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....

 

[littlespark]

What does that sticker say? £5 fine for cutting the seals?
I think I can live with that.

 

[Geoff101]

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 for this - I've now tried this theory, sadly with no success - but may lead uis nearer a diagnosis possibly....

I tried switching off both the IH and oven. They are both on DP isolators so this will have fully removed both their L and N from the circuits. With them removed both the RCD would properly reset with their MCBs on. (they both have their own individual circuits with nothing else connected). However, in both cases, when the isolator was put on again the RCD would not reset for either circuit unless the MCB was off. Also, as before with the oven and IH isolated none of the other circuits in the house would allow an RCD reset unless the MCB was off.

Which I guess for me is again swinging me back to asking the question, do I have a faulty RCD as any sort of a live load on it appears to prevent a reset action but once reset it appears to work fine?

 

[Geoff101]

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 for this - I've now tried this theory, sadly with no success - but may lead uis nearer a diagnosis possibly....

I tried switching off both the IH and oven. They are both on DP isolators so this will have fully removed both their L and N from the circuits. With them removed both the RCD would properly reset with their MCBs on. (they both have their own individual circuits with nothing else connected). However, in both cases, when the isolator was put on again the RCD would not reset for either circuit unless the MCB was off. Also, as before with the oven and IH isolated none of the other circuits in the house would allow an RCD reset unless the MCB was off.

Which I guess for me is again swinging me back to asking the question, do I have a faulty RCD as any sort of a live load on it appears to prevent a reset action but once reset it appears to work fine?

 

[Lucien Nunes]

I thought this correlated to power interruptions but can’t be sure on all of them.

This changes the diagnosis quite significantly. If there have been random trip events while energised, then any or all possible types of fault need to be considered, such as N-E shorts. Further speculation is futile until we have IR measurements and RCD test results etc.

 

[Geoff101]

This changes the diagnosis quite significantly. If there have been random trip events while energised, then any or all possible types of fault need to be considered, such as N-E shorts. Further speculation is futile until we have IR measurements and RCD test results etc.

Although I can’t be 100%, the times it has tripped when we have been around has been related to power outages. They’ve recently been working on power cables in the street and this is when it has started. It has tripped twice when we’ve not been here and will I can’t be sure I’m pretty certain it will have been a power glitch which then tripped in re engergisation which is the part I guess I am reproducing.
It clearly could be n e fault but I’m still thinking if it were a permanent n e problem it would stay tripped and not reset at all. If it was an intermittent n e fault then it would be tripping randomly and also letting me reset it via the rcd. ????

 

[telectrix]

them smart meters are the invention of the devil. just as bad as google earth being able to see what's going on in my bedroom at night. blast it with a 12 bore. see what supplier thinks of that.

 

[marconi]

Re Geoff101 #32: Thank you for reporting back. I think it does take us forward. I reread your opening post and thought about the changes to the MK Sentry CU when you had your extension works - you said 'a new RCD' was fitted - by which I think you mean the original switch disconnector (Main Switch) was replaced by a combined RCD and Switch Connector from the MK78xxs series - this would neatly fit in the space vacated by the Main Switch.

Then I studied the picture of you CU - which is the old set up of Main Switch on the right - nowadays they are on the left - I think it has been standardised to be so. There is a consequence then to simply swapping the MS for the RCD and having the line mcb busbar to the left: it means that the L and N meter tails to the RCD enter to the left and right respectively which is NOT what MK intends for the MK78xxs RCD (and indeed for most other brands). Why? Because like modern switched sockets the neutral contacts are arranged to close first and open last - that aint going to happen in your CU. The line contact is made first and then the neutral afterwards. This will play havoc with the transient potential differences between the installations Line to Earth conductors and Neutral to Earth conductors. So, as I said in one of my earlier emails, in earthed neutral systems switching should be such that the neutral is made live first (by connecting to the supply N) and then the installation Line made live and energised to mains potential.

So, your installation's RCD I believe is satisfactory - but needs to proven to be so nevertheless - it is simply responding to the reversed order of Line and Neutral RCD switching and the effect this has on brief transient leakage currents through L-cpc and N-cpc of the final circuits . By turning off all the mcbs (and the oven, cooker, IK isolators) and then resetting the RCD the final circuits are not being energised to mains potential until the RCD L and N contacts have completed making. Then once final circuits have their neutrals referenced to earth potential through low conductance paths the closing of their mcb and energisation does not cause a residual current problem the RCD cannot tolerate. Simply put for a given final circuit all the wiring of it leaps to 240V with respect to earth and then the neutral side drops to neutral/earth potential. When correctly switched, only the line side leaps to 240V. Much more transient leakage occurs in the former case than the latter.

So, I reckon this is a possible reason why the replacement of the Main Switch for an RCD/Disconnector has inadvertently introduced the problem you describe. The solution is to arrange for the correct order of switching of L and N by the RCD.

 

[Geoff101]

Hi Marconi, I think you might have cracked it!
It did previously have an rcd. Not sure if the exact model but I recall it was 4 modules wide similar to this pictured.
Rather than change the cu when we needed extra ways for the extension I remember the spark telling me he was changing the rcd to a later version to free up extra ways which is logical. By changing to the 7880 we had 2 extra ways for MCBs. However the diagram on the old one pictured does appear to show the live on the left where as the current one shows it on the right. As you point out, that would explain why it functions perfectly once reset but struggles to reset with any load. Thanks!
The next question of course is how to solve it in a simple way! I’ve attached a pic showing the inside of the current cu.
1) simple but fear not likely is does anyone know of an rcd of this size that has the live on the left? Even in a past model? Then I could do a straight swap.
2) I assume that this rcd can be put on the left and I can shuffle the MCBs along to the right to make space. However, that obviously requires moving the neutral bus to the left with it which looks fun, and whether there is enough neutral cable on all the circuits to do this is debatable.
3) I could obviously go the whole hog and put a completely new cu in.
4) I guess a slightly bodged approach would be to replace the rcd with a plain isolating switch and put a stand alone rcd in the tails before the consumer unit. Seems messy but would it meet requirements?

What would you do?!

Any ideas greatfully received!

 

[Geoff101]

A further query I’ve just thought of....
The initial problem with prompted this was the rcd sometimes tripping after a power interruption. Would this recevrsed layout lead to this or not. Once the rcd is actually switched is there any difference in terms of whether it’s l and n are reversed? In this case it’s not actually switching as it is already on. Will reversal affect it’s proper operation?
If not then is it possible that we’ve just been unlucky in that the last few power cuts have been accompanied but some sort of surge no reconnection which has tripped the rcd?
I certainly know that one was accompanied by a significant surge as it killed a wireless thermostat and a heated towel rail timer in one go!

 

[Dave OCD]

Geoff ,a BG 80a/30ma RCD is arranged with the neutral on the right, they are also reasonably priced and I've never had an issue with them. However this is just for information as I'm not condoning fitting different makes of device in a CU, the regs frown upon it.

 

[Geoff101]

Been doing some searching and see that this Hager rcd which is the same spec as the mk on fitted has the neutral on the right which would save a whole load of pain of switching it to the other end of the cu.

80 Amp 30mA RCCB RCD Double Pole Trip Switch Circuit Breaker 80A Hager CD280W 3250611645085 | eBay - https://www.ebay.co.uk/itm/80-Amp-30mA-RCCB-RCD-Double-Pole-Trip-Switch-Circuit-Breaker-80A-Hager-CD280W/301759392855?epid=1826248222&hash=item464242ec57:g:E0IAAOSwetxa6d~N

It’s states it a standard din fitting. Can anyone confirm if it will fit the sentry cu? Does a swap to this sound like a sensible and ok solution?

Thanks

 

[marconi]

My last post is still only theory until an electrician makes the re-arrangement and you see how everything operates. I stand by my initial recommendation of dividing up the final circuits over 2 or more RCDs/RCBOs and introducing some spare ways at the same time. You run at risk of nuisance tripping by protecting all final circuits by one RCD because of accumulated earth leakage eating into the residual current trip level of 30mA.

I looked to see if you have any electrician's qualification but nothing was posted. Assuming you are not it is time to employ one - if I am wrong then forgive me. EF members can advise you on someone local to you who 'cuts the mustard' and will give you some options and prices.

Yes, I think unusual electrical disturbances may have caused trips and the effects of some mains disturbances may be exacerbated by accumulated earth leakage and only having the one RCD.

 

[Geoff101]

Geoff ,a BG 80a/30ma RCD is arranged with the neutral on the right, they are also reasonably priced and I've never had an issue with them. However this is just for information as I'm not condoning fitting different makes of device in a CU, the regs frown upon it.

Ah thanks. I just saw this after I posted the reply about the Hager one too. I might have a look at this too. Sounds like a simple solution. I still cant work out what the prob is with a correctly fitting and Proper specced module in a different case is but there you go. Thanks

 

[Dave OCD]

I agree Geoff, the only thing to watch out for is that the outgoing busbar and neutral connections line up nicely without having to force anything - some devices have the cage clamps arranged at different depths in relation to the din rail. And this isn't a new installation, could well be a maintenance job.

 

[Geoff101]

My last post is still only theory until an electrician makes the re-arrangement and you see how everything operates. I stand by my initial recommendation of dividing up the final circuits over 2 or more RCDs/RCBOs and introducing some spare ways at the same time. You run at risk of nuisance tripping by protecting all final circuits by one RCD because of accumulated earth leakage eating into the residual current trip level of 30mA.

I looked to see if you have any electrician's qualification but nothing was posted. Assuming you are not it is time to employ one - if I am wrong then forgive me. EF members can advise you on someone local to you who 'cuts the mustard' and will give you some options and prices.

Yes, I think unusual electrical disturbances may have caused trips and the effects of some mains disturbances may be exacerbated by accumulated earth leakage and only having the one RCD.

Thanks, yes noted. I just like to have a bit of an idea about the options and potential solutions before I engage someone. Thanks for all your help everyone.