Power source switching device and storage battery system

A power source switching device comprising:

• a third relay for switching between an output of a first AC power source fed via a first relay and an output of a second AC power source fed via a second relay, and supplying the output after the switching to a load;
• a welding detection unit for detecting welding of the first relay;
• and
• a power source switching control unit for,
• in order to switch the output of the first AC power source to the output of the second AC power source, and supply the output of the second AC power source to the load,
• switching the first relay from a closed state to an open state, switching the third relay so as to select and feed the output of the second AC power source, and then prohibiting switching of the second relay from the open state to the closed state until a first-relay welding detection processing by the welding detection unit is completed,
• wherein the first relay and the third relay are simultaneously controlled so that switching of the first relay and switching of the third relay are performed in parallel.