"Distributed" MET, all wrong, but a pain to fix

[1Justin]

I’ve found a couple of installations (TNCS) where the location of the MET is not clear.
In both cases, gas/water bonding has been run back to either/or the DNO break-out “MET” or to the Consumer Unit. "There can only be one MET"

The first one I sorted out and created just one MET.

The new one, I find gas bonding to service head break-out location, and water bonding to CU. In this case it will be hard to extend water bond back to the service head break-out located MET, or the gas bond up to CU - which would make the CU the MET.
This is effectively making the 16mm Earthing Conductor a combined earthing Conductor and bonding for the water, - but not for the gas.

So I am thinking that if a “open supply neutral” current happened to pass up water pipe, it could create a voltage down the length of the earthing conductor, which would then put gas bond at a different potential to water bond.

So “There is only one MET”..Of course. But can I /should I leave this one since it’s tricky to get right? - Or attempt to get (say) water bonding back to break-out MET (albeit would need a crimped joint in the bond, and make a mess of the Décor).

 

[Richard Burns]

So the CU has an earth marshaling terminal for the water bond and as you say the conductor from the CU to the service head is a bonding and earthing conductor, however, unless there is a very long run, then the PD between the gas and water bond would be much less than 1V so should not seriously affect the touch voltages.
I would leave it as it is so long as your resistance measurements are similar.

 

[1Justin]

Hi Richard.
The distance between the two is not long, probably 3.5 metres of cable.

Might you make a note on the EIC? if left as is?

I'll be doing a CU upgrade and have added a cost in to fix it. I may knock that one out if I see it's all sensible enough (thanks).

Rgds

 

[Richard Burns]

Bonding conductors are not intended to carry fault current in the case of a normal fault to earth so if you assume a current flowing of, for convenience, 1 A then the volt drop across that 3.5m would be approx 0.01 V hardly significant compared to the 50 V touch voltage.
You could also consider it as the main water bond has been jointed at the CU, OK joints need to be secure from persons inadvertently disconnecting a bonding conductor but if someone is messing around in the CU they should know what they are doing!

Personally I would not note it, however it may be of use to later sparks who are thinking: where is the water bond? so probably cover yourself and note the water bonding runs through the CU via the earthing conductor.

If it is easy to change then yes you can do so to make things clear and more compliant, but it is a very minor point to me.

 

[Engineer54]

Hi Richard.
The distance between the two is not long, probably 3.5 metres of cable.

Might you make a note on the EIC? if left as is?

I'll be doing a CU upgrade and have added a cost in to fix it. I may knock that one out if I see it's all sensible enough (thanks).

Rgds

Nothing wrong in making a through crimped connection on a main bonding conductor to extend it's length....

 

[Welchyboy1]

I’ve found a couple of installations (TNCS) where the location of the MET is not clear.
In both cases, gas/water bonding has been run back to either/or the DNO break-out “MET” or to the Consumer Unit. "There can only be one MET"

The first one I sorted out and created just one MET.

The new one, I find gas bonding to service head break-out location, and water bonding to CU. In this case it will be hard to extend water bond back to the service head break-out located MET, or the gas bond up to CU - which would make the CU the MET.
This is effectively making the 16mm Earthing Conductor a combined earthing Conductor and bonding for the water, - but not for the gas.

So I am thinking that if a “open supply neutral” current happened to pass up water pipe, it could create a voltage down the length of the earthing conductor, which would then put gas bond at a different potential to water bond.

So “There is only one MET”..Of course. But can I /should I leave this one since it’s tricky to get right? - Or attempt to get (say) water bonding back to break-out MET (albeit would need a crimped joint in the bond, and make a mess of the Décor).

I think you are getting a little carried away here

You can combine a main/sub-main earthing conductor with a main bonding conductor

on the basic domestic set up your talking about i wouldnt give it a second thought, as long as the earthing conductor is big enough to account for the biggest CSA required then whats the problem

as said above its no problem to have an MET, then marshalling earthing/bonding connection points further down the installation as long as the CSA of the conductors are sufficient

How do you think its done on larger installations? i installed the main bonding in a school this year, 255m of 50mm cable,1x MET, 6x marshalling points and about 16 different points of connection

think of the nightmare that would of been to run them all back to a single point!

and i cant think of a reason why you would have to note anything on the cert

 

[jamie-spark]

I’ve found a couple of installations (TNCS) where the location of the MET is not clear.
In both cases, gas/water bonding has been run back to either/or the DNO break-out “MET” or to the Consumer Unit. "There can only be one MET"

The first one I sorted out and created just one MET.

The new one, I find gas bonding to service head break-out location, and water bonding to CU. In this case it will be hard to extend water bond back to the service head break-out located MET, or the gas bond up to CU - which would make the CU the MET.
This is effectively making the 16mm Earthing Conductor a combined earthing Conductor and bonding for the water, - but not for the gas.

So I am thinking that if a “open supply neutral” current happened to pass up water pipe, it could create a voltage down the length of the earthing conductor, which would then put gas bond at a different potential to water bond.

So “There is only one MET”..Of course. But can I /should I leave this one since it’s tricky to get right? - Or attempt to get (say) water bonding back to break-out MET (albeit would need a crimped joint in the bond, and make a mess of the Décor).

Not all installations have bonding conductors connect straight to the met. On large Installations covering various buildings u will usually find a EMT where a cpc supplied from the MET ( that has two functions 1 x cpc 2 x bonding conductor ) is terminated along side bonding conductors for that particular building

 

[jamie-spark]

Oops should have read welchyboys post