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-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 pass is possible.