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??

 

[davesparks]

Sorry, No. when doing my figure 8 for R1+R2. So I was getting a R1+R2 reading and was open circuit when switching.

when switching what? I cant think what part of an R1+R2 test involves switches?

 

[TheCodboy]

when switching what? I cant think what part of an R1+R2 test involves switches?

The switch at the socket.

 

[Strima]

Are you testing at the sockets using a plug adaptor for your MFT? If so try taking your readings from the terminations on the back, this will also help identify any loose terminations, incorrect polarity etc.

Could be an accessory screw has nicked a line when an accessory is fitted and this is causing the voltage to rise on the gas pipe as it has a better Ra than the electrode.

 

[Lucien Nunes]

Polarity reversal means two wires swapped over. They can't swap and unswap themselves on one circuit as you connect and disconnect another, therefore polarity reversal is probably a red herring inspired by the fairy lights on the tester. Line and neutral are probably fine, but some or all of the CPC is floating about. When the tester sees a hugher voltage CPC->N than CPC->L, it suggests reverse polarity because that's one of a number of possible interpretations, but it's not the only one.

I think you need to ignore all that and take some careful continuity measurements of various earthy things. Rod, MET, CPCs, extraneous services.... what is and isn't solidly connected. It's no good saying there's 230V 'to earth' if there are multiple unconnected versions of 'earth' at different potentials.

 

[TheCodboy]

Found it. 0.00 IR L-E

 

[pc1966]

Was it caught by a screw/nail? Or just a lack of grommet?
[automerge]1597757247[/automerge]
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.
[automerge]1597757315[/automerge]

Was it caught by a screw/nail? Or just a lack of grommet?
[automerge]1597757247[/automerge]
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.