In the power supply and distribution system, short-circuit current refers to the direct connection failure between phase and phase or between phase and ground without load. Its value is more than ten times or even dozens of times of the rated current. In the process of the circuit changing from the normal state to the transient state of short circuit, there may be an impact current whose value is 1.8 to 2.5 times higher than the steady short circuit current. Therefore, it can be seen that its short circuit will cause serious damage to the power supply and distribution system.