Faulty RCD tripping on the button but not via the tester

[tony9i9i9i]

What should you do if a client has a CU that is a split board protected by 2 * 63Amp 30mA double pole RCCDs, you took Zs and tested both RCCDs by their buttons before starting work.

But on completion of work, one of the RCCDs, when tested via tester, would not trip, the other operated as expected!

 

[timhoward]

I 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.

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.

 

[tony9i9i9i]

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.

but at 1 * delta it has to trip within 300ms.

 

[timhoward]

but at 1 * delta it has to trip within 300ms.

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.

 

[tony9i9i9i]

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.

I see, thanks again, but there is no PV.

 

[HappyHippyDad]

Thanks Westward. Is there a typical fault I should be looking for?

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).

It is always unlikely that the RCD is faulty, not impossible, just unlikely. I have only come across a handful of faulty RCD's in the last 10 years.

Here is the link/thread Why isnt the RCD tripping? - https://www.electriciansforums.net/threads/why-isnt-the-rcd-tripping.176881/#gsc.tab=0

 

[tony9i9i9i]

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).

It is always unlikely that the RCD is faulty, not impossible, just unlikely. I have only come across a handful of faulty RCD's in the last 10 years.

Here is the link/thread Why isnt the RCD tripping? - https://www.electriciansforums.net/threads/why-isnt-the-rcd-tripping.176881/#gsc.tab=0

Will do thanks.

 

[OLDBOY]

RCD's sometimes seemingly dont operate within the specfied time when tested with loads in circuit because of an appliance with a capacitor. Fridges and freezers in particular are prime candidates. This is because for a split second after the supply is disconnected the appliance capacitor holds on to a charge, so the tester still sees a voltage even though the RCD has operated.

 

[tony9i9i9i]

Thanks everyone for your help on this one, I've bookmarked the other thread.

Turns out on this one it was a faulty RCD, I went back with a new one, tested the original as advised and it would not operate. Swapped it out and it's all tickety.