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Lack of visible supplementary Bonding

aligarjon

Hi Guys.   I looked a a job today with a 16th edition CU so no rcd on the bathroom lights.  There is no visible supplementary bonding. Lots of pipes boxed in, bath panel not removable.  I had a reading of pretty much zero ohms from the hot and cold below the sink to the radiator so suspect it is done somewhere.  However i suspect the lights are not done as i dropped the down lights out and the switch.   There is a class one shaver point over the sink although it doesn't have any exposed metal parts.  I am getting 0.25 ohms from the earth at the shaver to the sink pipes.  The bathroom is next to the CU, well it's outside the door so the cpc run is pretty short.  What sort of reading do i need to be seeing here before i don't need to worry about a supplementary bond.  Is 0.25 low enough.

Thanks

Gary

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  • 0

    The supplementary bonding conductor must be no smaller than the smallest cpc, so that could be 1 mm². Appendix I of OSG shows that about 14 m of  1 mm² have a resistance of 0.25 Ω. Could the bonding be that long?

    The alternative path is cpc back to the CU, from there to MET, main bonding to pipe work.

    With the CU adjacent to the bathroom, I do not think that you would be able to measure the resistance sufficiently accurately to determine whether or not there is any supplementary bonding. Either way, 0.25 Ω seems a little high.

  • 0 in reply to Chris Pearson

    I suspect it's the cpc. The CU is only about 5 to 6m away. The circuit is the bathroom and a bedroom, the bedroom is farther away. I had an r1 + r2 of .80Ω in the bedroom.  It's a first floor flat.  

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  • 0 in reply to Chris Pearson

    I suspect it's the cpc. The CU is only about 5 to 6m away. The circuit is the bathroom and a bedroom, the bedroom is farther away. I had an r1 + r2 of .80Ω in the bedroom.  It's a first floor flat.  

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  • 0 in reply to aligarjon

    0.80 Ω seems rather high for a small property, even for a 1 mm² circuit.

  • 0 in reply to Chris Pearson

    especially for dc tests - if that was a  Zs result it might be at least party in the street. Recall the 'rule of  16' i.e. 1.6mm = 16SWG = 1/16 inch  and 16 milliohms per metre per mm2 of cross- section copper single core. (cold - use 19 or 20 for hot )
    In that rule of thumb, 0.8 ohms is 50metres of 1mm core (25 m there and 25m back) or proportionally further at higher cross- section (2.5 times further in 2,5mm2, 10 times further in10mm2 etc.)

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