You could bypass the rcd, either by moving the circuit under test onto the otherside of the cu, or otherwise, then take your reading, and put it back, but this is frowned upon, because the circuit is energised without the rcd protection that has been fitted to it, but under controlled conditions (you know for certain the circuit won't be used, and I suppose, that the RCD is not providing the only means of protection against indirect contact (so not on a TT system)) then I suppose it could be used...