MCB Failure or something else ?

[bracknelldiy]

Hi, apologies if the answer is already elsewhere on the forum but I could not immediately find it.

Main bedroom sockets is its own circuit and MCB (it was built as an extension post house build). Short story: none of the sockets are working with the MCB lever up. Longer story: I was about to do some work on the lights in the room. Plugged a lamp into one of the sockets (it worked). Switched off the MCB at the consumer board. Realised I switched off the Extn Skts MCB not the Extn Lights MCB, and switched it back on. Then found the sockets were US.

Can MCBs "fail" like this ? I have tried putting the lever up and down again a couple of times - yes I know that sounds a bit 'desperate' but so does coming on here and then being told I'm an idiot for not trying that first

If they can fail, how do you check to get a compatible MCB to replace it ? I ask that, because its an Alto CU and a quick search is failing other than hits on flea bay. I have noticed some comments on the forum about not using MCBs other than from the CU manufacturer.

This is potentially quite a problematic situation for us: we are having the bedroom ripped out and replaced*, with electrician coming in to do some preparatory electrical works. The works due to start in a weeks time and is going to be problematic if there is another problem to sort out first. I also have only a little spare time between now and then due to unfortunate circumstances.

*this is why I was about to do some work on the lights: removing the 'chandalier' lights and put in temporary pendant lights so they are not a hazard/get whacked with boards etc.

 

[Taylortwocities]

First off, the B16 MCB is in the wrong half of the board. It should be on the RCD-protected side. Have your electrician remedy that when he comes.

Next, before condemning the MCB, is to check FOR CERTAIN that there is no volts coming out of the MCB. Do you have any sort of voltage test gear? If you do, remove the front cover of the CU and test if there is 230v at the top terminal of the MCB.

If you dont have any thing that tests voltage then you are a bit like having a puncture in your car tyre and not owning a wheel spanner.

A very dirty way to do this: turn the main switch off on the CU (thats the big one on the left)
There should be only one wire in the top of the B16 MCB. Move that to the B10 (lighting) MCB. Turn everything back on and test your sockets using a small load (a light eg). If the sockets now work, then the MCB is knackered.
Put everything back and wait for your electrician.
Dont buy a replacement MCB, the sockets should have been provided with RCD protection, so you will probably need an RCBO.

 

[Murdochcat]

MCBs can fail but usually the movement of the lever often gives a hint.

Have you got any test kit ?

As for the lack of RCD protection - when was the extension done ?

 

[Adam_151]

MCBs can fail, and while they can fail in the way you describe its probably not that common and generally a result of the board having large holes on top and being installed near a building void where dust is falling down and somehow makes it way into the MCB and stops the contacts fully closing (It sounds very unlikely, and probably is, but I've seen it happen)

Normally when they go faulty mechanically, they go limp and they unlatch before reaching the on position when every you try and push them up.

More likely possibilities in your case...
1) That sockets MCB is installed on the non-RCD side of the board, its possible there is another RCD in some form between it and the sockets
2) Sometimes when MCBs are installed, the busbar tab ends up behind the cage terminal when in it, its possible that this has happened, especially if the MCB was added after.

Thats an alto brand consumer unit, probably about 15-20 years old old. Its effectivly a wylex unit which has been branded up for WF Senate/Wades. I personnally wouldn't have a problem with the 'correct' generation/shape wylex MCB in there. Others might.

 

[bracknelldiy]

First off, the B16 MCB is in the wrong half of the board. It should be on the RCD-protected side. Have your electrician remedy that when he comes.

Next, before condemning the MCB, is to check FOR CERTAIN that there is no volts coming out of the MCB. Do you have any sort of voltage test gear? If you do, remove the front cover of the CU and test if there is 230v at the top terminal of the MCB.

If you dont have any thing that tests voltage then you are a bit like having a puncture in your car tyre and not owning a wheel spanner.

A very dirty way to do this: turn the main switch off on the CU (thats the big one on the left)
There should be only one wire in the top of the B16 MCB. Move that to the B10 (lighting) MCB. Turn everything back on and test your sockets using a small load (a light eg). If the sockets now work, then the MCB is knackered.
Put everything back and wait for your electrician.
Dont buy a replacement MCB, the sockets should have been provided with RCD protection, so you will probably need an RCBO.

1. Yes I have a multi meter. I also have a flat head screwdriver voltage tester - which I see in the pinned posts get a bit of a panning! But is what i would typically use when changing switches/sockets etc. to do voltage checks). Quick question, and bearing in mind I haven't yet looked inside the CU so am asking from a position of ignorance: if I'm placing one of the test leads to the top of the MCB, where should I expect to be placing the other ? (I actually normally use the multi meter for DC work, it lives in the motorhome outside...).

1b) Should I be switching of the CU (the switches on the left) before removing the front panel, making myself 'familiar' with what is inside and then switching back on before doing voltage check (I suspect the answer is no need to).

2. Get the idea of switching the out line of the B16 to the the lighting MCB and then putting a light load (no two bar electric fire !) on to it. Very cool. But I'll go with the direct voltage testing method.

3. I had noticed the position of that MCB on the LHS before. I had wondered whether that meant that it was not RCD protected, which would seem odd. If all the slots on the RCD protected side are taken up, can an RCBO be fitted to the slot it is in now, or are we talking need for fitting of a new CU ? [from my quick look on the net, I'm thinking the answer is yes, but...]

 

[bracknelldiy]

MCBs can fail but usually the movement of the lever often gives a hint.

Have you got any test kit ?

As for the lack of RCD protection - when was the extension done ?

No hint from the lever: it feels mechanically sound.

Not 100% certain when the extension was done. but my memory is house build 1986 and the extension done a remarkably short time afterwards (like maybe 87/88). The extension is essentially a top floor extension further over the garage.

I have multimeter.

 

[chivers67]

Place one test probe on Earth and one on the Live and you should get 230v on AC . I don't think you can get an RCBO for that? might be able to get a longer bus bar and modify the board. Take the cover off carefully and take a photo of the whole board. buy yourself a proper non contact volt stick those Neon screwdrivers edit CAN be dangerous

 

[Taylortwocities]

1. If u using a multimeter it needs to be set to 500v AC. One prod on MCB, the other on earth.
You could use your test screwdriver. Try it one of the working MCBs first to check it works. Then try the suspect one.
If u not familiar with the insides of a CU, turn off the main switch before u remove the cover!
2. Tick
3. Your electrician can advise. It looks like a metal CU, so probsbly just swop the 4 MCBs on the left to RCBOs may get it up to date.

 

[Murdochcat]

No hint from the lever: it feels mechanically sound.

Not 100% certain when the extension was done. but my memory is house build 1986 and the extension done a remarkably short time afterwards (like maybe 87/88). The extension is essentially a top floor extension further over the garage.

There were no requirements for RCDs in the 1980’s

 

[Taylortwocities]

those Neon screwdrivers are dangerous

Umm. I’m not a fan either. They CAN be dangerous, ARE dangerous? If they were then they would be banned.

 

[bracknelldiy]

MCBs can fail, and while they can fail in the way you describe its probably not that common and generally a result of the board having large holes on top and being installed near a building void where dust is falling down and somehow makes it way into the MCB and stops the contacts fully closing (It sounds very unlikely, and probably is, but I've seen it happen)

Normally when they go faulty mechanically, they go limp and they unlatch before reaching the on position when every you try and push them up.

More likely possibilities in your case...
1) That sockets MCB is installed on the non-RCD side of the board, its possible there is another RCD in some form between it and the sockets
2) Sometimes when MCBs are installed, the busbar tab ends up behind the cage terminal when in it, its possible that this has happened, especially if the MCB was added after.

Thats an alto brand consumer unit, probably about 15-20 years old old. Its effectivly a wylex unit which has been branded up for WF Senate/Wades. I personnally wouldn't have a problem with the 'correct' generation/shape wylex MCB in there. Others might.

Interestingly it is in a building void and dust could potentially get in I think. Whether that could be the cause or not is another matter.

Mechanically it feels very sound.

An RCD on the other side of the circuit between it and the sockets is a very interesting (and PITA) thought. And sounds quite feasible given its not on the RCD side of the board. I wonder where on earth to start looking. If someone was fitting a separate RCD, where might one think about putting it ? I mean, it would have to be put in a place you would expect to look for it and be able find it.

By the way, I would have times previously switched that MCB off/on as I've changed the socket plates in that room.

 

[Adam_151]

I'd like to think most people would fit an external RCD near to the board if theres room, bear in mind its possible an RCD fused spur was used https://www.toolstation.com/safetysure-rcd-spur-white/p45636?gclsrc= These were often a favourite for conservtaories when the house wasn't RCD protected and they were extending existing circuit, and might be found local to the conservatory in that case, but I would doubt anything like that would be in a non obvious place in your situation

 

[Taylortwocities]

No telling where an RCD might be. If it were me I would have it adjacent to the CU.

 

[bracknelldiy]

1. If u using a multimeter it needs to be set to 500v AC. One prod on MCB, the other on earth.
You could use your test screwdriver. Try it one of the working MCBs first to check it works. Then try the suspect one.

Many Thanks. Yes, when using an (apparently not very liked) test screwdriver, if I only get a "not-live" from it I always go test it on something I know defo should be. Never had one fail yet, but also don't want to get fried.

 

[bracknelldiy]

I'd like to think most people would fit an external RCD near to the board if theres room, bear in mind its possible an RCD fused spur was used https://www.toolstation.com/safetysure-rcd-spur-white/p45636?gclsrc= These were often a favourite for conservtaories when the house wasn't RCD protected and they were extending existing circuit, and might be found local to the conservatory in that case, but I would doubt anything like that would be in a non obvious place in your situation

Thanks. There is definitely nothing like that fitted in anything remotely a "likely to find" place. We've been in the house for 13 years so it would probably have been unearthed by now. I'm going to test the feed out from the MCB tomorrow morning, and see what that is telling me.