In the U.K., we have TN-S, TN-C-S, and TT, although those designations were not adopted here until the 1980s.
In times past, almost all urban areas used TN-S. The neutral was grounded only
at the star point of the transformer secondary. There were no multiple grounds, and no neutral-ground bonds at each service entrance. The most usual distribution arrangement in such systems employed armored underground cables, with the cable armor providing a separate fault path back to the sub-station. TN-S diagram
In rural areas, the system which would now be called TT was used. Originally, these networks also had only a single ground on the neutral at the transformer. Again, there were no neutral-ground bonds at each service, and the sole ground fault path was actually by way of a local rod and the earth itself (an arrangement which in the U.S. of course, is prohibited by the NEC).
Because the fault path on a TT system can be of relatively high impedance, some form of earth-leakage (ground fault) circuit breaker is needed to provide proper protection. TT diagram
Although the current-balance ELCB (Earth Leakage Circuit Breaker) was used, a voltage-operated ELCB was the norm for many years on domestic systems. It detects faults a different way, but the supply distribution arrangement is still the same. The voltage ELCB was abandoned in the 1970s, but many are still in service. TT diagram with voltage ELCB
The TN-C-S system is the one which most closely resembles American practice. The neutral is grounded at the xfmr, then again at regular intervals along its route. At the service entrance, the EGCs and bonds to water pipes etc. are then all bonded to the incoming neutral. We still have a separate neutral and ground bars in our panels -- The bond is made to the neutral at the service head before the meter.
This arrangement has been in use since at least the 1930s, and was named PME -- Protective Multiple Earthing. In the original scheme of things, PME distribution was used only in rural areas in which soil conditions made it very difficult to achieve fault protection with a TT arrangement. In fact at one time, the utilities needed special permission from the government for each PME distribution network installed. PME/ TN-C-S diagram
Over time, PME/TN-C-S has become more widespread and utilities have added multiple grounds to their distribution lines so as to make PME available. The PME-ing of rural lines gradually accelerated over the last 20 to 25 years, and I believe that all
lines might now be PME. They certainly are in my area,
Although the distribution lines might be PME, that doesn't necessarily mean that the house has to be wired for PME as well. It just means that PME connections may
be used at the service if required. Even though the lines are now PME, there are still many services in rural homes which are wired for TT. Our regs. specify stricter bonding requirements for PME, so to change a house from TT to TN-C-S can often mean considerable work. In fact there are still new services being installed as TT even though TN-C-S could be employed.
Even the old TN-S urban systems have now been multiple earthed, so PME bonding is available in those areas as well.
As far as MV/HV distribution is concerned, British practice is to distribute with no neutral. Thus all 3-ph xfmr primaries are delta connected, and 1-ph spur lines are always run as two phases.
[This message has been edited by pauluk (edited 01-21-2006).]