PURPOSE: To enhance safety by connecting capacitor current of each of two groups differentially with the primary winding of a current transformer having two sets of primary windings and further connecting the secondary winding of the current transformer with a relay which functions upon detection of a current or a voltage thereby preventing short circuit of a capacitor which may cause eruption of oil.

CONSTITUTION: Since DC capacitors 1, 2 comprise a plurality of capacitor elements connected in series, capacitance of the capacitors varies upon breakdown of a part of the capacitor elements thus causing variation of ripple current or pulse current produced through charge/discharge of the capacitor. When a part of the capacitor elements in the capacitor 1 is broken down, current flowing through the capacitor 1 exceeds the current flowing through the capacitor 2 and they are not canceled in a current transformer 6 thus inducing a current and a voltage in the secondary winding. Thus induced current and voltage are detected through a relay 7 which then produces a contact signal for opening a power supply.