For those new to the forum, we have diagrams of the different systems used in the U.K. in the technical reference area
here. In the modern nomenclature, the systems are designated as follows:
#1: TN-S. Seperate earth path right back to the transformer, usually via the armor of the underground cable.
#2: TN-C-S. Also known as PME (Protective Multiple Earthing), the same as the MEN system down-under, with a neutral-ground bond at the service entrance and multiple grounds on the neutral along the distribution route.
#3: TT. Local earth only, with an RCD (*).
#4: TT. Local earth with a voltage-operated ELCB.
As David pointed out, the latter system is obsolete and was removed from the IEE Regs. as a permissible arrangement in the 1980s.
Certainly in this area of Britain though, the soil conditions enable quite acceptable earth resistance values to be obtained, and there are still many rural houses with one of these devices in use.
(*) As pointed out, the term ELCB was also used to refer to the forerunner of the modern RCD, qualified as a current-operated or current-balance ELCB where distinction between it and the voltage-operated ELCB was necessary.
In later years the current-operated ELCB was renamed RCCB (Residual Current Circuit Breaker) before finally changing to RCD (Residual Current Device).
I've never liked the term RCD, as I feel it to be far less descriptive than the earlier terminology.
