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

Geoff101 · Nov 21, 2018

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....

Deleted member 9648 · Nov 21, 2018

I've come across the problem a few times over the years where the RCD (If I recall always a 4293 not a 61008) will only reset with all mcb's off. My recollection is it has invariably been a N-E fault, I cant remember having to change an RCD to rectify the problem. That said it's been a while and my memories not what it was......

Geoff101 · Nov 21, 2018

No, it used to have an rcd but I think it was changed when the work was done. Could it have had a wider rcd which was changed for a narrower one to open up more ways on the board?
I recall we needed addional ways and I think this is how it might have been achieved without changing the cu. make sense or not?

Geoff101 · Nov 21, 2018

No, it used to have an rcd but I think it was changed when the work was done. Could it have had a wider rcd which was changed for a narrower one to open up more ways on the board?
I recall we needed addional ways and I think this is how it might have been achieved without changing the cu. make sense or not?

markythesparky · Nov 21, 2018

Yes that’s possible with that fuseboard although if your electrician has changed that rcd, it doesn’t comply with regulations.

There are regulations regarding discrimination which your installation falls short of. He/ she should not have issues certs staying that it complies.

However, getting back to the troubles at hand, it may be prudent to unplug everything within the house, leave the mcb’s on and then try and reset the rcd just to see if it’s an appliance or a number of appliances causing the issue. The start up of a lot of things at once may cause an issue.

Just trying to narrow it down!

If you eliminate the impossible, then all that remains, however improbable, must be the truth!

marconi · Nov 22, 2018

Alas and alack, simply opening a single-pole mcb does not disconnect the neutral wiring. So, with any but only one mcb turned on and then closing the double pole RCD all the final circuits line and neutral circuitry are in play as far as electrons are concerned including current paths to earth and especially transient currents as these circuits are made live but not energised to full mains(240V) potential - apart from the circuit of the closed mcb.

My simple experiment would be to disconnect the L, N and cpc of the induction hob circuit at the CU ; it's my number one suspect for your problem because of its in-line power EMI filters and how they respond to transient voltages between L and N and L and E and N and E. Then, with all mcbs closed try re-energising the board - I suspect the problem will go away. Put the induction hob on its own rcbo. If not I think you will find a number of mcbs can now be closed and then energised. Final circuits which might give you a problem will have white goods, microwaves and some types of computer equipment which use the earth/cpc for EMI filtering purposes.

Herein lies a problem - the sole RCD is also the Main Switch. What needs to be arranged is a Main Switch which supplies the RCBO for IH and the RCD for all the remaining mcbs. I don't think you have room fit an a new Main Switch and an new RCBO unless one combines some final circuits. If it was me I would replace the CU with a Main Switch and dual RCD board plus RCBO for IH or plump for the top end aka Murdoch solution - Main Switch and all RCBOs for final circuits.

(Please take a photo of your cut out, earthing bar and meter arrangements so we can try and discover the earthing system in use.)

DPG · Nov 22, 2018

Good post from Marconi.

And from now on the all RCBO board will be known as the 'Murdoch Solution'. Brill.

littlespark · Nov 22, 2018

Yeh Macaroni’s posts are always full of big long technical words. Unlike some. Eh Buzz

Hopefully you can find the culprit.
Turn off the power. Do all the street lights outside go off too?

Geoff101 · Nov 22, 2018

marconi said:
Alas and alack, simply opening a single-pole mcb does not disconnect the neutral wiring. So, with any but only one mcb turned on and then closing the double pole RCD all the final circuits line and neutral circuitry are in play as far as electrons are concerned including current paths to earth and especially transient currents as these circuits are made live but not energised to full mains(240V) potential - apart from the circuit of the closed mcb.

My simple experiment would be to disconnect the L, N and cpc of the induction hob circuit at the CU ; it's my number one suspect for your problem because of its in-line power EMI filters and how they respond to transient voltages between L and N and L and E and N and E. Then, with all mcbs closed try re-energising the board - I suspect the problem will go away. Put the induction hob on its own rcbo. If not I think you will find a number of mcbs can now be closed and then energised. Final circuits which might give you a problem will have white goods, microwaves and some types of computer equipment which use the earth/cpc for EMI filtering purposes.

Herein lies a problem - the sole RCD is also the Main Switch. What needs to be arranged is a Main Switch which supplies the RCBO for IH and the RCD for all the remaining mcbs. I don't think you have room fit an a new Main Switch and an new RCBO unless one combines some final circuits. If it was me I would replace the CU with a Main Switch and dual RCD board plus RCBO for IH or plump for the top end aka Murdoch solution - Main Switch and all RCBOs for final circuits.

(Please take a photo of your cut out, earthing bar and meter arrangements so we can try and discover the earthing system in use.)

Thanks. Really helpful. I’ll try that tomorrow when I get time.
In terms of following your advice for the future set up and keeping things as simple as possible would it be in order to add a main switch which then fed the existing consumer unit and also split to feed a new unit with single RCBO to feed the IH. Obviously then moving the IH circuit from the old CU and leaving a spare way?
Prob not the neatest way of doing it but would it meet requirements?

[ElectriciansForums.net] Can't reset the RCD with any MCB's on - Faulty RCD?

Deleted member 9648 · Nov 22, 2018

You also have a standard bonding clamp connecting your main earth to the cable sheath. This is not acceptable as a means of connection and can be dangerous as it will compress the cable. Do not disturb it, and contact your supplier to arrange for a proper clamp to be installed.

Wilko · Nov 22, 2018

Picking up on Wirey's thought - I hope there is a connection back up to the Consumer Unit somehow. I could be totally incorrect, but I see the 2 conductors going down (to gas and water perhaps) but none coming up with the meter tails. Is there one in that trunking perhaps?

Geoff101 · Nov 22, 2018

Geoff101 said:
Thanks. Really helpful. I’ll try that tomorrow when I get time.
In terms of following your advice for the future set up and keeping things as simple as possible would it be in order to add a main switch which then fed the existing consumer unit and also split to feed a new unit with single RCBO to feed the IH. Obviously then moving the IH circuit from the old CU and leaving a spare way?
Prob not the neatest way of doing it but would it meet requirements?

View attachment 45909

View attachment 45910

Following my previous post maybe something like this and consider moving both the IH and oven on this with RCBOs in it?

I Wylex NM206/63 | 2 Way Consumer Unit | Fast UK Delivery | Buy Now - https://www.electricpoint.com/wylex-nm206-63-2-way-63a-main-switch-metal-consumer-unit.html?utm_source=googlebase&utm_medium=datafeed&utm_campaign=google-shopping&gclid=Cj0KCQiAxNnfBRDwARIsAJlH29DCxq2ooi0EIAXSIGX2hcWs2gkE2dsm1F58XDREtFkBLCZmw_lrIOkaAnITEALw_wcB

Geoff101 · Nov 22, 2018

wirepuller said:
You also have a standard bonding clamp connecting your main earth to the cable sheath. This is not acceptable as a means of connection and can be dangerous as it will compress the cable. Do not disturb it, and contact your supplier to arrange for a proper clamp to be installed.

Interesting. This was fitted by the supplier about 4 months ago.....!

Geoff101 · Nov 22, 2018

Wilko said:
Picking up on Wirey's thought - I hope there is a connection back up to the Consumer Unit somehow. I could be totally incorrect, but I see the 2 conductors going down (to gas and water perhaps) but none coming up with the meter tails. Is there one in that trunking perhaps?

It’s in the trunking....

Silly Sausage · Nov 22, 2018

Ooooh, is that a 'smart meter' I see before me?
There's a recent thread along a similar line of RCDs tripping in proximity to SMs.
Is there any correlation here?

Geoff101 · Nov 22, 2018

Archy Styrigg said:
Ooooh, is that a 'smart meter' I see before me?
There's a recent thread along a similar line of RCDs tripping in proximity to SMs.
Is there any correlation here?

Interesting! It’s always tripped on reset but only recently has been tripping at other times. I thought this correlated to power interruptions but can’t be sure on all of them. The smart meter has only been there a few months so there is a possible connection..... how to tell though.....?!