The most likely reason is that when you solder the circuit board, the two solder joints are too big and short-circuited. Another reason is that the load capacity of power supply is small. Inspection method: Without opening the switch, adjust the multimeter to current 10A, and directly measure the current with two short-circuit switches. The current is too large and the circuit board is short-circuited; If the current is small, the load capacity of the power supply is small. Depends on what your circuit board is for. Generally speaking, if the current exceeds several hundred milliamps, a short circuit will occur.