The short circuit types of three-phase AC system mainly include three-phase short circuit, two-phase short circuit, single-phase ground short circuit and two-phase ground short circuitThe three-phase short circuit is also called symmetrical short circuit. Each phase of the system is still in symmetrical state as it is in normal operation. Other types of short circuits are asymmetrical.According to the operation experience and statistical data, the probability of all kinds of short circuit is different, among which the single-phase to ground short circuit is the most, the three-phase short circuit is the least. Although the probability of three-phase short-circuit is the smallest, the short-circuit current of three-phase short-circuit is usually the largest and the most serious harm. At the same time, it is the basis of analyzing asymmetric short-circuit, so the focus of short-circuit current calculation is three-phase short-circuit analysis and calculation.