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Bonding maximum resistance

Ricki Carter

Apprentice electrician here and I would really like help on clearing up a confusion about bonding. I'm coming towards the end of my apprenticeship and have always been told that the maximum reading for a bonding to water and gas is 0.05ohms. I have been fine with this for the last 2 years but have this weekend been confused as GN3 and 7671 doesn't actually state this. Any help clearing this up you be amazing thank you. I know why we bond and what it is used for it's just where the 0.05 comes from. 

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    It was also confused by some earlier editions of GN 3 which suggested a particular value (0.1Ω I think) for the overall resistance of a bond (e.g. from pipe to disconnected end at the MET) - the idea being if you got this value (or lower) it could be deemed satisfactory without any further ado. It did seem to get misinterpreted though - as many read it as a requirement that it should always be ≤0.1Ω rather than you needed to do some calculation (based on conductor length & c.s.a) to decide if the value obtained was reasonable if you had a higher reading. As you've already noted, BS 7671 only specifies min c.s.a for bonding conductors - there's no requirement as to their length or resistance - so higher readings may well be perfectly acceptable, indeed unavoidable, in some situations.

       - Andy.

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    It was also confused by some earlier editions of GN 3 which suggested a particular value (0.1Ω I think) for the overall resistance of a bond (e.g. from pipe to disconnected end at the MET) - the idea being if you got this value (or lower) it could be deemed satisfactory without any further ado. It did seem to get misinterpreted though - as many read it as a requirement that it should always be ≤0.1Ω rather than you needed to do some calculation (based on conductor length & c.s.a) to decide if the value obtained was reasonable if you had a higher reading. As you've already noted, BS 7671 only specifies min c.s.a for bonding conductors - there's no requirement as to their length or resistance - so higher readings may well be perfectly acceptable, indeed unavoidable, in some situations.

       - Andy.

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