RCD tripped & will not reset

Disaster has struck. For no apparent reason the RCD on my MK Senrty CU has tripped. I cannot get it to reset which suggests to me that either a) the DCD is knackered or b) there is a fault on the fixed wiring. I've tripped all the MCB's to try to isolate the circuit with the fault but this has not helped as even with all the MCB's to off the RCD will ot reset. Nobody in the house was using/doing anything out of the ordinary when it tripped. House was rewired 9 years ago.Any ideas ?
Neil
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MCBs only switch off the supply, but there could still be current flow neutral to earth.
Chances are however the RCD is stuffed (that's a technical term by the way.)
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Have you **unplugged** all appliances to be certain one of them is not responsible.
James

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Disconnect the load bus bar and load neutral from the RCD. If it doesn't reset now, then the RCD is definitely off to silicon heaven. As it trips with all MCBs off, I suspect this is the likely cause, although it is by no means guaranteed.
Christian.
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Thanks for all the pointers guys. Now sorted - it was a neutral - earth leak caused when my wife hung a very heavy picture on a copper panel pin she had inserted through the 1.5mm T&E feed to a wall light ! Clearly when I asked if anyone had been doing anything when the power went off I should have been more specific !! Took 2 hours to trace.
Neil
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