16A MCB trips when tumble dryer is on

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Hi,

I have a garage CU that has two MCBs, B16 for mains and B6 for lighting.

Recently, the B16 started tripping when the tumble dryer is on. There's an old fridge plugged into the same double socket, which I assumed was the culprit. So I unplugged it, and everything else apart from the dryer. MCB still tripped. It all used to be fine, nothing obvious changed but the MCB just started tripping.

I replaced the MCB today, the old one was very hot when I took it out, but no melted insulation on the wiring. All the cores are done up tightly inside the CU. I've unplugged everything in the garage apart from the tumble dryer. The MCB still trips. The RCD doesn't trip so unless it's faulty, I'm assuming there is no short or earth fault on the garage electrics.

I don't get it. Why would a 16A MCB trip before a 13A fuse blew? Indeed, why would a dryer trip a 16A MCB? I can't believe a dryer draws over 3.5kW...

The dryer is only a month old so shouldn't be faulty. Connecting it to the main house main by an extension reel, it run just fine and nothing trips - although the house main is on a 32A MCB.

I guess I'll have to disconnect all junction boxes and sockets in the garage and reconnect them one by one to eliminate a fault. Can anybody suggest a quicker way to find what's wrong? Would it be worth putting a multimeter across the MCB terminals to see whether any current is being drawn with all appliances unplugged?

Any advice appreciated. Unless it's, "call an electrician." :)
 
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I replaced the MCB today, the old one was very hot when I took it out, but no melted insulation on the wiring. All the cores are done up tightly inside the CU. I've unplugged everything in the garage apart from the tumble dryer. The MCB still trips. The RCD doesn't trip...
An MCB should not get hot under normal (i.e. not overloaded) circumstances - so either (a) the MCB was faulty or (b) the connections were not done up tightly enough of (c) an excessive current (well in excess of 16A) was flowing through it. You appear to have eliminated (a) and (b), and cannot really afford a reason why (c) would be the case!
I don't get it. Why would a 16A MCB trip before a 13A fuse blew? Indeed, why would a dryer trip a 16A MCB? I can't believe a dryer draws over 3.5kW...
One needs a current of at least 22-23A (a bit over 5kW) to cause either a 13A fuse or a 16A MCB to 'operate' - and even that may take up to an hour. For guaranteed faster operation of the devices, appreciably higher currents than that is required.
Would it be worth putting a multimeter across the MCB terminals to see whether any current is being drawn with all appliances unplugged?
It would obviously be good to know if any current is flowing when all appliances are unplugged, but you could not determine that by 'putting a multimeter across the MCB terminals'.

On the basis of what you've told us, it's all a bit mysterious. If something else was adding an additional current to that used by the dryer which was sufficient to cause an MCB to get hot and to trip, that 'additional current' would have to be so high that it would almost certainly have 'shown itself'. I'll be interested to hear what other ideas people might have!

Kind Regards, John
 
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Have you had the socket open?

Not something bizarre spurred off it, maybe the other side of the wall, that with the new dryer is pulling too much total current?
 
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Not something bizarre spurred off it, maybe the other side of the wall, that with the new dryer is pulling too much total current?
Nothing is impossible, but I would think pretty unlikely in a garage - although, I suppose, not unthinkable if it were an 'attached' garage!

Kind Regards, John
 
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It would obviously be good to know if any current is flowing when all appliances are unplugged, but you could not determine that by 'putting a multimeter across the MCB terminals'.

Thanks John,

My error - when I said "current" I meant "voltage" wrt potential difference across the terminals.

I have two circuits coming out of the MCB, in a radial configuration. One of them goes to a single double socket, so I'll check that out, disconnect the other circuit, and try again. If THAT pops the MCB then I guess it must be the dryer.

Incidentally, I didn't mention before, the garage CU is served from a non-RCD-protected B16 way on my house CU. That MCB hasn't tripped at all.

Cheers
Jim
 
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If the MCB is hot - and if overcurrent is not present - does it suggest that a connection inside has become loose and causing it to heat as if there were an overcurrent?
 
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If the MCB is hot - and if overcurrent is not present - does it suggest that a connection inside has become loose and causing it to heat as if there were an overcurrent?
Indeed [my (b)] - but we've been told that "everything was done up tightly".

Kind Regards, John
 
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Have you had the socket open?

Not something bizarre spurred off it, maybe the other side of the wall, that with the new dryer is pulling too much total current?

No - I put the wiring in myself so I know where it all goes. It's been running fine for 10 years and was checked by a professional so there should be nothing dodgy about it.

There is a party wall with my next door neighbour's garage, but none of the electrics run across that wall and he wouldn't know how to tap into my supply even if he was the type! ;)
 
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If the MCB is hot - and if overcurrent is not present - does it suggest that a connection inside has become loose and causing it to heat as if there were an overcurrent?
That was my first thought when I touched it and felt how hot it was. But no, it was all nipped up nice and tight. I've seen a melted MCB (not one of mine) due to loose connections and doesn't look or smell nice!

I suspect that it was hot because I reset it a few times before biting the bullet and starting to pull it apart.
 
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It would obviously be good to know if any current is flowing when all appliances are unplugged, but you could not determine that by 'putting a multimeter across the MCB terminals'.
Thanks John, My error - when I said "current" I meant "voltage" wrt potential difference across the terminals.
If you mean 'across the MCB' (i.e. from L to L or N to N), I suspect that you would be hard pressed to measure much of a pd, even if appreciable current was flowing through it.
Incidentally, I didn't mention before, the garage CU is served from a non-RCD-protected B16 way on my house CU. That MCB hasn't tripped at all.
That, of course, is also rather odd, given that the same current should be flowing through both MCBs.

Kind Regards, John
 
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Indeed [my (b)] - but we've been told that "everything was done up tightly".
No. I meant a part of the MCB inner workings.
Fiar enough, but unless the getting hot and tripping are 'unrelated' ('co-incidental') it would not be compatible with the tripping persisting when the MCB was replaced, would it?

Kind Regards, John
 

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