under-rated MCB tripping on Tumble dryer

[jkn]

Hi all a little advice please.

Been helping out at my (aging) Mum's house. I have had recent reports that the 'Tumble Dryer fuse' has been tripping. It turns out that the tumble dryer is fed on a radial circuit that is labelled 'Fridge', and is protected with a 6Amp type B (Hager) MCB. The tumble dryer has been in place for many years connected here.

I have confirmed that (not unsurprisingly) the MCB indeed trips frequently when the Dryer is used; it typically lasts about 5mins. The circuit seems to be wired with 2.5mm cable and I have ordered a 10Amp replacement MCB with a view to fitting this in place of the 6Amp one.

However ... why has the MCB lasted so long in place up to now, seemingly without tripping?

I have cleaned the front filter on the T/Dryer etc. and opened up the back. Ours gathers a load of fluff and periodically needs cleaning out. However this (Creda) one seems fine at the fan and heating element areas. I did notice a _tiny_ spot of damp in one corner, but nowhere near the electrics.

I am wondering if either (a) the MCB is starting to fail, after many years of being operating 'somewhat' above its limit (possibly with other family members helping out and using the dryer rather more than otherwise), or (b) if I should be more concerned that there is something else wrong with that circuit.

Thanks for any thoughts.

Jon N

[meow2222]

If you can confirm its a 2.5mm run, a 20A mcb would be good if not buried in inulsation, 16A if it is or will be. The dryer's plug fuse should suffice for the dryer.

NT

[jkn]

Thanks for the advice about the MCB rating - I was reconsidering my first thought of a 10A one. It is 2.5mm cable coming from the MCB in the CU.

However this doesn't address why it has only started tripping recently, given that it's been in place and in use with the T/Dryer for > 10 years...

Cheers J^n

[fred]

In article , jkn writes

You don't mention the rating of the appliance which would have been useful :-)

Assuming it is a 2.4kW 10A rating then the type B curve for a 6A MCB requires it to trip within 550 odd seconds or just over 9 mins.

As this is a thermal mechanism I don't see it as unusual for the breaker to operate anywhere in the 5 - 9 minute range so I'd say that you are experiencing perfectly normal operation and within spec for either a new or aged component..

I don't see anything significant in the perceived change in operation, it has been operated in overload for an extended period and still appears to be operating as intended. Compare with, "I've been poking a large bear like animal with a stick for years and it suddenly lashed out and ripped my arm off at the shoulder, can anyone suggest a cause for this change in behaviour" :-)

Alternatively, has mum switched from using it on 'low' to 'high'.

MCB info & curves here:

[meow2222]

for good reason. With the right mcb either it will still trip or it wont. If it does, come tell us.

NT

[jkn]

Yes, rather embarrassingly I forgot to make a note of this when I was looking round the back ;-(. Probably fairly safe to assume it's a 2400W one.

TBH that is possible, thanks for the thought. Or another family member is doing that, instead of using the most 'economical' setting...

Thanks, I was hoping to track those down.

Cheers jon

[ARW]

And very logical.

[John Rumm]

Economical does not really come into it, as long as you stop drying once the clothes are actually dry. (the amount of energy required to evaporate a given amount of water does not change with the speed you do it)

[meow2222]

Well... the total energy used to boil the water doesnt change, but other en ergy uses do. Then again even the evap energy probably changes: if one some how boiled it all off in 1 second, it would all leave by boiling. OTOH if o ne ran the dryer with no heat at all, it would all leave with zero temp ris e.

In practice I expect most of a dryer's heat leaves without even touching th e clothes. So I'm not convinced its as simple as twice the heat half the ti me.

NT

[jkn]

Update: at the weekend I managed to pay another visit, with some MCBs in hand. Having confirmed that the spur wiring was 2.5mm (and the fuse in the 'pattress fuse holder' had been 13A all along, I replaced the 6A MCB at the consumer unit with a 16A one. Interestingly, I then spotted that the MCB in the adjacent position, (marked 'upstairs lights' and wired with 1.5mm cable) had a 16A MCB fitted! So I ended up changing that one as well, down to a 6A one.

The Tumble dryer now seems to run fine, and the upstairs lights are also OK ;-). I suspect that perhaps these two MCBs had been swapped over at the time the house was originally wired. Maybe the 6A one had been close to tripping for a long time and had finally worn out in some way.

Thanks for the advice previously given ;-)

Cheers Jon N

[ARW]

Bloody handy that you had that spare 6A MCB you had just removed knocking about:-)

It sounds like Mr Cockup was at home and the CU had a slight error.

[jkn]

yes, I was quite tickled about that ;-)

J^n