[Bruce Tanner]

RCDs trip on excessive earth leakage current. Earth leakage current is accumulative among all the equipment powered on. So you might find that turning something else off in your household will also stop the tripping because it will keep the accumulative leakage to below the RCD threshold (30 mA).

That said, old fridge/freezers (and cookers) are notorious for excessive earth leakage currents. The only sure way of finding the culprit is some equipment that can measure it. But think of anything old that might have grot/dirt/damp around its electrical connections. Things in garages and kitchens are often the culprit!

 

[Bruce Tanner]

Yes each circuit has its own breaker, or MCB, which protects against too much current being drawn for each circuit. But an RCD protects several circuits against earth leakage currents (which is to stop you being killed by an electric shock if you touch something that is live).

In most modern installations there is one RCD that protects the socket circuits and perhaps another for an electric shower, but the current regulations also require a further separate RCD for the lighting circuits in new installations.