Electro-optical voltage sensor circuit monitoring leakage or loss of vacuum of a vacuum interrupter and vacuum circuit interrupter including the same

A vacuum circuit breaker includes a line terminal, a load terminal and a vacuum interrupter having a vacuum envelope containing a fixed contact assembly and a moveable contact assembly moveable between closed and open circuit positions. The fixed contact assembly is electrically interconnected with the line terminal and the moveable contact assembly is electrically connected to the load terminal. An operating mechanism moves the moveable contact assembly between the closed and open circuit positions. An auxiliary contact indicates one of the open or closed circuit positions. An electro-optical voltage sensor determines a voltage difference between the line and load terminals, or the shield and one of the line and load terminals. A circuit determines no leakage or loss of vacuum of the vacuum envelope when the vacuum switch is intended to be in the open circuit position and when the voltage difference is greater than a predetermined amount.