Fire sensing system protected from noise factors

Abstract

Fire detection signals are produced with high reliability based upon a quantity smaller than a predetermined quantity from a fire sensing system which detects predetermined standard quantities of combustion products (or physical quantities) such as heat, smoke or light produced by a fire. The system includes a fire sensor connected in parallel with a first semiconductor switching element and a second semiconductor switching element contained in two independent power-supply circuits. The first power-supply circuit contains the first semiconductor switching element which detects a combustion quantity smaller than the standard quantities and which is rendered conductive upon receipt of an early output produced by the fire sensor. The first power supply circuit is connected in parallel with a second power-supply circuit containing the second semi-conductor switching element which detects the standard quantities and which is rendered conductive upon receipt of an alarm output produced by the fire sensor. A substantial delay time based upon a statistical time difference expected to exist from an early output corresponding to the initial stage of the occurrence of fire to the production of an alarm output corresponding to the decision that a fire exists is provided between the operations of the first semiconductor switching element and the second semiconductor switching element. Noise energy induced in the system is absorbed by the first semi-conductor switching element rendered conductive after the substantial delay time has passed, such that the second power-supply circuit including the second semiconductor switching element is maintained in a proper condition to control the production of the fire alarm.