RCD tripping

May 29, 2020 Last reply: 6 years ago 4 Replies
J

One of the RCDs in the split consumer unit tripped yesterday. It was the side I'd previously had a problem with concerning the lighting wiring. Not a good time for a wiring check...



However, a quick check turning off and then on each MCB in turn on that side showed it wasn't the lighting wiring. I'd got out a 10" desk fan which hadn't been used since last summer, and that had been on an hour when it tripped the RCD. Resetting the RCD and turning on the fan had it tripping again in a few minutes.



I've just dismantled the fan (not easy - many hidden plastic catches and unreachable circlips) and put it on in free air. After an hour the outside of the windings were at 87 deg C, and the laminations furthest from them were at 67 dec C. I assume that if left on the enamel on the wires would have reached a high enough temperature to fail and rubbed against the laminations (which are earthed), so tripping the RCD.



Fortunately, I've always got a couple of spare fans and the tip is now open for electrical goods.


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T

My last and worst RCD tripper was an earth neutral short. Other culprit have been wet electrics due to weeping plumbing, a stove element whose insulation was breaking down, and a burned out washing machine motor that showed a 2k short to earth on the armature windings.

Its easy enough to pop a meter across a disconnected fan switch it to 'on' and see if it has leakage

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S

Not so obvious when the unit is cold.

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T

I have yet to see a direct short to earth that vanished when cold as a failure mode on ANYTHING

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B

I had a uk made fan in which the oilite bearings were so soft they wore in a year and the thing started to get hot as it started to brush the outside of the laminations. I'd imagine it would eventually have seized up and the Eddie currents would have overheated the winding. The motor in it was a bit like the old induction motors you saw in autochanger BSR record players in the 60s, but his was made in the 90s out of crap. When I think of the ever video machine I had having two of those motors, which was hot enough to keep your food warm, yet the bearings outlasted the electronics and apart from cleaning never had any trouble despite the Eddie current breaking speed control used. Brian

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