Earth wire on gas main pipe - advice on regs sought

[derrickbowman]

it gets more interesting.

If there is some current flowing in the bond to the gas pipe ( assuming plastic supply to the meter ) then it is flowing along the internal gas pipe to where the gas pipe meets a water pipe. ( probably at the boiler gland plate ).

Assume the gas fitter shuts of the gas supply at the gas meter and then disconnects the gas pipe from the boiler. When the pipe is disconnected the current flowing along the pipe will be interrupted. That could create a spark. There will be gas still in the pipe mixing with air as the pipe is disconnected.

Is it possible that prolonged and heavy current along the gas pipe could result in over heating and possible melting of s soldered joint leading to gas leaks and sparking ?

Having spoken with the guy who sent me the report we have deduced that such a problem may have happened in the past. The protective bond wire runs from lead water pipe (true ground) to the gas pipe then on to the MET. The gas pipe is not grounded as it changes to plastic after the meter and into the driveway.

If the wire from the gas pipe bond termination to the MET was broken then the route for the equi-potential current is through the gas pipe to the boiler. At the boiler the gas pipe is bonded to the water and heating pipes but they become plastic once under the floor so do not provide a metallic path back to either the MET or the lead water pipe.

As a result the only path to the MET is the boiler chassis earth which is a 0.75 to 1.0 multistrand back to the isolator then 1.5 mm back to the CU. This gauge of earth wire would be okay to blow the boilers 5 amp fuse in a short to earth scenario but if the current is the equipotential current (>16 amp at peak load) then the earth wire gets hot inside the boiler electrics area with no fuse protection !!! Sure enough the earth wire is singed quite badly.

So at some time the protective bond wire from the gas termination point to the MET was disconnected and the gas pipe and boiler chassis earth/boiler twin and earth cable were the main bond wire.

This highlights to me the importance of maintaining the protective bond wire where a location may have big difference between supply earth and extraneous earths and why 543.3.1 stress this so. The protective bond wire has to take a secure and reliable route back to the MET and whether being clipped to the gas pipe or clipped to a wall or routed in trunking/conduit is the most reliable I would not like to judge. I am almost inclined to say that multiple wires taking separate safe routes might even be appropriate in some installations.

A call to the supplier has bought the response that we may need an additional local earth to the supply side entry and have this cabled externally to said water pipe (the supply cable is two core so no chance of a decent earth through that or its sheaf armour). One suggestion is a reversion to TT as it once was when the feeds were overhead to the houses. It does also bring up the the question of whether the other houses in the hamlet or the farm complex have similar problems. They may have replaced their lead water pipe with plastic and so not have the problem. The supplier did suggest that we have the lead pipe broken outside and use a plastic tail to remove that extraneous earth from within the building boundary.

 

[bernardgreen]

The supplier did suggest that we have the lead pipe broken outside and use a plastic tail to remove that extraneous earth from within the building boundary.

I would think very carefully about that. When there is a potential difference between pipes either side of the isolating section there will be electrolysis action between water and pipe. That could put lead compounds into the water supply.

I would replace the lead pipe with plastic for the entire run. On health grounds there are ( or were ) grants or low cost routes to getting lead pipes replaced.