Earth fault in no earthed system

[m.s.j]

In one of the industrial site, the no earthed system has been chosen in electrical designing stage. Therefore nobody anticipated a strong Earth fault current in the system.
On the other hand, the primary earthing system was not installed very well and related standards recommendations weren't applied in the erection stage; because the related engineers thought that the primary earthing was not very important in” no earthed systems” and they could ignor its erection in some area.
Many years after start up and operation of plant , some cable insulations were damaged due to no suitable operation conditions. One day a short circuit happened between one old cable and related electromotor casing.
Unlike anybody imagined, a sever short circuit current passed through electrical system and ground surface which could be very dangerous for anybody in site area.


How can you explain the reason of that accident?

 

[Astronuc]

This seems like a homework problem.

If so, one should attempt an explanation.


Failure of insulation can account for a short circuit, among other things.

 

[Newton1Law]

Interesting. You do not give a very good explanation of how the system was connected to the supply circuit but here is an example of how an industrial plant could get into trouble. Let's say the plant is very large and has multiple service points. (Service points meaning locations where they have transformers that step down a primary voltage to a lower magnitude secondary voltage and supply power to that portion of the plant.) Assuming that all the service voltages are connected to a three pahse delta connected system, no specific Earth ground or neutral would be needed since under normal conditions, all current flows would return to the wires making up the delta. This does not mean there would be no equipment grounds, just that the system would be considered ungrounded since there is no intended neutral in the service points or wires. Now let's say after many years, that a motor in one part of the plant has a ware spot on one of the three pahse wires that supply power to it. Let's call that phase, Phase A. Let's also assume that the motor in question is 100 yards away from another location we will call location #2. Now at location #2 the same problem develops on another motor similar to our first motor, but on this motor, Phase B shorts to the grounded case of the motor. We now have two grounded conductors from two different phases. If the voltages between the phases is say 480 volts then you now have 480 volts trying to push current through ground system across the 100 yards in the plant.

This of course is only one of a number of possible explanations. Hope this helps.

 

[m.s.j]

Thank you very much for your good answer.

I can add to your good answer as following:
As you know in non-grounded system, when one of three phase conductors connected to earth, the rms of other phases-earth voltage increase 1.73 time, therefore simultaneously Earth fault on other phases in other places is possible specially when insulation material of electrical equipments are weakened. So in non-earthed systems two phase short circuit fault via Earth path is possible.