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]

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.

 

[TheCodboy]

Sound like there is a L-E short on the dodgy RFC, and the TT nature 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.

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.

 

[davesparks]

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?

 

[pc1966]

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.

 

[TheCodboy]

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?

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.
[automerge]1597684934[/automerge]

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.

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.

 

[Wilko]

Hi - I’m just very concerned you’ve reported 240V on the gas pipe. As all internal gas pipes are copper this could be very dangerous ... perhaps I’ve misunderstood ?

 

[pc1966]

Hi - I’m just very concerned you’ve reported 240V on the gas pipe. As all internal gas pipes are copper this could be very dangerous ... perhaps I’ve misunderstood ?

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)

 

[TheCodboy]

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.

 

[davesparks]

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.

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.

 

[TheCodboy]

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.

 

[davesparks]

240 to earth. Used my MFT for that.
Using my voltage indicator I’m getting 230 N-E and ZeroV L-E.

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?

 

[streamer]

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.

 

[Pete999]

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??

What do you mean > 999 ohms is that an IR reading?

 

[TheCodboy]

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.

 

[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