Consumer unit, can a fuse blow permanently?

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MisterBoy

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We had a guy doing some work today who turned off a fuse as they needed to move a cable and wanted to be safe. It's in an outhouse so only just realised it was still off, text them and they advised it was fine.
Big flash from the fuse on the consumer unit and now the switch won't go up at all, no 'click' to it. All other fuses are just fine.

This isn't mine but it's this style:
Consumer-Unit.jpg


Which makes me think the fuse has 'gone' except I thought the whole point of these fuses was when they blew it just flipped the switch. How do they work? It's a unit from about 2000 I think.

Don't worry I'm not about to start tinkering, I'm just interested how they work.
 
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They've done something wrong.

The MCB can break when switching or a fault occurs. It's not particularly common though
 
A dead short can cause enough current to blow the MCB (Fuse) permanently, it shouldnt but it does especially with some cheaper ones, sometimes pushing them towards off manages to engage them but unlikely, you may need a new one, but the cause of the short needs locating first.
You may find with the output disconnected it will reset, but not advisable unless your competent.
 
Yeah I'm not worried what the fault itself is, it'll be something obvious as there's only one cable. Nail through a wire or something. I'd just never seen an MCB pop and then do nothing if you tried again.

Is the idea it might have physically moved something so much it has broken inside the MCB then, rather than 'fusing'?
 
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"Chint" is very much a budget brand so I expect it is done for.

There is a way of calculating prospective fault current for a dead short but I have completely forgotten how. It is considered acceptable for an MCB to sacrifice itself in breaking a bad fault. The contacts may be burned, or even welded together, so it needs replacing.

I remember being told that mine are capable of breaking a 15kA fault current. But only once (!)
 
Some Chint seem to have a lower breaking capacity than most, possibly 3000A or 4500A.

https://chint.co.uk/image_dump/EBG Datasheet.pdf

Many Din rail mounted ones are either 6000A or 10000A rated, this will be marked on the front of your Mcb, i believe thats the maximum it should cope with and still work.
 
333rocky333 said:
Some Chint seem to have a lower breaking capacity than most, possibly 3000A or 4500A. .... Many Din rail mounted ones are either 6000A or 10000A rated, this will be marked on the front of your Mcb, i believe thats the maximum it should cope with and still work.
Click to expand...
As I've said before, although I don't doubt there are some (probably in London?), I don't think I've personally ever come across a domestic installation with a PFC (even at the origin of the installation) a lot over 1,000 A, let alone 3,000, 6,000 or 10,000 A.

6000A (which is a very common rating) implies (at 230V) a loop impedance of around 0.038Ω, which (at the origin of the installation) equates to under 14m of 2 x 16mm² conductors back to the DNO's transformer. At least in my house, 14m wouldn't even get beyond the boundary of my property

Kind Regards, John
 
MisterBoy said:
Yeah I'm not worried what the fault itself is, it'll be something obvious as there's only one cable. Nail through a wire or something. I'd just never seen an MCB pop and then do nothing if you tried again.

Is the idea it might have physically moved something so much it has broken inside the MCB then, rather than 'fusing'?
Click to expand...

As the circuit was working before they disturbed my guess would be they missed something. An MCB has a number on the front which is the maximum breaking capacity In amps. In domestic this is normally 6000. Up to 6000 amps should trip the breaker leaving it functional. A current greater than 6000 amps can generate enough heat to self the contacts shut. You need the fault fixing before trying to switch on a new breaker. Even if you manage to get the breaker fixed, I would recommend replacing it. A short circuit can result in hundreds of thousands of amps flowing through the breaker.
 
So we found the problem, when terminating a cable someone had borrowed one of my Wago connectors thinking it was like a chocolate block, when actually the one they grabbed was a shared conductor. I don't think I need to explain further to you gents what happened next :)
I should say it wasn't a sparky, they were simply removing a light to put in a new ceiling so the sort of thing any DIY-er might do... I'm just glad it wasn't me though you lot might understandably be scathing :)

Not sure if the MCB survived yet but they're not pricey as far as I can see.
 
Yeah. New MCD for a tenner from Screwfix, hadn't realised you can just pull and replace them without having to unscrew anything. Nice design.
 
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