Abstract: A switch unit is movably arranged in a housing for selective connection of two power sources to a load circuit. When the switch unit is extracted out of the housing, one power source is connected to the load circuit via one by-pass switch unit arranged on the switch unit, thereby avoiding interruption of power supply. When such power supply from the one power source is interrupted, connection is automatically shifted to the other power source via the other by-pass switch unit arranged on the switch unit. When one by-pass switch unit is closed to connect one power source to the load circuit, one interlocking element movable with that by-pass switch unit stops one engaging element movable with one contact, thereby inhibiting concurrent connection of two power sources to the load circuit.

Abstract: A change-over switch performs switching between two different power supplies after connecting both power supplies simultaneously for a given time period. A controller controls the change-over switch to perform the switching operation around a synchronous point of the power supplies. Voltages of the power supplies are monitored to derive a difference voltage therebetween. From the difference voltage, a phase difference between the voltages of the power supplies is monitored. A plurality of low voltage detectors are provided for monitoring the difference voltage. The low voltage detectors are arranged to be operated at voltage values no more than preset values, respectively. The controller outputs a switching command to the change-over switch when time intervals between adjacent operation start time points of the low voltage detectors are all greater than preset values, respectively, and when the low voltage detectors are all operated.