to assess the state of the system and predict failures before they occur. Other methods include partial discharge (PD) testing, using either fixed or portable testers, and acoustic emission testing using surface-mounted transducers (for oil equipment) or ultrasonic detectors used in outdoor switchyards. Temperature sensors fitted to cables to the switchgear can permanently monitor temperature build-up. SF6 equipment is invariably fitted with alarms and interlocks to warn of loss of pressure, and to prevent operation if the pressure falls too low. The increasing awareness of dangers associated with high fault levels has resulted in network operators specifying closed-door operations for earth switches and racking breakers. Many European power companies have banned operators from switch rooms while operating. Remote racking systems are available which allow an operator to rack switchgear from a remote location without the need to wear a protective arc flash hazard suit. Switchgear systems require continuous maintenance and servicing to remain safe to use and fully optimized to provide such high voltages. See also Arc flash Circuit breaker Disconnector Electrical safety Electric arc High voltage Remote racking system Short circuit References External links Electric power systems components Electric power infrastructure