Why does my current clamp meter show large current in gas pipe?

[loz2754]

Went to a house this morning to locate a fault causing the RCD in a split load board to trip.
Going through the usual tests, I decided to put my earth leakage clamp meter on the earthing conductor.
Here is where it gets weird: with the main switch in the off position, the meter was reading nearly 3 amps through the earthing conductor!!!
So I decided for a giggle to put the clamp around a nearby gas pipe, and got around 1.7 amps through the gas pipe!!!
Ze was 0.30 ohms on what appears to be a TNC-S. Polarity is fine.
Can anyone explain this please?

 

[Darkwood]

Think more outside the box (assuming the clamp meter is correctly calibrated and not low on battery) fault currents may not be from within the property but using the local gas mains as a route to the good earthing of the property, there may be a fault locally in ground around the property, this may have no relationship to the property electrics at all but regardless this can be very dangerous situation especially to other tradespeople like plumbers who may alter the pipe work and expose themselves to deadly currents.

 

[stevethesparks]

How well does it zero when standalone?
I had one of this make, and did find it a little unstable at times.

 

[loz2754]

How well does it zero when standalone?
I had one of this make, and did find it a little unstable at times.

It zeros ok. I also tried the Fluke T5 1000 current clamp on the main earth which read 2.5 amps.

 

[telectrix]

this can be very dangerous situation especially to other tradespeople like plumbers who may alter the pipe work and expose themselves to deadly currents.

we can only hope. ?. maybe there is a God after all.

 

[loz2754]

Incidentally, in case anyone was wondering, I traced the RCD tripping fault to a dodgy cable in the upstairs sockets circuit. Nothing to do with my question about the current flow in the gas pipe.

 

[telectrix]

Incidentally, in case anyone was wondering, I traced the RCD tripping fault to a dodgy cable in the upstairs sockets circuit. Nothing to do with my question about the current flow in the had pipe.

so did you check the current in the gas pipe/MEC after fixing the fault?

 

[loz2754]

Think more outside the box (assuming the clamp meter is correctly calibrated and not low on battery) fault currents may not be from within the property but using the local gas mains as a route to the good earthing of the property, there may be a fault locally in ground around the property, this may have no relationship to the property electrics at all but regardless this can be very dangerous situation especially to other tradespeople like plumbers who may alter the pipe work and expose themselves to deadly currents.

Yes this what I concluded.
I disconnected the main earth and tested for voltage between the main earth and neutral- 0 volts. Tested between gas pipe and neutral- 0 volts.

Not sure what I should to go about making this safe...

 

[loz2754]

so did you check the current in the gas pipe/MEC after fixing the fault?

Yes it's still there.
And remember, this current is there even with the main switch in the off position.

 

[stevethesparks]

Assuming both Main protective Bonding connections are in place, and your confirmation that Ze is good, I cant see there is a lot you can do.
Its always possible its originating from a neighbouring property with a fault maybe?
I watch this thread with interest.

 

[loz2754]

Assuming both Main protective Bonding connections are in place, and your confirmation that Ze is good, I cant see there is a lot you can do.
Its always possible its originating from a neighbouring property with a fault maybe?
I watch this thread with interest.

I'm guessing it originates outside this property, as the current was there with the main switch off.
Maybe I should recommend the customer gets in touch with the DNO.

 

[Pretty Mouth]

I read about this somewhere before. IIRC it's to do with it being TNC-S.

Your neutral back to the transformer is shared with your neighbour. Your neutral is also connected to the gas pipe via bonding. Your gas pipe is shared with your neighbour, who also has it connected to neutral via bonding. So you have in effect 2 conductors in parallel, the neutral, and the loop bypassing part of the neutral via bonding + gas pipe.

Some current making its way home will flow through the gas pipe + bonding. That's kinda what i read, anyway

 

[DPG]

I would report it. This could cause issues if anyone ever disconnects that gas pipe to replace the meter etc. Wonder what potential difference you would get between the two sections once disconnected?

 

[loz2754]

Wonder what potential difference you would get between the two sections once disconnected?

I don't know. I measured the disconnected ends of the bonding and earth cables to neutral, but of course the neutral is connected to the supply main earth, so without a reference earth, I got 0 volts.

 

[DPG]

Definitely would advise a jumper lead be connected if anyone ever disconnects that pipe!

 

[telectrix]

I would report it. This could cause issues if anyone ever disconnects that gas pipe to replace the meter etc. Wonder what potential difference you would get between the two sections once disconnected?

get a wet-pants to hold both ends. estimate the effect by the dB level of the scream.

 

[DPG]

get a wet-pants to hold both ends. estimate the effect by the dB level of the scream.

It's probably in the gas regs that!

 

[James]

I have 2 theories

1 supply is tncs I believe

If the combined neutral and earth cable is broken between transformer and some properties the following could happen.
N current from properties downstream of fault can’t flow to transformer directly.
They will then try and find an alternative path, this could be through all customers earth terminals and flowing to real earth via metal pipework buried in the ground.
The 3A you are measuring could be a small part of a huge current with multiple paths via many houses.

Or

2 an underground cable fault is raising the ground voltage around a section of the gas pipe, this is being held at close to zero volts as the pipe is bonded to earth at many houses, however the current would be shared amongst many so you don’t see a voltage rise when disconnecting main earth cable.

I think either scenario would warrant a call from the dno to check it out.

 

[UNG]

Reminds me of one a DNO guy told me recently, homeowner got a plumber out because she was getting very warm water from the cold tap, plumber confirmed that the cold feed was bringing warm water into the house and called United Utilities who then got ENWL involved, it was found that a faulty cable joint in the street adjacent to the water pipe was the cause of the problem
It did make wonder what an electrician would have found in the property

 

[littlespark]

Just fuse the gas pipe.

simples

?