Should I change a 2.5mm Main bonding conductor?

[riveralt]

I am working out the costs for a fitting a new gas boiler (no new circuit involved) and was checking out the sizing of the main protective bonding conductors. Its a TNCS system with the incoming tails 16mm and what looks like a similar size earthing cable.

The gas bonding conductor is 10mm but the water appears to be approx 2.5mm (or the imperial equivalent).

The main earthing cable does not go to the Main earthing terminal, there isn't one, but goes directly to the earthing strip in the Consumer Unit where it is joined by the Gas and water bonding conductor.

I've the checked the red book and Part P of the building regs but its not clear whether or not I am obliged to bring the water bonding up to 17th edition speck or simply record the deviation on the minor works electrical certificate?

It is also not clear whether I am obliged to provide an MET?

Any help would be appreciated.

 

[electronicsuk]

If there are no changes to the fixed wiring involved (and I wouldn't count a flex from an FCU to the boiler as fixed wiring, personally) then you're not obliged to update the PEBs. However, if the route is short and relatively easy to run, you might as well install a new 10mm earth while you have the chance.

There is no requirement for an earth block external to the CU. Running the bonds back to the earth bar at the consumer unit is standard practice and absolutely fine.

 

[Spark123]

Check out regulation 131.8 in the red book.

 

[riveralt]

Thanks for that quick response.


The problem is that 131.8 appears to be contradicted by Part P in so much that Part P states,

'Regulation 4(2) also means that, when extending or altering an installation, only the new work must meet the current requirements and there is no obligation to upgrade the existing installation unless the new work would adversely affect the safety of the existing installation, or the state of the existing installation was such that the new work could not be operated safely.....'

Unfortunately, as is usual, the location of the incoming water pipe and the CU is such that at least 10metres of cable would be required and a lot of holes drilled in that nice new shiney kitchen.

So is 2.5mm bonding conductor on the water inlet adequate?

 

[Spark123]

I take it you are talking of Approved Document P which is a guidance document?

In my opinion a 2.5mm MPB will not comply if you are working to BS7671 and are carrying out any additions or alterations.

 

[electronicsuk]

Thanks for that quick response.


The problem is that 131.8 appears to be contradicted by Part P in so much that Part P states,

'Regulation 4(2) also means that, when extending or altering an installation, only the new work must meet the current requirements and there is no obligation to upgrade the existing installation unless the new work would adversely affect the safety of the existing installation, or the state of the existing installation was such that the new work could not be operated safely.....'

And there is the crux. If doing work on the fixed installation then you have no choice but to bring the PEBs up to scratch. It then comes down to your interpretation of the regs and 131.8 as to what constitutes a part of the installation.

If you're just swapping the boiler out like for like, removing the existing flex from the old boiler and wiring into the terminals on the new one, it might be tempting to try and say that you are not altering a part of the fixed installation. After all, it's not all that different to buying a new appliance and plugging it in. However, having re-read 131.8, I would probably go back on my previous statement, as I can't see how the above thinking would provide justification for not updating the PEBs.

 

[Taylortwocities]

542.3

 

[riveralt]

10mm cable it is then - many thanks