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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. 

 

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    There is nothing inherently wrong here, but it might be considered slightly unusual. Can you ask for the Electrical Installation Certificate and send me a copy? Type C breakers are used where there may be a significant inrush current, typically electronic lamps and appliances. The bonding is curious, as you have plastic service pipes, and the 6mm cable is undersized if they were metal. It is not wrong to bond the plumbing, and 6mm is satisfactory as supplementary bonding, but it is not a requirement. What RCD protection is there, or are the breakers RCBOs?

    Kind regards

    David CEng etc.

    email davezawadi(at)yahoo.co.uk

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    Type C RCBOs are in my view a good idea on lighting as the inrush current of LED lamps is liable to trip B types.

    Likewise for 20 amp power circuits, C type preferable if circumstances permit.  B type is liable to trip on the inrush current of say a portable welder or a garden waste shredder. 

    A  32 amp B curve device should withstand almost anything that is legitimately fitted with a 13 amp plug.

  • 0

    Thanks David. 
     

    I am an electrician, so raised this flag with the vendor (a small house builder) who just said “it complies“. I think even asking the question upset them slightly! 

    I have asked for the certs and will share the details once I have them.

    There are RCBOs on each and every circuit, with a 100A cut-out feeding the DB. 
     

    There is also an EV charger, supplied with an earth stake, which looks very well installed.

    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. There is a lot of LED lighting, so perhaps the use of a type C here would make sense to overcome in-rush. 

    What doesn’t add up is why 2 of the 20A socket circuits use type B and the others type C. 
     

    Thanks. 

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    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.

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    Makes sense! 

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    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? 

  • 0

    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?  

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    What brand of consumer unit is it? If something like the old Square D Kq or Qo range, then type C's may have been the only type whoever added any extra circuits after the initial install could get hold of.

     

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    An I.E.T. Guidance Note 8 used to mention Rcp. This relates to identifying extraneous-conductive-parts. It represents the measured resistance of a conductive part to the main earthing terminal of an installation. A plastic pipe may not be electrically conductive but its liquid contents may be conductive. This could be the case with a central heating system running to metal radiators.

    A reference to B.S. published document PD 6519 and IEC 60479 is made.

    If the measured reading of a conductive part is more than 22kOhms then it is not considered to be an extraneous-conductive-part so does not need bonding.

     

    Certain special locations such as mentioned in reg.701.415.2 require supplementary protective equipotential bonding. But the need to run the cables back to the main earth terminal is not required.

     

    Z.

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

    So, how can the water affect the result?

     

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