 40% (or whatever) bigger, I've even tried talking them through my calcs and explaining the adiabatic to them, and why their bonding needs to be adequate. It's a hard call to make, if you've done the calcs and it needs upgrading, then do you note it down on the certs that customer refused to have the work done, but do the rest of the work anyway, or do you walk away from the whole thing. Personally, I need to make a living, so as long as I'm not making anything worse, and my paperwork is in order, then I do my best and crack on. Sorry for waffling on.

 

[Deleted member 26818]

Regulation 132.14.2: No addition or alteration, temporary or permanent, shall be made to an existing installation, unless it has been ascertained that the rating and the condition of any existing equipment, including that of the distributor, will be adequate for the altered circumstances.
Further more, the earthing and bonding arrangements, if necessary for the protective measure applied for the safety of the addition or alteration, shall be adequate.

There is as yet no requirement in BS7671 for the use of a 16mm² earthing conductor with 25mm² circuit conductors in every case (may become a requirement in the future with an amendment or new edition).
In many instances a 10mm² earthing conductor will be sufficient (if determined by use of the adiabatic equation).
It is something that has to be determined by the person designing or assessing the installation.

The selection of 10mm² as the minimum CSA for earthing and bonding conductors for use with TN-C-S has not been determined in relation to a network neutral fault (if that were the case, it would be woefully inadequate).
It has been determined to allow the earthing and bonding conductors to withstand both diverted neutral currents and installation earth faults.

Determining whether earthing and bonding is necessary, will depend on the the addition or alteration being made.
For instance SELV lighting would not require either earthing or bonding.
Technically it would comply with the Regulations if you were to install such lighting in an installation which had no main earth conductor.
It could also be argued that a main earthing conductor is not required if the protective measure is by means of an RCD.

Industry advice on determining whether earthing and bonding arrangements are adequate, basically consists on checking the conductors for signs of heat damage.
If there are no signs of heat damage, and the addition or alteration will not entail new loads such as in the case of a replacement CU, then the conductors are considered as adequate.
However if the addition or alteration would entail new/ higher loads, for instance a new circuit or higher rated shower, then the conductors would not be considered adequate.

 

[diditrain]

You can`t force them to do something if they don`t want to, you can only give your professional opinion. Many will tell you to simply walk away, others will say do the works and then leave a duplicate covering letter signed by the customer, stating that they would not allow you to do the proposed works. Only you can decide.
Think about who you may have to justify yourself to. The next Electrician who comes along and refers to you as a man who wears stirrups, or a man/woman with a white wooly think on their head.

 

[Heisenberg]

others will say do the works and then leave a duplicate covering letter signed by the customer, stating that they would not allow you to do the proposed works.

It is my understanding that no letter from the customer will absolve you of your responsibilities regarding BS7671. You are the competent person not them, they are in no position to make any form of judgement.

Of course everyone still needs to pay the bills and it is down to the individual to decide, I just wanted to point out that if you believe that having some sort of disclaimer signed by the customer will help when everything goes south, you can as they say.. 'tell that to the judge' :wink_smile:

 

[SJD]

Thanks spinlondon for a really helpful reply.

I have a query regarding your comment about the selection of 10mm² CSA:

The selection of 10mm² as the minimum CSA for earthing and bonding conductors for use with TN-C-S has not been determined in relation to a network neutral fault (if that were the case, it would be woefully inadequate). It has been determined to allow the earthing and bonding conductors to withstand both diverted neutral currents and installation earth faults.

My GN8, section 5.2.4, says "the minimum CSAs given in table 54.8 of BS7671 were intended to obviate the likelihood of protective bonding conductors overheating due to diverted load currents in the event of an open PEN conductor in the PME supply". I was also thinking that 10mm² will be somewhat inadequate for this purpose, possibly GN8 is wrong, or I misunderstand something?

Is there written down somewhere how the values in table 54.8 were arrived at, and what fault conditions exactly they are to protect against (without damage)?

 

[kingeri]

I think the way the regs will eventually end up is with a standard size for all installations.....probably 16mm2, so no room for interpretation!

 

[Deleted member 26818]

Thanks spinlondon for a really helpful reply.

I have a query regarding your comment about the selection of 10mm² CSA:



My GN8, section 5.2.4, says "the minimum CSAs given in table 54.8 of BS7671 were intended to obviate the likelihood of protective bonding conductors overheating due to diverted load currents in the event of an open PEN conductor in the PME supply". I was also thinking that 10mm² will be somewhat inadequate for this purpose, possibly GN8 is wrong, or I misunderstand something?

Is there written down somewhere how the values in table 54.8 were arrived at, and what fault conditions exactly they are to protect against (without damage)?

You have to take on board, that the GN8 is written based on the author's opinion.
Unfortunately no, not as far as I'm aware.
My comment is based on my opinion.
Whilst I have much respect for Geoff, occasionally my opinion will differ from his.
However the consensus of opinion throughout the industry, is that we should not take account of a network neutral failure, as it is such a rarity and there are supposed to be such safeguards present as to make it not worth consideration.

 

[daverogers]

As far as I take it as long as the bonding conductor is 6mm and there is no signs of thermal damage than you may keep the bonding conductor as is (domestic). But you must list this as a departure from BS7671. Saying that I only leave this if the installation of a new one would not be practical.

When choosing or assessing the earthing conductor adiabatic is the only way to go IMO.

Cheers

 

[oldtimer]

Clarification is the problem here 6mm is fine by but go into say a 20 year old flat where there is a PME supply and 6mm to gas and water only problem is the main gas water pipes are plastic but are copper as they pass through the flats then to make matters worse because of the chaos of the 15th edition and earthing, contractors just ran 6mm earth cables to the gas water so in this block of flats so when you test the pipework you have to earth bond it as you have in a sense a borrowed earth.