Occasionally, the workshop RCD used to trip and I could never find out what was causing it.
In my defence, I did look over the whole lot and made sure there was no water in anything, and everything looked sensible.
It only ever happened after a couple of hours of use, and when workshop heater was set to 2kW.
So for want of something more intelligent to do, I blamed the RCD which is of the plug-in type and which naturally runs fairly warm.
On the rare occasions that it tripped, I would turn the heater down to 1kW, reset the RCD, and all would be well.
Recently it tripped again, and this time I removed the RCD from the circuit.
A few minutes later there was a soft pop noise.
I turned everything off and, head torch on, followed the faint burning smell.
Eventually I tracked it down to a burst Y capacitor in a filtered extension lead – I use one to for a drill in the hope it will eat transients that would otherwise get into the test gear.
Not a cheap extension lead, but a Made in England extension lead.
And a capacitor covered in international qualifications.
Sorry RCD, you were blameless and you were saving me.
Must buy a Megger.
By the way, I am pretty careful what I leave plugged into the mains permanently – particularly since I once came home to find a telephone answering phone with a hole melted in the top.
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Ps, sorry heater as well.