Emergency supervisory system

An emergency supervisory system comprising:

• a central signal station and a pair of power lines connected thereto in a loop for receiving therefrom a supply voltage and call signals corresponding to individual addresses;
• a plurality of individual supervisory means each having a respective address and connected in parallel to said lines for receiving therefrom said supply voltage and a respective call signal corresponding to the respective address thereof;
• each supervisory means comprising detector means disposed in a corresponding supervisory region for monitoring physical variables for emergency conditions, and trunk means for receiving data acquired by said detector means relating to the physical variables being monitored;
• such trunk means further receiving the corresponding call signals as instructions to transmit the acquired supervisory data to the central signal station;
• a plurality of switching means respectively connecting said plurality of supervisory means to said power lines in parallel;
• each switching means comprising detection circuitry connected to one power line for electrically detecting the presence and absence of fault conditions on a corresponding load side thereof, a switch connected to said one power line, and switching control circuitry for actuating said switch to an open state for effectively disconnecting the corresponding supervisory means from said one power line to thereby electrically isolate it from the system when a fault condition is detected and for actuating the switch to a closed state maintaining connection of the corresponding supervisory means to the power line when the absence of a fault condition is detected;
• and
• means in the central signal station for receiving the supervisory data transmitted by the supervisory means and for effecting transmission of said call signals and said supervisory data to an opposite end of the power line loop other than the end thereof in use preceding the detection of a fault condition when any of said switching means determines a fault condition and has disconnected a supervisor means;
• said switching means detection circuitry comprising:
• a first short-circuit detecting circuit for detecting a short circuit between the power lines when the line voltage at the connection of said switch to said one power line is lower than a first predetermined voltage which is lower than the line voltage following such connection;
• and a second short-circuit detecting circuit for detecting a short circuit between the power lines when the line voltage at such connection is lower than a second predetermined voltage which exceeds the line voltage following such connection;
• said switching control circuitry comprising a switching control circuit responsive to outputs of the first and second short-circuit detecting circuits.