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RCD may well be faulty. Try it with the mcbs off and the other RCD off and if no success ditch the circuit neutrals also, if it trips a circuit fault is causing the problem.
Discuss Faulty RCD tripping on the button but not via the tester in the UK Electrical Forum area at ElectriciansForums.net
Thanks Westward. Is there a typical fault I should be looking for?RCD may well be faulty. Try it with the mcbs off and the other RCD off and if no success ditch the circuit neutrals also, if it trips a circuit fault is causing the problem.
Thanks Loz.I have found that a high resistance neutral to earth fault can cause an RCD not to operate. I have also found that a doorbell transformer connected to one of the circuits can cause an RCD not to operate. And of course, a type AC RCD can be blinded by an appliance or other device that happens to be leaking DC current and thus the RCD will not operate.
Guidance Note 3 indicates that the test should be carried out at the RCD with the load disconnected to avoid spurious results.
This is a very obscure possibility but it might beI tested from a ring main on that side of the board, the tester ran through the cycle 1/2 and 1/2 at 180, then 1 ran over 300ms and the tester aborted the rest of the tests. At that point, I assumed it was a faulty device. As I say when testing the other side of the board the RCCD operated properly.
but at 1 * delta it has to trip within 300ms.This is a very obscure possibility but it might be
-the tester is giving up at 300ms
-it actually would have tripped at say 600ms
-it is taking more that 300ms because there is a PV installation on an MCB that side of the board supplying the installation from the other side.
In any case, testing without loads as suggested above would lead you to discovering this anyway.
I agree.but at 1 * delta it has to trip within 300ms.
I see, thanks again, but there is no PV.I agree.
My point is that if PV is incorrectly installed, it can back-feed an RCD until the inverter notices lack of grid supply and shuts down and this can take longer than 300ms. So in this (unlikely) scenario it could have possibly caused an RCD test to fail.
The first time I encountered this was when RCD's passed at the beginning of a job and then one of them didn't at the end. Though in my case it did trip, it just took far too long. But a different MFT might give up sooner.
Have a thorough read through the thread I've linked below Tony. I had a similar problem to you (ignore the fact I wrote that the RCD was faulty in my first post in the linked thread, I was wrong as I suspect you may be) and this thread helped me to understand why a perfectly good RCD will not trip when tested from a socket or light rather than at the RCD itself (with outgoing terminals disconnected).Thanks Westward. Is there a typical fault I should be looking for?
Reply to Faulty RCD tripping on the button but not via the tester in the UK Electrical Forum area at ElectriciansForums.net
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