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 !

 

[Wilko]

Hi - I was just going to suggest to check through the installation earthing. Perhaps a wander lead from met to that outlet so it's 100% that earth is where it's supposed to be

 

[Octopus]

I was fault finding a 15 year old house recently .... the garage CU had reversed polarity and no earth connection .......................

 

[Dave OCD]

Hi - I was just going to suggest to check through the installation earthing. Perhaps a wander lead from met to that outlet so it's 100% that earth is where it's supposed to be

Thanks Wilko, yes I'm certain about that, with the new spur in place the garage lights wouldn't come on either. I'm 99% certain it's a dodgy unit but doubts creep in as it's brand new and I can't see how the internal circuitry could create this. But if I'm right it's a very dangerous RCD.

 

[telectrix]

with the RCD out of circuit, then test for polarity. if it's right then then it is a duff RCD. dangerous and should be brought to manufacturers attention in case they have made a faulty batch.

 

[Octopus]

Does it need to be an RCD?

Does the socket circuit have rcd protection?

 

[ipf]

with the RCD out of circuit, then test for polarity. if it's right then then it is a duff RCD. dangerous and should be brought to manufacturers attention in case they have made a faulty batch.

That's it. By pass the RCD for testing purposes.

 

[Dave OCD]

with the RCD out of circuit, then test for polarity. if it's right then then it is a duff RCD. dangerous and should be brought to manufacturers attention in case they have made a faulty batch.

That's exactly what I did Tel, I know it has to be the RCD but it just seems so odd. I have another to fit in the morning so will update tomorrow, thanks.

 

[Dave OCD]

Does it need to be an RCD?

Does the socket circuit have rcd protection?

Unfortunately it's needed Murdoch as the socket circuit isn't RCD protected, I'll be doing a new CU later in the year but the Customer is a pensioner and can't afford all that just yet.

 

[Flanders]

Has the rcd unit also got surge protection as I have seen surge protection exstention leads showing crossed polarity when tested ,just a thought as I have notice some rcd sockets have surge protection fitted now

 

[Dave OCD]

I don't think so, also this one appears to be reversing L & E, so if you want to commit murder by RCD, this unit is just the job.

 

[SparkyChick]

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.

 

[Dave OCD]

Yes SC but only after I'd removed the RCD and temporarily joined the supply and load together, with the RCD connected the MFT complained loudly.

 

[SparkyChick]

Sorry Dave, I came back to the thread and forgot what I'd read in your OP

Bizarre situation. Definitely want to know the outcome

 

[Dave OCD]

You can see how it had me puzzled eh ?! I'll post an update tomorrow.