James,
The electrode resistance (as distinct from Ze) is required to be such tah the current required to operate the RCD, say 100ma, will not cause a voltage drop along cpc and over electrode resistance greater than 50v or 25v depending on the circumstances. Greater voltage drops may occur but with corresponding quicker disconnection. Effectively, this means a max electrode resistance of 500ohms for a 100ma device in normal locations and 250 ohms for other special locations such as farms. IEE GN3 also recommends values over 200 ohms should not be regarded as stable, so I guess that you have to look at both recommendations since using a 500ma rcd would not comply with the first provision found in BS7671 2001
