Can't reset the RCD with any MCB's on - Faulty RCD?

[Geoff101]

We’ve had a few power glitches/cuts recently by our supplier. Each time, on power restoration our RCD has tripped. Irritatingly we were on holiday when it happened last - not good for frozen food!

When we had an extension 3 years ago some changes were made to the consumer unit and a new RCD was fitted into it. This was done by a qualified electrician with all the tests etc carried out. Since then I've never just been able to reset the RCD when tripped - I've always needed to switch all the MCBs off, reset the RCD and then reset the MCBs one by one.
I've now done some investigation trying to reset the RCD with just one of the MCBs in the on position and it appears that almost any individual circuit will prevent the RCD resetting if the MCB is left on.

We don't generally have any problems with other nuisance RCD trips - once it's reset everything is fine.

I haven't been round to remove everything that is plugged in, and some of the circuits that prevent resetting are lighting circuits anyway. I did try one experiment as an example - The oven circuit MCB, if on, prevented the RCD resetting. This circuit only has the oven connected to it. With the oven isolator switched off but the MCB on it allowed the RCD to reset - so clearly not an issue with the circuit itself. Switch the oven isolator on again and the RCD couldn't be reset. It's a fairly new built-in AEG dual oven and am pretty confident it's not faulty.

So the big question is, does this sound like it could be a faulty RCD which doesn't like being reset with any sort of load on it? In the case of the oven it obviously wasn't actually on and heating, just in standby with the electronics active. I'm guessing whatever earth leakage surge there is on power up is triggering the RCD - as it appears is happening in most the other circuits. Interestingly though, using the oven as an example the 'surge' is enough to prevent the RCD being reset, but clearly not enough to trigger it once reset and the MCB is put on.

I don't know how common it is for the RCD to be suspect like this - and if faulty it appears it has been from the start. Worth a swap to see?

Thank for any thoughts....

 

[Wilko]

Hi - just looking at the innards of your board - I think it's time for it to be retired. Several reasons, but "quart in a pint bottle" comes to mind .

 

[coolsparx]

I only fit RCBO boards, resolves all the nusiance tripping issues - acculimative normal internal leakages & external supply fluctuations. The Lewden boards are very resonably priced.

 

[chrisdungan]

In my experience it's most unlikely to be a faulty RCD.... hardly ever happens.
RCDs trip when some of the neutral current flows in the earth conductor. So
Cumulative leakages of all the combined neutrals downstream from that

 

[chrisdungan]

In my experience it's most unlikely to be a faulty RCD.... hardly ever happens.
RCDs trip when some of the neutral current flows in the earth conductor. Remember these suckers trip when the leakage current is the size of a gnats dick.......So
1 Cumulative leakages of all the combined neutrals downstream from that RCD. are you able to split some circuits to an additional RCD?
2 excessive leakage on one of the circuits. look at anything with an element that might have absorbed moisture OR a water leak that has got into a termination somewhere. OR even the end of a cable that's unterminated that gets moisture across neutral and earth
3 out in the house somewhere is there a neutral that's either crossed over with another RCD ie phase on one RCD neutral on another or shared onto another RCD. These things can happen at renovation time.

Just had a look at the photo. FAAAAR too much cumulative leakage will be the issue. Upgrade the board. Swap all the MCBs for RCBOs on each circuit. that'll sort it
Best of luck

 

[freddo]

Hi - just looking at the innards of your board - I think it's time for it to be retired. Several reasons, but "quart in a pint bottle" comes to mind .

Yes, the consumer unit is obsolete and any new parts if available will be some years old now. The connection to MCB 4 is rather poor, and the large neutral wire is likely to be a poor fit in the small terminal 12. It would be a waste spending money on this old unit, it has already been modified more than once.

 

[Geoff101]

So for those following this exciting thread who like a happy ending....(!)
Managed to get a local spark out this morning. Ran the circuit tests - all ok.
Swapped the rcd for a BG one which has the L N connections on the opposite sides.
Rcd reset first time with all MCBs on! Result!
I’ll look at the options for a new CU with a bit more space in the new year.
So I think Marconi wins the prize for spotting the wrong handed rcd. Cheers!
Thanks to everyone who contributed and helped move the thinking on. At least you got to find out the ending!

 

[DPG]

Great stuff. Thanks for letting us know.

 

[Bul]

Hi - I might be the only person that didn't quite understand the wrong handed rcd. Could anyone explain it again or do a line drawing. I might be slow but I'm honest and interested.

 

[Geoff101]

Hi - I might be the only person that didn't quite understand the wrong handed rcd. Could anyone explain it again or do a line drawing. I might be slow but I'm honest and interested.

If you look back at my pic of the CU you'll see the RCD is mounted on the right hand side of the CU. The live is therefore goes on the left of the RCD as this has to connect to the live bus and the neutral goes on the right. In my case the RCD had been replaced with a more recent model to that originally fitted. The newer CU's (which this was designed for mount the RCD at the left hand end). These therefore need need the RCD designed to have the live on the right hand side to connect to the live bus and the neutral on the left.
As my old style one had been replace with a new style one without the person realising the difference it meant that the RCD had it's live and neutrals reversed which was causing the tripping problem when trying to reset it. The house supply was not reversed - just the RCD. Hope this makes more sense. No problems since!

 

[Squid]

Haven't read the full thread but always reset the board with all breakers in the off position then turn them on one by one as certain circuits may have a high load connected to them,, think PC's firing up as on auto reboot etc etc.

 

[DPG]

Haven't read the full thread but always reset the board with all breakers in the off position then turn them on one by one as certain circuits may have a high load connected to them,, think PC's firing up as on auto reboot etc etc.

Good practice.

 

[Bul]

If you look back at my pic of the CU you'll see the RCD is mounted on the right hand side of the CU. The live is therefore goes on the left of the RCD as this has to connect to the live bus and the neutral goes on the right. In my case the RCD had been replaced with a more recent model to that originally fitted. The newer CU's (which this was designed for mount the RCD at the left hand end). These therefore need need the RCD designed to have the live on the right hand side to connect to the live bus and the neutral on the left.
As my old style one had been replace with a new style one without the person realising the difference it meant that the RCD had it's live and neutrals reversed which was causing the tripping problem when trying to reset it. The house supply was not reversed - just the RCD. Hope this makes more sense. No problems since!

Totally understand now - Thanks Geoff. This has been an extremely interesting thread and really shows the forum at it's best.