L-N Reverse and 240v on gas pipe

[TheCodboy]

Hi all, been called out to a job today where the customer says they are getting a tingle from the gas meter and some appliances. Its a TT system, got a ze of 48 ohms. Gas bonding is 10mm and is continuous. Dual rcd board with 3 RFC's on one side of the board. There's a L-N reverse when i go around and check the sockets with my plug in, also voltage on the gas pipe. I've narrowed it down to one ring, that when i turn off, the L-N fault on the other rings goes and the voltage goes from the gas pipe. Now, ive been around the faulty RFC and checked the sockets and they are all wired up correctly. I've got end to end readings on L,N & E. When I've done a figure 8 with L-E and went around the sockets, i have as expected R1+R2 readings and >999 when i switch the socket (as you would normally expect). Connect one leg into the board and I have L-E 0v N-E 230v and L-N 230v, same when i connect the other leg.With the faulty ring connected back in, we have 240 on the gas again. Reverse polarity somewhere. The guy reckons there cant be any JB's as he pulled the cables in himself. They had some plastering work done recently but I've checked those sockets and they are wired correctly. The RCD for that side of the board is also duff, cant get it to trip. Ordered a new rcd which I'll fit tomorrow, but still doesnt solve my L-N reverse problem! Any pointers folks??

 

[pc1966]

Was it caught by a screw/nail? Or just a lack of grommet?
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Hopefully new RCD, short fixed and all happy once more!

 

[TheCodboy]

Was it caught by a screw/nail? Or just a lack of grommet?

The the look of it, plasterers float. It was at a socket where a plasterer has been working. Grommets in the back box so it’s all I can think only would be.
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Was it caught by a screw/nail? Or just a lack of grommet?
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Hopefully new RCD, short fixed and all happy once more!

Yep,new rcd working correctly now too. ????

 

[Lucien Nunes]

So what exactly was happening when that faulty circuit was energised? Was the MET swinging up to 230V and the gas pipe remaining at true earth? If all the earthing and bonding is OK elsewhere, it would also have been connected to a Ze of 48 ohms and dumping 5A into the ground. If it wasn't a solid short (hard to tell from your '0.00', which if it meant megohms, could still be 10k.) then why was it having such a marked effect at the MET?

What I'm getting at is that it's nice to find a fault, but unless one can prove that it was directly responsible for the symptoms, there could be more to find.

 

[TheCodboy]

So what exactly was happening when that faulty circuit was energised? Was the MET swinging up to 230V and the gas pipe remaining at true earth? If all the earthing and bonding is OK elsewhere, it would also have been connected to a Ze of 48 ohms and dumping 5A into the ground. If it wasn't a solid short (hard to tell from your '0.00', which if it meant megohms, could still be 10k.) then why was it having such a marked effect at the MET?

What I'm getting at is that it's nice to find a fault, but unless one can prove that it was directly responsible for the symptoms, there could be more to find.

It was a dead short live to earth.

 

[Lucien Nunes]

What was the actual resistance, or did you see the conductors physically welded together?

 

[TheCodboy]

What was the actual resistance, or did you see the conductors physically welded together?

0.00 reading on the mft.

 

[Lucien Nunes]

0.00 what? Megohms on an IR test? If so, that can still be 10kΩ which is more consistent with a carbonised insulation fault and is far from being a dead short, and shouldn't have much impact on the system elsewhere. If it's 0.0Ω (which you can't tell from an IR test, needs to be low ohms) then the only reason you were able to close the breaker was that the entire earthing system throughout the installation became fully live WRT real earth. Anywhere in the middle, the fault would have got much hotter.

 

[ipf]

It looks more like a nail than float damage. If so, until you released it, the nail could have had an affect on your readings. As Lucien asks, we’re you continuity or IR testing?

 

[TheCodboy]

It looks more like a nail than float damage. If so, until you released it, the nail could have had an affect on your readings. As Lucien asks, we’re you continuity or IR testing?

These were IR readings.

 

[Lucien Nunes]

From an IR reading on a MΩ scale you can't say whether or not it was a dead short, only that it was not more than tens of kΩ. It could be 10,000 times higher than what one would really consider to be a short, although it sounds like it probably was from the other evidence.

 

[davesparks]

0.00 reading on the mft.

MFT's have a variety of different ranges so just staying 0.00 could be ohms, megohms, volts, amps, milliamps, seconds.

An electrician should understand the need to, and be able to, quote the units as well as the measurement for the measurement to actually mean anything at all.
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These were IR readings.

Then you haven't measured a dead short, you have measured 0.00 megohms which could be many thousands of ohms.
Depending on your exact tester if you had switched your tester to the normal resistance range it may have given a result suggesting that it was open circuit.

 

[TheCodboy]

0.00 what? Megohms on an IR test? If so, that can still be 10kΩ which is more consistent with a carbonised insulation fault and is far from being a dead short, and shouldn't have much impact on the system elsewhere. If it's 0.0Ω (which you can't tell from an IR test, needs to be low ohms) then the only reason you were able to close the breaker was that the entire earthing system throughout the installation became fully live WRT real earth. Anywhere in the middle, the fault would have got much hotter.

I did state in the photo I posted that it was 0.00 IR between L/E.
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MFT's have a variety of different ranges so just staying 0.00 could be ohms, megohms, volts, amps, milliamps, seconds.

An electrician should understand the need to, and be able to, quote the units as well as the measurement for the measurement to actually mean anything at all.
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Then you haven't measured a dead short, you have measured 0.00 megohms which could be many thousands of ohms.
Depending on your exact tester if you had switched your tester to the normal resistance range it may have given a result suggesting that it was open circuit.

In my original post I stated that I had end to end continuity on L N & E. So now I’ve seen the damaged cable, that to me that would suggest that the live and earth are touching together/fused together, however you choose to name it. That was the baffling thing,I had continuity of the ring. When I got to the job,I didn’t have a lot of spare time. I knew I had to go back and replace the RCD anyway which was faulty. So I disconnected the circuit which was causing the gas pipe to become live, leaving the site safer than when I arrived.

 

[Lucien Nunes]

I'm afraid this is pedantry, but it's important pedantry. I have an insulation tester for HV circuits that measures in gigaohms. If I said it read 0.00 would you consider that to be a 'dead short'? Even though it could be ten megohms? Would you have guessed I was talking about gigaohms and not megohms? Always state the units!
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the circuit which was causing the gas pipe to become live

230V between gas pipe and MET could be either way round - how do you know that the gas pipe was not remaining at earth potential and the whole earthing system of the house was becoming live? From my reading of the evidence I would say that is more likely. I'm not criticizing here, just looking at other technically valid interpretations of the data that you've posted.

 

[davesparks]

230V between gas pipe and MET could be either way round -

If we are taking the information exactly as it has been reported here then it was apparently 240V at the gas pipe and 230V at the CU. Which could be a clue, or just poor reporting of test results again.