David,
Thanks for the endorsement, but we must also give plenty of credit to our American friend Nick for taking the time to convert my rough sketches into the nice neat graphics you see on that link.
Tom,
I didn't put the new-style designations on the diagrams at the time, as most of you will be unfamiliar with them in the States and I thought it might add to the confusion, however for reference: #1 is TN-S, #2 is TN-C-S, and #3 is TT.
The TN-S system was installed widely in built-up urban areas where underground armored cables were employed. Such services are still to be found by the thousands in towns here.
The TT system is very common in rural areas with overhead distribution. When originally installed, many of these distribution networks also had the neutral grounded only at the star-point of the xfmr. (The RCD/GFI protection is standard now, but up until the 1970s the arrangement shown in diagram #4 was more common for residential services.)
The TN-C-S, or PME system has been in use since the 1930s, but was originally used only in certain rural areas where local soil conditions made it difficult to obtain a low-impedance ground connection. (David, I imagine this would have been the case in many areas of the Scottish Highlands ???)
In recent years, PME has gradually become more widespread, and many rural distribution networks have had the extra ground rods added along the route. Here in Eastern Electricity's region at least, all the old overhead networks have now been converted to PME.
Many buildings still have their original TT arrangement, but it means that any service can be converted to PME now if required.
As you said, our PME/TN-C-S system is the same basic arrangement as the standard U.S. service. The only real difference is the location of the neutral-ground bond at the house -- Before the meter here, at the main panel in the States.
The possibiliy of parallel paths with PME raise the same sort of arguments as you have in America. Our Regs., for example, specify larger size bonding conductors where PME is used to allow for the possibility of an open neutral on the service.
