In the event of fire or flooding, operation of switches from bridge/fire control area sends a signal to an oil diverter valve. Oil from a pressurised hydraulic system is sent to a ram moving the door.
The door may also be operated locally by a manual diverter valve. In addition, in the event of loss of system pressure the door may be operated by a local manual hand pump
remote door position indicators are fitted as well as were appropriate alarms to indicate operation.