Main Earth bonding to a sub-board.

[BigSim]

Is there a reg that states the main earth bonding cable (gas,water,etc), needs to go to the MET? I've checked the regs, apart from being continuous and within 600mm of the incoming supply, I couldn't find anything specific. I've been told (by somebody with a higher pay grade), that you can run it to a sub-board. If this was the case (something which I've never done, and thought it was frowned upon), I'm assuming the requirements for the min size of the cpc supplying said DB would also have to be adequate?

 

[GEOFF S]

Is there a reg that states the main earth bonding cable (gas,water,etc), needs to go to the MET? I've checked the regs, apart from being continuous and within 600mm of the incoming supply, I couldn't find anything specific. I've been told (by somebody with a higher pay grade), that you can run it to a sub-board. If this was the case (something which I've never done, and thought it was frowned upon), I'm assuming the requirements for the min size of the cpc supplying said DB would also have to be adequate?

Hi

Effectively connected to the MET is what you need to ensure. And yes however that is achieved it needs to be adequate.

Cheers

 

[davesparks]

Yes main bonding does need to connect back to the MET, but this does not have to be a direct connection. a main bond can be connected to an EMT (earth marshalling terminal) which would then be connected back to the MET via a conductor of at least the size required for a main bond.


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[bigspark17]

You may run main equapotetial bonding to an EMT, providing the supply cpc into that EMT is large enough to be a cpc and bonding conductor. You are correct in thinking best practice would be to run to MET but as you say it's not always possible or how another firm has installed it. If you are using an swa fed EMT you may need to check that the swa armouring is of an adequate sizing in relation to a copper conductor, what is the set up in question?

 

[BigSim]

It was in a school where I had to install a new gas main bonding conductor (gas pipe had been moved). Managed to run it back to the MET at the origin (pain to install), only to be told later that I could of run it to a sub-board near by. The sub-board was only a 10 way, so I pretty sure the csa would of been undersized for this purpose. Was trying to find a reg detailing this for future reference, but couldn't find one. Like I said earlier, every installation I've worked in or installed the main earth bonding cables have always been run to a MET. I've installed earth reference bars but they have always been near the origin.

 

[bigspark17]

It was in a school where I had to install a new gas main bonding conductor (gas pipe had been moved). Managed to run it back to the MET at the origin (pain to install), only to be told later that I could of run it to a sub-board near by. The sub-board was only a 10 way, so I pretty sure the csa would of been undersized for this purpose. Was trying to find a reg detailing this for future reference, but couldn't find one. Like I said earlier, every installation I've worked in or installed the main earth bonding cables have always been run to a MET. I've installed earth reference bars but they have always been near the origin.

What was supplying the 10way db.? How long roughly was your run back to MET.? What size earth did you use.?

 

[BigSim]

Let's see if I can remember......I used 16mm earth cable as the main supply tails were 35mm (junior school), the sub-board from what I can remember was 10mm T&E (4mm cpc) with no additional earth cable, the DB was about 50 or so metres away (gas bond about 5 metres from the sub-board).

 

[Leesparkykent]

Let's see if I can remember......I used 16mm earth cable as the main supply tails were 35mm (junior school), the sub-board from what I can remember was 10mm T&E (4mm cpc) with no additional earth cable, the DB was about 50 or so metres away (gas bond about 5 metres from the sub-board).

It wouldn't of been adequate then and you have done the correct thing.

 

[bigspark17]

Did you take a continuity reading of your cable once installed using a wander lead.? Was it close to 0.05 :/

 

[BigSim]

Took an R2 reading using a wander lead for the cert, think it was around the 0.045 mark. Thinking about it, it was a 50m wander lead with a few turns left, so the cable run was probably around the 40m mark. But back to my original question, is there a specific reg for this as it may be handy to know if the same situation arises again in the future.

 

[Risteard]

the sub-board from what I can remember was 10mm T&E

The idea of T&E in a school (or indeed in any non-domestic installation) appals me.

 

[BigSim]

Know what you mean! It appears in schools, hospitals, factories....what ever happened to trunking, conduit & singles!

 

[davesparks]

Did you take a continuity reading of your cable once installed using a wander lead.? Was it close to 0.05 :/

Why 0.05?

 

[bigspark17]

Max continuity resistance for an effective bonding conductor.

 

[Risteard]

Max continuity resistance for an effective bonding conductor.

That it isn't.

0.05 Ohms is merely to demonstrate a negligible impedance between the connection between the main protective bonding conductor and the extraneous conductive part, i.e. you would measure between the pipework itself and the adjacent BS951 clamp to ensure that it is satisfactorily connected.

The actual resistance measured with a low-reading ohmmeter between the Main Earthing Terminal and the end of the main protective bonding conductor will be a product of its length and cross-sectional area. There is no specified maximum ohmic value for this.