Paul,
In some areas, over here, we used to have what was known as a Voltage Controlled Leakage Breaker, these were a real pain in the rear, they were only installed where the basics of Earthing could not be maintained, like really stony soil.
This device used to have a seperate Electrode, spaced outside the Earth Radii(normally on the other side of the house).
And it would trip on the Earth leakage between the rods,reaching 26V RMS, but it would cut the supply to the whole house,
this is one reason why I am against the use of an RCD as a main switch.
I would personally use a minimum of 4 RCD's in a house, saves call-outs, late at night, when the faulty kettle(and there are a few),
keeps tripping the whole house. [Linked Image]