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??
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Discuss L-N Reverse and 240v on gas pipe in the UK Electrical Forum area at ElectriciansForums.net
Sound like there is a L-E short on the dodgy RFC, and the TT nature (Ze not enough to take out the OCPD) and lack of working RCD means no trip to shut stuff off.
No idea why that would not be showing up on your IR test though, but when you did the IR test on the RFC, was the ring CPC connected to the installation's CPC?
I.e. a live to other-earthed-something short could do that sort of thing.
No idea why that would not be showing up on your IR test though, but when you did the IR test on the RFC, was the ring CPC connected to the installation's CPC?
I.e. a live to other-earthed-something short could do that sort of thing.
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I didn’t get a chance to IR it today. I’ve gotta go back tomorrow to replace the Rcd as it’s shot. Won’t trip with the test button or with a ramp test. I’ll insulation test it as soon as I get there.
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In all that testing did you check the polarity of the incoming supply?
That RCD may not be tripping as the result of the fault, it may not be faulty itself.
L-E 0V L-N 230V N-E 230V the 240V on the gas pipe, what is the gas pipe voltage measured from? Is it to neutral?
Are those actual measured values?
Did you carry out any insulation resistance testing?
That RCD may not be tripping as the result of the fault, it may not be faulty itself.
L-E 0V L-N 230V N-E 230V the 240V on the gas pipe, what is the gas pipe voltage measured from? Is it to neutral?
Are those actual measured values?
Did you carry out any insulation resistance testing?
Davesparks has a very good point there - you need to be absolutely sure the supply polarity is good first.
For that you need the installation to be off as any short from phase (L, or N if reversed) to the CPC will make the Earth rod live and fool any convenient checks on polarity. With the incomer switch or RCD off you can check volts L-E and N-E at the incoming supply to be sure that you are looking for the right thing downstream for any other fault-finding.
For that you need the installation to be off as any short from phase (L, or N if reversed) to the CPC will make the Earth rod live and fool any convenient checks on polarity. With the incomer switch or RCD off you can check volts L-E and N-E at the incoming supply to be sure that you are looking for the right thing downstream for any other fault-finding.
Incoming supply polarity checked ok
With the faulty RFC completely disconnected from the board L-N & E it wont trip
The voltages are taken from my voltage indicator. I've connected one leg in and tested at the end of the other.
Haven't carried out IR test yet.
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I isolated at the main switch, checked L-E 230v N-E 0v L-N 230v. Incoming polarity is good. All other circuits are showing no L-N reverse when the suspect RFC is disconnected.
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I think all bonded stuff to the CPC is going live - hence the volts on the gas pipe.
No idea if the pipe has an isolation break, is short and to a plastic gas main, etc, but to trip the RFC's OCPD it would need to be down to a couple of ohm Ra (even assuming the figure given above was just the rod, and not a Zs for whole system)
It was just the rod. Main earth (rod) disconnected from the DB.
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Youve misunderstood my question, I asked where the 240V on the gas pipe was measured from, not what type of tester did you use. Is it 240V between the gas pipe and the live, the neutral, the earth, something else?
What is the fault on the RFC? As far as I can tell from your posts you haven't yet found a fault on the RFC? Plugging in a socket tester is not a valid test for identifying a type of fault, it is a rough estimate. Those things will show up a L-N reverse for other faults than just reversed polarity.
240 to earth. Used my MFT for that.
Using my voltage indicator I’m getting 230 N-E and ZeroV L-E.
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do you mean 240V to earth as in the actual physical surface of the ground? I assume you don't mean to the electrical earth as the gas is bonded so should be at the same potential as the electrical earth.
Where did you measure these 230V measurements? Are they appearing in different parts of the installation or just in one place?
Hi TheCodboy. I know you've checked the incoming polarity. Have you checked the polarity at the mcbs? Is it possible there has been a crossover somehow within the consumer unit? I saw a schnieder consumer unit recently where it looked quite easy to get things the wrong way around.
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What do you mean > 999 ohms is that an IR reading?
Sorry, No. when doing my figure 8 for R1+R2. So I was getting a R1+R2 reading and was open circuit when switching.
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when switching what? I cant think what part of an R1+R2 test involves switches?
The switch at the socket.
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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.
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.
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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.
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.
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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Yep,new rcd working correctly now too. ????
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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.
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.
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What was the actual resistance, or did you see the conductors physically welded together?
0.00 reading on the mft.
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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.
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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.
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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.
I did state in the photo I posted that it was 0.00 IR between L/E.
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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.
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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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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.
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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.
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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.
Reply to L-N Reverse and 240v on gas pipe in the UK Electrical Forum area at ElectriciansForums.net
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