13.09mA in uninsulated gas bonding

Rozdog · Jul 23, 2025

Good morning. I am always willing to be embarrassed but not willing to get an electric shock. A mate of mine has bought a house with 1960s wiring. He will be doing a whole rewire but he has asked me to check for existing safety. There are C1s and C2s all over the place but there is 13.09mA present in the uninsulated (bare) cable used as gas pipe bonding when the board is energized and no change when both boards are switched off. Therefore something is providing current and it can only be:

A) The gas pipe is energised by the ground it passes through
B) A house near by has a fault which it is passing down the gas pipe close by
C) Open PEN fault is not applicable as it is a TN-S system

Any ideas? Thoughts? Next steps.

PS Electrics · Jul 23, 2025

Rozdog said:
Good morning. I am always willing to be embarrassed but not willing to get an electric shock. A mate of mine has bought a house with 1960s wiring. He will be doing a whole rewire but he has asked me to check for existing safety. There are C1s and C2s all over the place but there is 13.09mA present in the uninsulated (bare) cable used as gas pipe bonding when the board is energized and no change when both boards are switched off. Therefore something is providing current and it can only be:

A) The gas pipe is energised by the ground it passes through
B) A house near by has a fault which it is passing down the gas pipe close by
C) Open PEN fault is not applicable as it is a TN-S system

Not necessarily just because it is TN-S at the House doesn't mean the earthing is not TN-C on the DNO side due to network repairs.

You have Isolated the supply so have determined the Diverted Current is Imported.

I would contact the DNO they should be able to tell you if you are on a true TN-S Earthing system. Thus ruling out a PEN fault.

I suspect a PEN fault and part of the supply network is TN-C and the fault is some way away because you are getting mA and not Amps flowing.