RCD not tripping under test

[manoworcs]

Hi all. New user, but have lurked around here and used advice in other threads before, but haven't been able to find this problem anywhere, so figured I'd register and give it a shot.

I've put in a new dual RCD consumer unit for a customer, went through all the testing with no problems, tested one RCD which was fine, then tested the other and it didn't trip at I. Out of curiosity, I tested it at 5I and it tripped in 9.4ms, then did the ramp test and it didn't trip (reading was >33mA). Figured it must be an RCD problem, so put the other one in its place and had the same issue. The one I thought was faulty worked without any problems in the other place, too.

I'm assuming it must be something in the wiring somewhere, but don't understand what would cause this, particularly as it does actually trip in good time at 5I. I may also just be very stupid, the unit was a nightmare to replace and I was a bit weary when I got around to the testing...!

I'm going back there at the start of the week to finish off, so would appreciate any help or advice anybody else might have who has had this problem before.

Thanks.

 

[Lighthouse Phil]

I had a guy ask me the same question before. He had a split load CU with 2 RCD protected supplies. Every thing was ok with the domestic loading and testing. He couldn't understand why the night time only supply didn't trip the RCD test switch during the day. He's a plumber now.

 

[manoworcs]

I may be doing something stupid, but it's not quite that bad :lol: There's definitely a supply, the circuits protected by that RCD have power. And the RCD trips at 5I.

 

[Piratepete]

Hi
Lighthouse Phil, not very helpful! Have you had a bad week?

Manoworcs - IMHO your tests (swopping etc) do point to a wiring problem (though I'm not sure what would stop the RCD tripping - high Ze or Neutral resistance perhaps). Check your circuits, including the Neutrals

Pete

 

[Tridentsystems]

We're you testing at cu with loads disconnected ?

 

[Octopus]

What make of RCD is it? where are you testing it? Is everything disconnected?

 

[telectrix]

remove all loads from that side of the board, including neutrals. test RCD > if OK, then add each circuit, 1 at a time, testing the RCD each time. when it fails to trip, the last circuit added is the 1 with the fault. re=test that circuit.

 

[manoworcs]

Wylex board, both are 80A RCDs. Initially, I was testing it at the cooker outlet (in the same room), but tested at the CU when it didn't work. Disconnected the loads, but now it's been mentioned, I may have forgotten to disconnect the neutrals :mad2:

It was the last thing I was doing at the end of a tough week, so I'm blaming it on that. Cheers.

 

[ArseyDee]

Wylex board, both are 80A RCDs. Initially, I was testing it at the cooker outlet (in the same room), but tested at the CU when it didn't work. Disconnected the loads, but now it's been mentioned, I may have forgotten to disconnect the neutrals :mad2:

It was the last thing I was doing at the end of a tough week, so I'm blaming it on that. Cheers.

I think you may be mis-understanding tel's post. His advice to disconnect the neutrals was as a fault finding measure. Tel was not saying that the neutrals should be disconnected as part of the usual RCD test procedure.

 

[ArseyDee]

....or maybe i've just mis-understood the wording of your post #8......

....in which case, forget I was even on here!!

 

[Octopus]

When testing RCD's at the board, its a good idea to switch off all the MCB's

 

[uksparks]

I know this sounds ridiculous but have you got your leads the right way round on your tester? I did that once whilst scratching round in an understands cupboard with a dying head torch and not enough light to realise I'd plugged them on the wrong way round. Took me ages to realise thinking, this is a brand new shower ccu so what's wrong.

- - - Updated - - -

Under stairs cupboard sorry

 

[ArseyDee]

I'll just throw this into the arena:-

Is your tester set for the correct RCD type?

On my Megger, the default setting is Type AC RCD, but I accidentally changed this with one of the function buttons and it took me a while to figure out why the auto test was ok at 1/2 x, but timed out and stopped on x1.

I know this does not mirror your tripping / non-tripping issue, but it's just an idea.

 

[Midwest]

I've only just wizzed through this thread, so apologies if this has already been mentioned. Are you a DIY'er? What tests have you done on your circuits? Have you done a global insulation resistance test? A neutral to earth fault can cause an RCD not to function, especialy in a TN-C-S supply. Does the test button on the RCD function?

 

[Octopus]

I've only just wizzed through this thread, so apologies if this has already been mentioned. Are you a DIY'er? What tests have you done on your circuits? Have you done a global insulation resistance test? A neutral to earth fault can cause an RCD not to function, especialy in a TN-C-S supply. Does the test button on the RCD function?

The OP does say "he's going back next week" so I'm guessing he's a sparky

 

[Midwest]

Ok apologies, manowarcs. Just read his thread properly. But I would recommend doing IR test for neutral-earth fault, and check to make sure all appliances etc are disconnected before doing RCD test.

 

[Badged01]

I did that once whilst scratching round in an understands cupboard with a dying head torch and not enough light to realise I'd plugged them on the wrong way round

Think what you mean is that you once heard someone else had done than and you are just relaying the info ....

Is your tester set for the correct RCD type?

Ive also heard that this can happen if you're testing with the wrong RCD setting!

 

[manoworcs]

The test button on the RCD functions just fine, I think I ruled out it being a problem with the RCD. There's clearly a problem with the wiring somewhere, I suppose I was more curious about what sort of problem would cause the RCD to behave in that way. I'd have thought any sort of earth fault would make it impossible to set in the first place...no? It is a TN-C-S system though, so I'll go back and look at that again.

I didn't do a global insulation resistance test, I did each circuit separately. Some of the wiring is old, so I was expecting a problem. Was pleasantly surprised not to get one, but that's also part of what's baffled me now.

No problems with my tester settings, I quadruple check that sort of thing any time I get unexpected results.

Thanks to everyone that's taken the time to reply though.