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6mm bonding - new build and Type C RCBOs

vantech

Hi, 

I am in the process of buying a new house (new build), and of course checked out the electrical supply. 
 

The gas and water both have what looks like a 6mm bond on a supply clearly marked PME. I would expect that the gas and water are both plastic to the point of entry. 

I have also noticed that the consumer unit has a mixture of type B and type C MCBs. 
 

Kitchen ring = 32A C 

Down sockets 1 = 20A B 

Down sockets 2 = 20A C 

First sockets = 20A B

Down lights = C10 

Up lights = C10 

All RCBOs. 

I know there is nothing inherently wrong with type C in domestic, as long as the ZS is within limits, but I haven’t seen this done before.

Any suggestions or feedback? 

Obviously I haven’t been able to strip the system back and see how it’s been installed, quality etc, but the water/gas bond has thrown me a bit. The vendor said it’s been signed off by a contractor. The install does look very tidy. 
 

Thank you. 

 

  • 0

    So, how can the water affect the result?

    Clean water is a half decent insulator of course, but the water running around most central heating systems is far from clean - having either picked up a decent amount of metal oxide particles or is dosed with inhibitor (or both). Even with that a metre or two of plastic pipe will still likely provide at least many tens of k Ohms of resistance - but a much shorter length - say a push-fit aleration on otherwise copper pipework might not be enough to provide the requried 20-odd k Ohms.

        - Andy.

  • 0

    geoffsd: 
     

     

    An extraneous-conductive-part should be measured when disconnected from the installation.

    So, how can the water affect the result?

     

    A metal pipe, such as an oil or gas pipe entering a building can be insulating when the wall is dry. But after rain it could become conductive and become an extraneous-conductive-part due to salts in the brickwork or mortar.. Andy has covered the central heating issue.

     

    Z.

  • 0

    I thought you meant the water in the pipes.

  • 0

    vantech: 
     

    gkenyon: 
     

    In terms of main bonding … if PME conditions apply, any and all main bonding should comply with Table 54.8 (min 10 sq mm).

    If it's not main bonding, and not a cpc, why is it connected to MET? 

    The gas and water are bonded back to the MET in the DNOs cupboard. I had a quick look and it was a 6mm.

    If the water and gas come in via plastic pipes (assuming they could be as this is a new build), main equipotential bonding can be omitted, so it could be the case this is just supplementary bonding for everything inside of the dwelling?  

    If it's not “main protective bonding”, what is it? If “main protective bonding” is not provided, have you checked whether it's permissible to omit supplementary local equipotential bonding in rooms containing a bath or shower (or is it provided anyway)?

  • 0

    gkenyon: 
     

    vantech: 
     

    What has thrown me somewhat is the use of 20A radials for socket circuits rather than rings, again fully acceptable but unusual in the UK. 

    I know a number of people in the industry who absolutely hate ring final circuits. Some of those people have seen all too often the problems caused “when rings go wrong”.

    (Perhaps “When Rings Go Wrong” is a good title for a series of articles or videos in the industry media somewhere?)

    To be honest, the only reason we need a fused plug that's so clunky, is the fact we insist on continuing to use the ring final circuit. Aside from using extra ways, there are huge benefits in running 2 no. B20 radials over 1 no. B32 ring final circuit for general purpose socket-outlets.

    And if the incoming supply is mentioned maybe call it tale of the tails!!

  • 0

    gkenyon: 
     

    vantech: 
     

    gkenyon: 
     

    In terms of main bonding … if PME conditions apply, any and all main bonding should comply with Table 54.8 (min 10 sq mm).

    If it's not main bonding, and not a cpc, why is it connected to MET? 

    The gas and water are bonded back to the MET in the DNOs cupboard. I had a quick look and it was a 6mm.

    If the water and gas come in via plastic pipes (assuming they could be as this is a new build), main equipotential bonding can be omitted, so it could be the case this is just supplementary bonding for everything inside of the dwelling?  

    If it's not “main protective bonding”, what is it? If “main protective bonding” is not provided, have you checked whether it's permissible to omit supplementary local equipotential bonding in rooms containing a bath or shower (or is it provided anyway)?

    There are RCBOs for each and every circuit. 

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