Two new Wylex RCDs both failed tests

[steamboatwilly]

Folks,

A bit baffled (not difficult) ... added a light switch in a rental this week (empty property so no social distancing issues) and checked the RCD before I signed off. It was a Wylex 80a 30mA and it failed all tests (inc. ramp) so I like-for-like replaced it. The new one also failed all tests (1/2, 1, 5 and ramp). Figuring I'd got a faulty one I replaced it again with the same results.

So ... surely not 2 new dodgy Wylex RCDs? Must be a problem within the installation? Tried testing the RCD on its own with no loads, same result. RCD stays on and the manual test trip works.

Megger MFT1552, 30mA range, Ze 0.29ohms, PFC 0.78kA.

Do I need to go to dead tests?

 

[telectrix]

suspect a N-E fault on the installation. can do weird things to RCDS, does the RCD test button work?

 

[steamboatwilly]

suspect a N-E fault on the installation. can do weird things to RCDS, does the RCD test button work?

Thanks. It does indeed ... did on both new RCDs. So, need to do dead tests then I guess? Serves me right for checking the RCD.

 

[dave274]

Check you've not caught the neutral in the switch if you have one.

 

[steamboatwilly]

Check you've not caught the neutral in the switch if you have one.

Ta. There's a main switch outside of the Wylex CU. Do you mean that one? It's a 100a Wylex DP isolator.

 

[dave274]

no I mean the light switch, if it has a neutral in it then check you haven't caught it with the screw when replacing it.

 

[steamboatwilly]

no I mean the light switch, if it has a neutral in it then check you haven't caught it with the screw when replacing it.

Oh crap, of course. Apologies. Yes, good point, I will check that! It does have a neutral in it.

 

[Bob Geldoff1234]

Your test meter could be faulty.Maybe it doesn't like Wylex RCD's? Try it on a different make? You also need to be checking the RCD completely on it's own.ie take out the live and neutral terminals at the bottom of the RCD so it's completely stand alone and then try testing it again.If it now works then ,like others have said, it's probably a neutral fault.

 

[steamboatwilly]

Your test meter could be faulty.Maybe it doesn't like Wylex RCD's? Try it on a different make?

I did wonder. I've tested Wylex before and not had a problem. I also changed the batteries in case they were going bad. I'll check my RCDs here at home just to be sure though. Thanks
[automerge]1585507205[/automerge]

.... take out the live and neutral terminals at the bottom of the RCD so it's completely stand alone and then try testing it again.If it now works then ,like others have said, it's probably a neutral fault.

Aha, yes, makes sense to isolate the issue a bit more. I'll do that too.

 

[steamboatwilly]

OK, thought I'd update you all on this one.

Yes indeed, there was a fault between N and E. I isolated the RCD which then tested fine. I then disconnected the earth(s) for the lighting circuit - the first circuit I checked - and the RCD then tested fine. I reconnected each of the 4 (!) lighting radials one at a time and found the fault on the downstairs lights.

I'm currently looking for where exactly the fault is but it's certainly not in a light or a switch as I checked those already. Screw through a cable likely ...

Thanks for the help here guys.

 

[Farmelectrics]

if it was a neutral earth fault how come it wasn't tripping the rcd straight away. ive had a issiue with rcd that don't reset because of neutral earth fault

 

[steamboatwilly]

if it was a neutral earth fault how come it wasn't tripping the rcd straight away. ive had a issiue with rcd that don't reset because of neutral earth fault

No idea I'm afraid ... not been able to get back there yet to look / check. Maybe I disturbed something somewhere. It is now a new RCD so perhaps the original was indeed faulty

 

[Farmelectrics]

No idea I'm afraid ... not been able to get back there yet to look / check. Maybe I disturbed something somewhere. It is now a new RCD so perhaps the original was indeed faulty

seems a bit strange however if its all tested ok crack on

 

[steamboatwilly]

seems a bit strange however if its all tested ok crack on

It had me fooled. It's all fine now bar that downstairs light circuit (currently disconnected CPC). Luckily the house is vacant so I can pop back there sometime soon.

 

[westward10]

Depends on whether the neutral/earth fault causes an imbalance. Just because you have a neutral/earth fault it should not be assumed the circuit will follow the earth path, circuit impedance may dictate the circuit carries on down the neutral path hence no imbalance.

 

[Lucien Nunes]

The behaviour of an N-E fault is complex and can seem unpredictable. There are paths of different resistances to both neutral and earth for current from the faulty circuit, from other circuits and even from neutrals of other installations when there's no load on this one. How each of the currents divides can be hard to predict; sometimes they will cause nuisance tripping when there is heavy load on, sometimes just failure to test, sometimes the RCD won't reset at all.

What they should do consistently, though, is fail insulation tests, so I am a little surprised that wasn't picked up first.