There are 2 purposes for grounding (earthing): 1) to minimize the voltage which can be placed on exposed metal parts of electric equipment so people are safe when working around it, and 2) to provide a low-resistance path for a fault’s current to return to the source so the overcurrent protective devices can turn it off. A non-grounded (non-earthed) system may be chosen for a couple reasons–the circuit’s voltage between conductors is fairly high (such as over 600 volts), or you need the equipment to continue running even with a short to ground until it can be fixed (such as avoiding safety problems or equipment damage from an unexpected shutdown). When you have a non-grounded system, good design also includes a way to tell if any of the circuit conductors has accidentally become grounded. This would be some type of alarm or an indicator light which is monitored (tested and checked often). Then the ground can be found and fixed. The theory here is that the first fault to ground will be fixed before a second one occurs. In the situation you describe, I suspect that there was no way to detect a first fault to ground, so during the many years of this site’s operation, a fairly low-resistance fault to ground occurred and was not detected or fixed. This probably did not put enough voltage on any piece of equipment for it to be felt during use. Then, when a different conductor’s insulation