Malc,
Thank goodness I'm not the only one to have an "age thing". I feel as though I might have been around since the steaming lights were installed in Victory, one of my students suggested that I forgot about the Victory but then asked what were the conditions like on the Ark :'(
PMK,
1. True it is common practice to bond the neutral and earth together. This does not automatically make it PME. I wouldn't care to take a stab at the percentage that use genuine PME but I think that you'll find a higher proportion use a system known as CNE (combined neutral and earth). Similar principle but without the whistles and bells of a full PME setup. For info, PME was introduced primarily for use in areas where the ground conditions made it difficult to obtain a satisfactory earth electrode value. By using Multiple Earths (in parallel) this problem could be overcome. We found it not all sweetness and light back in the sixties!
2. Use of a R.C.D., the bond between neutral and earth is carried out at the supply authorities intake position. The installation is bog standard apart from this. The RCD will detect any out of balance between phase and neutral currents as a result of an earth fault being present on the circuit or installation. The conductors in the RCD pass through a laminated core which in healthy conditions is magnetically neutral (both currents being the same) but in the event of an earth fault occuring a resultant magnetic flux will circulate in the core and cause a current to flow through a separate trip coil.
3. Difference between RCD and ELCB, There used to be two types of ELCB, one current operated and one voltage operated. The current operated version is now what is classed as a RCD. The voltage type went out of use some time ago, they were very prone to failure if there was a parallel earth path on the installation as it would cause the tripping coil to be bypassed thus leaving the circuit / installation live with an earth fault on it with all the metalwork connected to the setup live.
I hope this answers your question.
Regards