Fire detector systems having false alarm prevention

Claim 1

A FIRE DETECTION SYSTEM INCLUDING A NEGATIVE TEMPERATURE COEFFICIENT RESISTOR DEVICE FOR SENSING FIRE CONDITIONS, SAID SENSING DEVICE BEING CHARACTERIZED AS HAVING A THERMAL TIME DELAY IN ITS RESPONSE, FIRST AND SECOND WHEATSTONE BRIDGES HAVE A COMMON BRANCH, SAID SENSING DEVICE BEING CONNECTED IN AN ARM OF SAID COMMON BRANCH, RESPECTIVE DIAGONAL OUTPUT CIRCUITS CONNECTED TO SAID BRIDGES EACH INCLUDING AN OUTPUT RELAY, SAID FIRST BRIDGE BEING PRE-SET IN A STATE OF UNBALANCE TO OPERATE ITS RESPECTIVE RELAY WHEN THE RESISTANCE OF SAID SENSOR DEVICE FALLS TO A FIRST PREDETERMINED VALUE, SAID SECOND BRIDGE BEING PRE-SET IN A GREATER STATE OF UNBALANCE TO CAUSE ITS RESPECTIVE RELAY TO BE OPERATED WHEN THE RESISTANCE OF SAID SENSING DEVICE FALLS TO A SECOND RESISTANCE VALUE SUBSTANTIALLY LOWER THAN SAID FIRST VALUE, MEANS FOR DELAYING SAID FIRST RELAY RELATIVE TO SAID SECOND RELAY BY AN OPERATE TIME WHICH IS SUBSTANTIALLY LESS THAN THE RESPONSE TIME OF SAID SENSING DEVICE IN FALLING FROM SAID FIRST TO SAID SECOND RESISTANCE VALUES RESPONSIVE TO THE SENSING DEVICE BEING SUBJECTED TO AN ELEVATED TEMPERATURE WHEREBY UPON EXPOSURE OF SAID SENSING DEVICE TO A FIRE CONDITION SAID FIRST RELAY IS THE FIRST TO BE OPERATED BUT UPON A SHORT CIRCUIT DEVELOPING IN SAID SENSING DEVICE SAID SECOND RELAY IS THE FIRST TO BE OPERATED, AN ALARM CIRCUIT, AND MEANS CONTROLLED JOINTLY BY SAID RELAYS FOR ACTIVATING SAID ALARM CIRCUIT.