Blown MCB

DogsBody

DogsBody

DogsBody

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Joined
6 Aug 2010
Posts
941
Location
Ashbourne, Derbyshire
I have an armoured cable running to my shed to a standard garage sized fuse box, lights running off a 6A MCB and sockets running off a 32A MCB.

Today randomly the sockets stopped working, confirmed that they are actually ok by using the 6A MCB instead.

Can an MCB just stop working like this ?

It doesn't have a HUGE draw on it, but we do run the tumble dryer off it, the clothes weren't dry when we got back so it has blown whilst the tumble dryer was on, it didn't trip though.

Not heard of this before all I wanted to know if it is normal.

I will be replacing it with a 16A now
 
Yeah MCBs sometimes just fail, you can usually tell by how they feel when resetting them, they tend to feel floppy / loose when compared to a working MCB next to it, sometimes you can feel that they are not latching.

Most common with 6A lighting MCBs, i guess as they trip the most with blown bulbs, eventually they get damaged and wont reset.

Usually rated at 6Ka but this is the rating where they will disconnect safety without blowing up etc, not a guarantee they will still be working fine afterwards.

PS. If a competent spark designed and fitted the elecs then you can replace with a 32A no need to worry about de-rating to 16A, as it should be correctly designed and installed to take 32A

pps. Its possible you will find the dryer to be faulty, worth keeping an eye on it when you get another MCB for the sockets or getting it checked over

ppps. Sounds like you don't have RCD protection anywhere for the shed supply or in the shed cu, you could replace with an RCBO for extra safety
 
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Also take the old MCB down to the elec wholesaler when you go for a replacement to make sure it matches, ideally you would be able to buy the exact same part, but if you have an obscure brand or older part you might have to match the fitting rather than be able to get the same MCB.

Even within the same brand they sometimes change the fitting between generations / ranges, and matching fitting across brands is worse.

Officially you should use the same brand of parts for the whole cu (tested to be working together & within spec etc), but sometimes its not possible.
 
Thanks for your help

FYI, I bought the garage consumer unit myself and it shipped with a 32a and a 6a MCB, the spark tried to change for a 16A when he fitted it but but his MCB was a different brand and it wouldn't fit so he told me to do it, I just hadn't got round to it yet.

The cable to my shed is run off a fused switched spur in the house to the consumer unit in the shed.

so it is protected by MCB and RCD in the shed, as well as MCB and RCD in the main consumer uit in the house and on top of that the 13A fuse in the spur.

Have been running the tumble dryer off this setup since electrics were installed (roughly a month) with no trips or issues whatsoever, last night we ran an extension lead from the house to the shed until the new MCB arrives and it still works fine so I don't think the tumble dryer is faulty (although not ruled out and will be keeping an eye on it)

The MCB itself still latches as it should but something doesn't quite feel right with it.
Replacement ordered for £2.99 off ebay anyway. :)
 
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