Dave Plowman (News) pretended :
How did you 'narrow' it down? Simply switching off an MCB doesn't disable
that circuit as far as an RCD is concerned - a neutral to earth fault will
still trip it. The only way to truly disable the circuit is to disconnect
line and neutral.

Just to qualify that a little....

If the RCD can be made to reset by turning MCB's off, the one which
does cause it to trip as it is turned on will most likely protect the
circuit with the problem.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk