Tumble dryer tripping MCB

[Pandora]

My utilities room has its own ring circuit protected by a 32A RCBO. The only loads on this ring are a Miele washing machine and a Miele tumble drier, which have been operating just fine for about two years.

This morning, I ran the washing machine no problem. When the wash had finished, I transferred the small pile of clothes to the tumble dryer. I set the tumble dryer going and after about 20s, the RCBO tripped.

I wanted to test whether the RCBO was faulty, so I ran an extension cable from another ring circuit (also protected by a 32A RCBO) and plugged the dryer into the extension cable. 20-30 seconds after switching on, the RCBO tripped.

I'm assuming that since the RCBO trips after almost half a minute, the fault is overcurrent rather than earth leakage.

The filter in the tumble dryer is clear. Any other things I could check to find out what is causing the overcurrent in the tumble dryer?

[Andrew Gabriel]

That's not a valid assumption. I would suspect it's actually earth leakage. It might be from the heater, only happening when it's got to a particular temperature (sagging element, possibly contacting dust or local overheating due to dust?). Another possiblilty might be condensation running somewhere it shouldn't.

I might try running it with the heater disconnected. Depends how safely you think you can do that.

[Pandora]

Thansk, Andrew.

The tumble dryer has a "cool air" setting.

I've tried running it on this with a light load - and after 15 minutes it hasn't tripped the RCBO.

Does this mean that I need a new heating element?

[Andrew Gabriel]

Possibly, but it might be fixable with just some simple maintenance, such as blowing the dust out. Can't tell without inspecting the element.

[John Rumm]

To trip a 32A type B MCB by over current in 20 seconds would take a sustained load of some 130A+ - that ought to take out the 13A fuse in the plug long before. The alternative might be if the heater is switching in after that delay, and it has gone dead short - that may take out the RCBO by fault current, but even then it would be unlikely the fuse would survive repeated attempts. Hence earth leakage is a more probable cause I would guess.

Have you got a non RCD protected circuit you could run it from? If so, just letting it get hot enough for one drying cycle may fix the problem if it is leakage caused by water being some place it should not be.

[Rumble]

More likely that the heater element is not switched on until the tumbling has settled down and the heater trips the RCBO as soon as it comes on...

[Pandora]

Just an update to close this thread:

After leaving the tumble dryer for a few days to "air", the dryer is now working again.

The dryer is a condensing type with a removable container for the water. Wonder if I spilled a bit into the back of the dryer when removing the container? That might account for the leakage current last weekend.

Anyway, thanks to all for comments.