Under-voltage release under-voltage (YU). The undervoltage release opens the circuit-breaker when there is a significant voltage drop or power failure in its power supply. It can be used for remote tripping (using normally-closed pushbuttons), as a lock on closing or for controlling voltage drops in the primary and secondary circuits. The power supply for the release is therefore obtained on the supply side of the circuit-breaker or from an independent source. Closing of the circuit-breaker is only allowed when the release is powered (the closing lock is implemented mechanically).