Grid protection apparatus, electrical circuit switching apparatus, and electric power supply system

The grid protection apparatus according to the present invention is provided with a transformer having a primary winding and a secondary winding that are electrically insulated from each other. The primary winding is connected to an external power source for supplying alternating-current electric power using a secondary cell, and is connected to a first electrical circuit for receiving the alternating-current electric power from the external power source. The secondary winding is connected to a second electrical circuit connected to the power-supply circuit of a load from a commercial electric power grid.