PURPOSE: To protect various electric loads by determining the operating voltage of an electric switching means connected between the 1st and 2nd input conductors and a low-rated transformer higher than the maximum allowable voltage of the low-rated transformer and lower than the variation lower limit value of high source voltage from a commercial power source.

CONSTITUTION: An electric switching means 120 is connected between the 1st and 2nd input conductors L1 and L2 and a low-rated transformer 130. The operating voltage of this switching means 120 is determined higher than the maximum allowable voltage of the low-rate transformer 130 and lower than the variation lower limit value of high source voltage from a commercial power source. Consequently, if the high source voltage is applied by mistake across the 1st and 2nd input conductors L1 and L2, the switching means 120 will instantaneously be opened to prohibit the high power source voltage from being applied to the low-rated transformer 130. Various electric loads such as the low-rate transformer 130, etc. can thereby be protected accurately against abnormal voltage.