Consumer Unit - replacing RCCB/RCD and MCB

nheather

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Is it a fairly straight-forward thing to change an RCCB/RCD in a consumer unit.

I'm getting a bit fed up with mine. The consumer unit is as old as the house - about 25 years - and has the original RCCB and MCBs.

At least once a day the RCCB will trip. I've been in the house when it does it and there is nothing that I can associate the tripping with.

I've also noticed that none of the MCBs trip but when I reset the RCCB it immediately causes the MCB on one of the two ring mains to trip - always the same one. But when I reset that MCB, everything is good until the next day or two.

I'm wondering whether the problem is just that the RCCB is old and is false triggering. If there is a problem with the wiring it is going to be a nightmare to track down because 99% of the time everything is fine and I can't match the tripping with any device turning on or off.

So thinking of just changing the RCCB and see if that resolves the issue - if not I can then call an electrician in.

Are they all standard fitting or do I need to find one that matches the rail in my particular consumer unit.

Cheers,

Nigel
 
I think you know the answer but I think you need a sparky to investigate, and change the unit and the same time.

When a friend did mine, he changed the board and then used a device to check all of the circuits for faults before he signed off on it. How it doesn't drive you insane tripping every day I'll never know.
 
Hi nheather, I'm an electrician.

RCD's are designed to trip because of Live/neutral to earth faults. They can also become faulty. Typically its because of an appliance (as yours seems to be tripping on one of your ring final). Unless you've already tried, you can unplug all of your appliance not being used, to see if there is any routine to the tripping.

You could also have a fault in the wiring, that when a high load appliance is used for example, can cause the tripping.

I would not recommend a DIY approach; you'll need competent electrician and a multi function tester, to test the circuits and the RCD itself. If you replace the RCD, that would need testing to ensure it functions correctly.

It the past, I've located similar faults to dodgy appliances, like dishwashers & fridge freezers, crushed cables behind faceplates, outside luminaire full of water, accumulative earth leakage & faulty RCD's, amongst other things. It does take a methodical approach to locate the fault.

If you have an aging CU and one RCD, you might wish to consider replacing it with something newer with dual RCD's, or even individual RCBO's for each circuit. Such an install will lessen the inconvenience of the whole house being isolated by such faults.
 
Thanks.

Yes I am considering getting the whole thing changed. The populated CUs are pretty cheap. My fear is that I will get an electrician in who can't find anything wrong - pay a few hundred to get the CU replaced only to find it is still tripping.

There is nothing obvious causing it to trip. Certainly not the kettle, microwave, washing machine or dryer.

It will trip when the house is empty so there isn't really much that would be running.

Fridge freezer and the central heating are the things that would spring to mind - but unplugging the fridge freezer for a day or two is not really an option.

What can an electrician tell if the fault is only happening once every day or two but perfectly fine in between.

Also had a look inside my CU - just took the top off, no more and my RCCB has a strange lump on the top that I don't see in any RCDs that Screwfix or TLC sell.

RCCB.jpg


Any ideas what it is - this isn't mine - just a photo I grabbed off the internet.

Cheers,

Nigel
 
If you get an electrician in, who'll replace your CU without any prior testing, especially as your are experiencing issues, then he'll tied his horse up outside somewhere and you need to show him the door.

Fridge freezer is a good culprit, the compressor kicking in & out and not always replicating the fault. You could get someone in to PAT test your appliances. I've earth clamped domestic properties, just to see what earth leakage is going on.

In your current install, is it just the sockets that are on the RCD? If so, could you temporally plug the fridge freezer, via extension lead into your cooker control switch, if you have one?

If you employed me to replace your CU, I would like to find the fault first (at your expense first).
 
So if I got someone like you in and said.

Here is my CU - it's 25 years old as are the modules.
Every day or two at no set time, the RCCB trips.
Happens whether there are people in the house or not - in other words trips when most devices not being used.
When I find it tripped it is just the RCCB, none of the MCBs have tripped.
When I reset the RCCB, the MCB on the ring main immediately trips - same MCB every time.
When I reset the MCB it stays set as does the RCCB and all will be fine for another day, two or more when it will repeat.

What would actually do? Serious question, in my mind if everything is working fine until it trips in a day or two won't any measurements you take say 'all is good" - in other words how would you detect a fault that only happens every day or two?

Cheers,

Nigel
 
I had a CU that tripped and when we had our kitchen done the new induction hob would trip everything out as soon as it was switched on. 3 different sparkies came out and tested the circuit. All confirmed that it was fine and the induction hob must be at fault. AEG sent out an engineer who couldn't fix the hob and they replaced it. Sparkie came back and wired it back in. Tripped the house out. New consumer unit fitted. No more tripping. Electricity is magically weird shit.
 
When I turn the light out in the garage it disconnects my Sky Q box in the living room, it doesn't put it in standby, the green light is still on but it says no signal, it's very strange :confused:
 
So if I got someone like you in and said.

Here is my CU - it's 25 years old as are the modules.
Every day or two at no set time, the RCCB trips.
Happens whether there are people in the house or not - in other words trips when most devices not being used.
When I find it tripped it is just the RCCB, none of the MCBs have tripped.
When I reset the RCCB, the MCB on the ring main immediately trips - same MCB every time.
When I reset the MCB it stays set as does the RCCB and all will be fine for another day, two or more when it will repeat.

What would actually do? Serious question, in my mind if everything is working fine until it trips in a day or two won't any measurements you take say 'all is good" - in other words how would you detect a fault that only happens every day or two?

Cheers,

Nigel

This sort of fault can present itself from a brand new install or it can be a simple as the RCD is old & tired and is tripping lower than the typical >21mA, and/or you have a cumulative earth leakage from your appliances. Its difficult to fathom the RCD reset, MCB trips thing. RCD is earth leakage, MCB is overcurrent, overload or over current (short circuit). It could be you have an appliance that has a fault thats manifesting itself in both aforementioned ways.

I had a dishwasher that had a design fault, where some of the front panel wires were shorting against metal casing. It was tripping my RCD, which I identified after isolating the appliance. Possibly, in your scenario where you are resetting the RCD and holding in the on position, the MCB is next in line to trip (over current).

To identify the fault, you have to rule out the causes. First stop is ensuring the house wiring is without fault which includes your current RCD, then appliances. Without suitable test equipment and knowledge, you are just guessing, as I am here.
 
My new CU has a MCB called garage, which funnily enough feeds electric to 2 MCB CU in the garage. My spark ran 2.5 SWA from the 20A MCB to my summer house. There is a fused spur from the sockets in the summer house to the lights. I was fitting soffit lights at the weekend and with the 20A switched off so no power to the summer house but wither fused spur still ‘on’ when I cut the cable connected to the light switch the main breaker in the house tripped. When I went all the way back down the garden to the house to reset it, it would stay on. I had to go back to the summer house and turn the spur ‘off’ for it to reset.
 
Firstly the fault should be found. Probably a fridge or freezer, as condensation inside can cause a leakage to earth causing the RCCB to trip. If the MCB trips then it is caused by overcurrent or short. It could be that the compressor start current is causing the problem.

I would recommend getting a new CU unit with RCCBs for all circuits. The price of RCCBs has come down now to a reasonable level. If one circuit has a fault, that will trip and leave the rest of the house on.
 
This sort of fault can present itself from a brand new install or it can be a simple as the RCD is old & tired and is tripping lower than the typical >21mA, and/or you have a cumulative earth leakage from your appliances. Its difficult to fathom the RCD reset, MCB trips thing. RCD is earth leakage, MCB is overcurrent, overload or over current (short circuit). It could be you have an appliance that has a fault thats manifesting itself in both aforementioned ways.

I had a dishwasher that had a design fault, where some of the front panel wires were shorting against metal casing. It was tripping my RCD, which I identified after isolating the appliance. Possibly, in your scenario where you are resetting the RCD and holding in the on position, the MCB is next in line to trip (over current).

I understand that but it doesn’t make sense in my case.

When I’m at home, I will get to the RCCB in under a minute, I reset it and instantly the MCB goes. A few seconds later I reset the MCB and it is fine. So if there was overcurrent situation it disappeared as soon as I reset the MCB.

When I’m not at home, I will get to the RCCB hours after it has tripped, I rest it and instantly the MCB goes. I reset the MCB and it is fine. That would suggest the overcurrent situation has hung around for a few hours waiting till I got home, but then disappears as soon as I rest the MCB.

Personnally I think the whole CU is getting tired and is just behaving oddly. So replacing it with a new one seems sensible but a little confused by your suggestion that an electrician who can’t find a fault but continues with replacing the CU is a bit of a cowboy.

Cheers,

Nigel
 
It could be a wiring or appliance fault and nothing to do with the CU or the RCCB/RCD in it.
 
It could be a wiring or appliance fault and nothing to do with the CU or the RCCB/RCD in it.

Absolutely, but it is a very weird one.

An overload is either present permanently (like something fully shorted out), or when certain conditions are met (like when the heating element in a washer dryer is turned on).

Mine isn’t permanent because it works fine for 99% of the time and I would not be able to reset the MCB.

But if it is tripped because of a certain condition it is very weird - because the second I reset the RCCB, whether I do it immediately or wait four hours, the MCB trips, but the second I reset it everything is fine. If someone can think of a device fault that could do that I’d welcome the suggestions because I can’t. More inclined to think it is some strange interplay within the aging CU.

Now the RCCB tripping is a different matter. That could be caused by some device, but all the same very curious.

It can happen when there is no one in the house which limits the devices that it could be, because most are off. So we have things like the fridge/freezer and the boiler - they are the only devices truely operating. Then you have things in standby, TVs, set-top boxes, chargers - it would be unusual for them to be the cause.

And then there is the fact that it can go by for days without a trip.

Cheers,

Nigel
 
Personnally I think the whole CU is getting tired and is just behaving oddly. So replacing it with a new one seems sensible but a little confused by your suggestion that an electrician who can’t find a fault but continues with replacing the CU is a bit of a cowboy.

Cheers,

Nigel

Thats because there's no point in progressing with anything, until the fault is looked into. Why bother replacing a CU with one RCD, to a dual one or RCBO's. One of them is still going to trip.
 
My new CU has a MCB called garage, which funnily enough feeds electric to 2 MCB CU in the garage. My spark ran 2.5 SWA from the 20A MCB to my summer house. There is a fused spur from the sockets in the summer house to the lights. I was fitting soffit lights at the weekend and with the 20A switched off so no power to the summer house but wither fused spur still ‘on’ when I cut the cable connected to the light switch the main breaker in the house tripped. When I went all the way back down the garden to the house to reset it, it would stay on. I had to go back to the summer house and turn the spur ‘off’ for it to reset.

Thats because the circuit still has the neutral unswitched. Cutting a cable in these circumstance, i.e. connecting neutral to earth, can cause the RCD to sense the imbalance between the two live conductors. If you had turned off the fused spur, both live conductors would have been isolated from the RCD.
 
Absolutely, but it is a very weird one.

An overload is either present permanently (like something fully shorted out), or when certain conditions are met (like when the heating element in a washer dryer is turned on).

Mine isn’t permanent because it works fine for 99% of the time and I would not be able to reset the MCB.

But if it is tripped because of a certain condition it is very weird - because the second I reset the RCCB, whether I do it immediately or wait four hours, the MCB trips, but the second I reset it everything is fine. If someone can think of a device fault that could do that I’d welcome the suggestions because I can’t. More inclined to think it is some strange interplay within the aging CU.

Now the RCCB tripping is a different matter. That could be caused by some device, but all the same very curious.

It can happen when there is no one in the house which limits the devices that it could be, because most are off. So we have things like the fridge/freezer and the boiler - they are the only devices truely operating. Then you have things in standby, TVs, set-top boxes, chargers - it would be unusual for them to be the cause.

And then there is the fact that it can go by for days without a trip.

Cheers,

Nigel

I understand how frustrating such faults can be, but you will need to have your installation inspected & tested by a competent electrician, with a multi function tester or testers & earth leakage clamp meter. If it is an appliance fault which it might be in your case, it may be difficult to locate. As suggested, try unplugging appliances where possible, relocating suspected appliance, and see if a pattern develops.

Where you are resetting the RCD & mcb trips, the fault has been presented identified by the RCD. When you reinstate the supply by resetting the RCD, the fault is still present and the MCB trips (you are holding the RCD In place). You may have some wiring in an appliance thats is shorting against metal casing (as I experienced), the electrical contact & reaction, might just be temporarily moving the wiring away from the casing, until the next time (appliance ) movement brings it back into contact (just a theory).
 
Before my CU was replaced it would take 3 resets of the MCB before it would stay on.

That’s why I’m inclined to beleive it’s the CU, though that doesn’t mean I have ruled out other causes.

But I can’t think of any device or wiring fault which would result in what I’m seeing with the RCCB and MCB.

My only hesitation is that I reckon it will cost £400 or more to get the CU replaced, money well spent if that is the cause (90%) but a partial waste of money if it doesn’t fix the problem (10%).

But with a fault that is so sporadic and relatively infrequent I can’t picture any tests that an electrician could do to detect it. All I can imagine doing is replacing the CU and see what happens.

Cheers,

Nigel
 
When the RCCB trips can you unplug everything before you reset it to see if the MCB stays on in that instance...
 
When the RCCB trips can you unplug everything before you reset it to see if the MCB stays on in that instance...

It would be a hell of an undertaking to physically unplug everything in the house - some of the wall plugs are dificult to get out.

I’d be more inclined to do it if I could at least half envisage a scenario that cold cause it.

I can’t get over, MCB doesn’t trip until I reset the RCCB. And then immediately I reset the MCB it is fine. That’s saying that the overload only exists at the moment I reset the RCCB which doesn’t make sense.

What I will try later today, is with everything turned off (but not disconnected), use the test button on the RCCB to trigger it. Then reset it and see if the MCB immediately trips. If it does surely that would point to something strange happening in the CU.

The other thing I could try at the weekend is to disconnect the feed from the MCB so it isn’t connected to anything and then see if it trips.

Or I might try swapping the two MCBs around and see if the tripping stays with the circuit or moves with the MCB.

Cheers,

Nigel
 

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