Main incoming earth bonding

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Hi,
I recently had a contractor install earth bonding for my water and gas mains.

I understand that 10mm should be used, except for the main incoming earth, which should be 16mm.

The contractor used 10mm for the main incoming earth, explaining that there was already a 6mm cable in place.

Is that correct practice?

I have attached a photo which shows the cabling connect to the main incoming earth.

Does the cabling look correct?

Any help would be much appreciated.

 
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Hi, yes i would say its 10mm, but what are the older green earths connected to ?

Regards,

DS
 
Do you mean the cable coming off the black supply cable, how has the new cable been connected to it
 
The greens will be to the CU.

10.0mm² is fine on a TN-S supply.

Main earth is attached to the sheath with an ancient BS951 clamp. Standard practice for years.
 
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I can't see 'the 6 mm that was already in place'.

Bit naughty using 10 mm for the main earth, he should have 16 mm.
 
Yes, i thought as much. I would have replaced them for the sake of an extra meter of cable.

Regards,

DS
 
I can't see 'the 6 mm that was already in place'.

Bit naughty using 10 mm for the main earth, he should have 16 mm.

Thanks to everyone for their responses so far.

I have attached a close up of the main incoming earth connection, in case it might show the 6mm cable (not sure if I am looking in the right place).

I was told by another contractor that, unless the main incoming earth cable is of greater capacity (and therefore the path of least resistance), the bonding will not be as effective as it should be.

Is that true?

Do the current regulations require that the incoming earth cable is 16mm (or at least a greater capacity than the other cables)?

 
I just looked back at his email and what he says is that the bonding should be 10mm, and that

"I also added a 10mm cable to the existing supply main earth cable which is 6mm"

Is that OK?

His email reads as if this cable was an extra, rather than an integral part of adding mains earth bonding?

Is the incoming earth cable an integral part of the bonding and has he done it to regs?

Thanks again for the advice so far.
 
In some (most) DNOs that sort of clamp should not be used on a lead sheathed cable owing to the risk of damage and it is designed for copper pipes.

The responsibility of making that earth connection rests with the DNO as it is their cable, not yours & not the electricians.

Is it not the case that the earth should be 50% of the live conductor, that cable is a 2c.0225 sqin. i.e. it is the equivalent of 16mm2 so the earth should (if I'm correct) be 8mm2 so a 10 on it's own is fine!

The responsibility for that first run of earth is the DNOs does not come under BS7671 so any earth sizes mentioned in that piece of advice are not relevant anyway.

So in answer to the electrician asking "Is that OK" is that from the point of view of the DNO is probably not!
Technically it isn't bad but it, as I've said, is not up to you or him to second guess the owner of the cable.
 
I was told by another contractor that, unless the main incoming earth cable is of greater capacity (and therefore the path of least resistance), the bonding will not be as effective as it should be.

Is that true?

Do the current regulations require that the incoming earth cable is 16mm (or at least a greater capacity than the other cables)?


No that's utter nonsense and the ramblings of someone who doesn't have the first clue about earthing, bonding, how they work or what they do.

As long as the main earth is of sufficient size to provide a low enough Ze to blow the main fuse in the event of a fault, and not get too hot in the process then it's fine. 10mm² will be fine on a TN-S supply.

It is only when you have a TN-C-S supply you are required to have at least 16.0mm² for the main earth.
 
whs.gif
 
Don't quote me; but i believe for TN-S/TT supplies you can use 6mm² as the main protective bonding conductor providing the neutral tails are not over 10mm².

For TN-C-S (PME) supplies, one should refer to table 54.8 of BS7671 for guidance on the minimum size for the main protective earthing conductors.
 
Don't quote me; but i believe for TN-S/TT supplies you can use 6mm² as the main protective bonding conductor providing the neutral tails are not over 10mm².
Nothing to do with the tails.
The main bonding for TN-S is half (with a minimum of 6mm²) the required earthing conductor which is calculated by the (our) adiabatic equation.

For TN-C-S (PME) supplies, one should refer to table 54.8 of BS7671 for guidance on the minimum size for the main protective earthing conductors.
Yes but the minimum is 10mm² .
 
Does anyone put a warning notice next to an earth block these days or am I reading the reg wrong
 
Nothing to do with the tails.

The main bonding for TN-S is half (with a minimum of 6mm²) the required earthing conductor which is calculated by the (our) adiabatic equation.

My mistake :( :oops:

I made the STUPID assumption that the size of the line/neutral tails would determine the maximum capacity of the incoming fuse and thus the maximum prospective fault current before the fuse would blow, and that the main earthing conductor had to handle half of this prospective fault current.

Don't know where i got that idea from?

Adiabatic equation as in this: http://niceic.com/en/account/media/17thpg15.pdf
 

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