AUXILIARY SWITCH DEVICE

PROBLEM TO BE SOLVED: To prevent overloading on the common bus due to an error made by an operator when switching power to the auxiliary bus from the common bus of the common power supply system in the case of a power generator accident or inspection. SOLUTION: In an auxiliary switch device connected through a connection breaker to a power system supplied power from a generator or through an auxiliary transformer and to a power system supplied common power stepping down from the main power system, a current converter 22 is provided to detect the load current from the above generator and to convert it into an analogue amount (a), together with a current converter 23 to detect the load current from the above common power supply system and to convert it into an analogue amount (b). A comparator 26 is also provided to compare the sum of the above analogue amounts and the allowance (d) determined in advance for the common power supply system, along with an output circuit to output an auxiliary switch lock signal and an alarm depending on the result of the above comparator.