everyone is a bit right - and some are a bit wrong here.
An RCD does not need an Earth per se, but needs an EARTH PATH
it is quite conceivable to have a house with no incoming earth, but there would be an alternaive path though the CPC to the MET, through the protective bonding and down to earth though any extraneous conductive paths (gas, water pipes etc)
as long as that path is 1667 ohms or less, the RCD would trip before the fault voltage reached the magical 50V, and even with a path of 7666 ohms or less would still trip.
Of course we need an earth path for CPD's to work correctly, but an RCD, with ANY path to earth as stated above, will trip off
and before anyone starts thumping the keyboard, I am NOT suggesting using an RCD means you dont have to have an incoming earth
