Very odd problem what do you think ?

[Dave OCD]

I might be overlooking something that's right under my nose but opinions would be much appreciated. I visited a customer today with a detached garage supplied with power from a 13a RCD fused spur in the conservatory [spurred off RFC].
The RCD is 'tripping' randomly so I went to investigate.
The unit in question must be around 15 years old or more and immediately I found that the test button did nothing, a test at a garage socket also failed at x1 and x5 and the RCD refused to operate and I then found the same with the load disconnected testing at the spur itself. [SFCU in case of pedantry].
So after isolating and doing an insulation resistance test at 250v with everything switched on in the garage [freezer and lights] which showed around 40 Megohms L-E & N-E I fitted a brand new BG Nexus RCD SFCU, upon powering up the test button worked perfectly but in the garage both my tester and Kewtech plug in socket tester are showing L-E reversed !
With the supply and load cables connected together everything is fine [Highest Zs is 0.79 at the furthest garage socket].
So I have another replacement unit to fit tomorrow, it has to be that, right ?
I can't think of anything else which could cause this but opinions are most welcome even if I'm being dumb.
Sorry about the long winded tale !

 

[SparkyChick]

Another question... did you prove the voltages across the 3 conductors using say an AVI?

Brain fart time I think... I'm just musing out loud

 

[Dave OCD]

No but that's a very good point, I'd convinced myself it had to be the RCD spur though as all was well with it removed and connected straight through from Conservatory to garage.

 

[loplug]

I was thinking check for voltage as well and you could do a R1+R2 and R1+Rn from the supply side of the rcd spur to the garage outlets to verify polarity that way

 

[Richard Burns]

When a tester says line and earth reversed the comparative voltages are being measured and when you have a situation where this reversal is unlikely, as in your case, it is useful to note that the same comparative voltages would be measured if the outgoing neutral from the RCD were disconnected and there was a load connected at the end of circuit.
At the garage Line-earth would be 230V, Line-Neutral would be both the same at 230V (as the load would not drop voltage) and so the pd would be 0V, Neutral-Earth would read 230V because the Neutral is at 230V. These are the potential differences you would measure if Line and earth were reversed.

So in your case I would expect the original RCD had failed and that the neutral connection on the outgoing side of the new RCD is faulty to cause the symptoms you describe.

 

[Dave OCD]

I'm wondering if it might be the RCD simply not putting the neutral through for example, then the internal tripping circuitry is fooling the MFT and socket tester into saying L-E reversal, I can't see how it could physically reverse L & E but convinced it's dodgy, I'll know for sure tomorrow.

 

[Dave OCD]

Did you do a Zs at a socket in the garage? If the L-E really are reversed I would have expected your MFT to complain.

When a tester says line and earth reversed the comparative voltages are being measured and when you have a situation where this reversal is unlikely, as in your case, it is useful to note that the same comparative voltages would be measured if the outgoing neutral from the RCD were disconnected and there was a load connected at the end of circuit.
At the garage Line-earth would be 230V, Line-Neutral would be both the same at 230V (as the load would not drop voltage) and so the pd would be 0V, Neutral-Earth would read 230V because the Neutral is at 230V. These are the potential differences you would measure if Line and earth were reversed.
So in your case I would expect the original RCD had failed and that the neutral connection on the outgoing side of the new RCD is faulty to cause the symptoms you describe.

Brilliant Richard ! Exactly what I was posting at the same time although your version is far better but you've confirmed my suspicions ! Thanks very much.

 

[Richard Burns]

Brilliant Richard ! Exactly what I was posting at the same time although your version is far better but you've confirmed my suspicions ! Thanks very much.

I am just glad I managed to hit post reply fractionally before you did so I was not repeating what you had just said.

 

[NDG Elecs]

Yeah Richard summed up the likely issue in my view. Test across the RCD itself to confirm.

I have had countless issues with icky BG RCDs I refuse to use them, or their boards, anymore.

 

[Dave OCD]

Thanks for all the replies everyone, especially Richard ! I fitted the replacement this morning and all was well. It operates in 8.8ms on both x1 and x5 tests and trips out at 27ma on the ramp test .
I felt confident enough to offer the customer my extended 48 hour guarantee [rather than standard 24 hr] in exchange for another cuppa.

 

[tomspark93]

I’ve had multiple BG rcd fused spurs fail in the past with the same issue. No continuity on the neutral poles of the unit. I had at least 4 faulty units within the space of a few months a couple of years ago! I refuse to fit them now! At the time I thought they were a bargain at £15 or so from screwfix but they are absolute rubbish in my opinion.

 

[PEG]

I’ve had multiple BG rcd fused spurs fail in the past with the same issue. No continuity on the neutral poles of the unit. I had at least 4 faulty units within the space of a few months a couple of years ago! I refuse to fit them now! At the time I thought they were a bargain at £15 or so from screwfix but they are absolute rubbish in my opinion.

There was also a run of the same brand's two pole SFU which had a pole not working...very anti-euro...