On 25 June, 14:47, Tim S wrote:
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

A 3mA RCD should trip within 10% of the rated triping current so it
might open at 27mA. It should not trip at 50% of the rated trip
current so it should remain on at 15mA. There are other conditions to
be satisfied but these are your two applicable ones.
A multi-meter is unlikely to be able to be of help in finding a high
resistance fault and I suggest you would need a Megger. I suppose I
return to my original response that you need assistance or you might
like to invest in an all-in-one multifunction installation tester
which can carry our insulation, continuity,loop impedance, RCD
operation and timing and other tests depending on the price you pay.