POWER SUPPLY PROTECTIVE DEVICE, POWER SUPPLY DEVICE AND SWITCH FAILURE DIAGNOSING METHOD

A power supply protective device, includes: a first switch provided between a first terminal to which at least one of a load and a charger is connected and a second terminal to which a power supply is connected; a switching circuit connected in parallel to the first switch and including a second switch and a voltage drop element connected in series to the second switch, the voltage drop element causing a voltage drop of a reference voltage by a current flow; a voltage detection unit which detects at least one voltage of a first voltage as a voltage of the first terminal, a second voltage as a voltage of the second terminal, and a third voltage as a voltage at a point between the second switch and the voltage drop element; and a control unit.