Bosch Double Oven MBS533BS0B Tripping MRB

[mvteng]

Bosch Double Oven MBS533BS0B – when using both ovens occasionally it will trip the MRB. Not often, maybe once a month.

The MRB is 16A (Type CUMB16)

The Bosch Data Sheet for the over shows current 32A

Is this just a simple case of the MRB being too small for that particular oven?

[electrofix]

16a is too low but before uprating breaker make sure the cable is able to take the extra load

Dave

[mvteng]

Perfect Dave. Thanks. Cable is 6mm2 so should be good i think

[electrofix]

no probs with that just uprate to 32A

Dave

[mvteng]

Thanks again Dave

[mvteng]

MRB uprated to 32A. been fine for a few days, but now when top grill is switched on it trips the RCD after about 2 minutes.

It doesnt trip straightaway, but pretty much trips everytime on the grill after its been on for a couple of minutes.

reset the RCD and its fine, it doesnt trip again.

Any ideas?

[electrofix]

does it only trip on the grill setting ?

most of the time this type of fault is an element with a slight earth fault

Dave

[mvteng]

Thanks Dave

I think its only the grill setting. I need to keep checking.

when you say a slight earth fault – is that a loose connection? or is that normally a fault in the element itself?

It switches on for a couple of minutes before it trips, so I wondered if it was some sort of thermostat issue i.e. when it gets to a certain temperature???

After reset, the grill heats up with no issue. Strange

[electrofix]

no one of the elements is faulty

had the identical fault on a creda last week. grill heated for 10 mins then blew trip, changed element now ok

Dave

[mvteng]

ok. Thanks Dave.

Element is fine after the RCD is reset.

switch on grill

wait 2 minutes – RCD trips

reset RCD

Grill fine after that. I can see its bright orange all around

If the element is faulty , would this happen? would it not just keep tripping the RCD?

Sorry – just trying to understand

[electrofix]

its a hard one when the fault does not repeat

I cant say 100{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d} thats its the element but its more then liklely.

to be certain you would have to test the element insulation resistance just after it trips

Dave

[mvteng]

So – Good news and bad news

On inspection the 2 grill elements look identical. Same shape, same connections, so I simply swapped them over to find out if the fault was the element.

Good news – the new top grill (old bottom grill element) no longer trips the RCD.

Bad news – The small fan behind the grill is now constantly on. This is not the big fan in the main oven – this is the one that circulates air around both ovens (cooling fan?) Normally it switches off after the oven has cooled down . Cant see any loose wires that I might have dislodged, and the procedure to swap the grill element was really straightforward

Any ideas

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