Read from page 23 of the On Site Guide.
In summary all circuits must be RCD protected.
All of these circuits need 30mA RCD; circuits with buried cables less than 50mm not in earthed steel conduit , all circuits of locations containing bath or shower, circuits with socket outlets not exceeding 20A, mobile equipment not exceeding 32A for use out doors.
All other circuits need 100mA RCD (time delay gives discrimination).
Because of the relatively high impedance of a TT earth rod there may not be sufficiently high current flow to earth to trip an MCB in case of a fault. The 100mA RCD in this situation will trip instead so giving protection to those circuits.
Also the consumer unit must be an all insulated type.
I would think that if you use a dual RCD board with 2x 30mA RCD and no unprotected ways available then you wouldn't need the 100mA and could just use an isolator.
In summary all circuits must be RCD protected.
All of these circuits need 30mA RCD; circuits with buried cables less than 50mm not in earthed steel conduit , all circuits of locations containing bath or shower, circuits with socket outlets not exceeding 20A, mobile equipment not exceeding 32A for use out doors.
All other circuits need 100mA RCD (time delay gives discrimination).
Because of the relatively high impedance of a TT earth rod there may not be sufficiently high current flow to earth to trip an MCB in case of a fault. The 100mA RCD in this situation will trip instead so giving protection to those circuits.
Also the consumer unit must be an all insulated type.
I would think that if you use a dual RCD board with 2x 30mA RCD and no unprotected ways available then you wouldn't need the 100mA and could just use an isolator.