My two cents...or pence
Assume a domestic single phase 230V supply. The neutral is connected to ground at the POCO supply transformer LV star point. So there are three conductors, live neutral and ground. The exposed conductive parts of appliances are connected to ground. It's a typical system for UK and elsewhere in the world.
If a live conductor has a fault and contacts a grounded part, fault current should take out the over current protection. If the neutral does, a residual earth current device should.
For an isolated supply you could possibly have a live to ground fault on one bit of kit and a neutral to ground on another. So possibly 230V for two adjacent bits of kit.
A grounded system takes that risk out.
This would be true in a grounded system where the POCO Earth the XO, but what if the XO wasn't grounded by the POCO, here in America we allow a un-grounded delta for industrial if a phase to earth monitor alarm is used, I would say if kept isolated, and a monitor it would be safer on the load end.
All most all electrocutions accrue between a ungrounded conductor and Earth or something connected to earth, with a reference of the XO to Earth this completes the path through your body, now if after the last transformer to your service was kept isolated, and a monitor with a bell or siren/red lights on it, and since there is no reference to Earth accidentally touching a current conductor and something that is referenced to earth such as concrete, there would not be a complete path for current.
In understanding why Earthing was brought into existence, we have to travel back to the days of telegraph, at first the telegraph wire was strung along wooden poles for miles connecting to wooden insulators or stand offs, the problem with this arrangement was it provided a resistance through the insulator and down the pole to earth, this placed a higher load on the batteries used to supply the power for the telegraph and limited the distance to each station.
Well someone invented a glass insulator and it was a success, it had such a high resistance to Earth the batteries lasted longer and distance could be now greater, but another problem arose, it seemed that line men and operators working on and repairing the lines was being electrocuted, at first they weren't sure what it was, but it was discovered, that winds, and later soler winds would cause a voltage to build up on the lines to a very high potential, while the wooden insulators would keep this voltage drained down, the glass ones would allow it to build till something came into contact with it and Earth.
so they found that by Earthing one of the conductors would allow the voltage to drain off without increasing the load on the batteries.
this was carried over to developing electrical transmission lines, and some how over to even secondaries of local transformers feeding loads less then 1,000 feet from it.
In turn it developed to a fault path to automatically open the line for voltages above 500 volts and by miss information over to lower voltages, when it was discovered that the Earth couldn't reliably open a OCPD it turned into voltage stabilization, and protection from lightning.
and like the energizer bunny it keeps on going from there.
the rest is in the PDF file Cold Fusion posted at the bottom of post 8
