Thank you very much for your good answer. In regard to this topic, 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 voltage increase1.4 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 pass is possible.