Bonding maximum resistance

Question

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.

gkenyon · Answer

Ricki Carter said: but have this weekend been confused as GN3 and 7671 doesn't actually state this. 
 Others have already pointed you in the right direction, but I'll see if I can provide a bit more information. 
 BS 7671 does not put a specific limit for main bonding conductors (although one is available via formula for supplementary protective bonding). 
 However, GN3 DOES include some guidance on what to expect when testing: 
 
 A suitable reading based on the expected conductor resistance per metre multiplied by the approximate length in metres 
 effectiveness of a "bonding clamp" of 0.05 &Omega; 
 
 So, the value of 0.05 &Omega; is mentioned, but as a guide for effectiveness of the connection of the conductor to the bonding clamp. Have a read on pages 60 to 62 of the 9th Edition. In particular, page 62 "Expected results": 
 
 Put in pictures, the 0.05 &Omega; is the maximum difference expected between the second measurement (dotted line on red lead, to the pipe) and first measurement (no dotted line on red lead, to the bonding clamp). The first measurement should be roughly (or less than) the length of the bonding cable from the MET (in metres) multiplied by the resistance per metre of the cable at 20 deg C in Appendix B to GN 3 - BUT the value may well be less than the overall "resolution plus accuracy" of your test instrument, which is where the 0.1 &Omega; suggestion comes in.

Bonding maximum resistance

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