The main reason for the short circuit is:
(1) the insulation selection is not appropriate, such as voltage grade is high, the insulation level is low;
(2) subjected to high temperature, humidity, corrosion loss of insulating ability;
(3) tied with wire conductor or hang insulation on the metal;
(4) long-term insulation wear or mechanical damage, aging;
(5) in oil equipment oil aging metamorphism or water, impurities, etc.;
(6) lightning overvoltage, line when no-load voltage rise and so on, also can make wire insulating weak insulation was strike and short-circuit.