george (dicegeorge) coughed up some electrons that declared:

cpc farnell have some 10ma rcds
which work at about 13 amps -
i use them for my extension leads to the garden,
as they should trip first...

[g]

Nice idea - but unfortunately it's not a given.

Once a fault 30mA has been seen by both devices, both will trip, unless one
of them has an integeral time delay (Type S).

However, your theory works for fault currents between 10 and 30mA given or
take device tolerances.

Cheers

Tim