GB1291755A - Ionization fire alarm with insulation monitoring - Google Patents

Abstract

1291755 Ionisation smoke detectors CERBERUS A G 24 June 1970 [24 June 1969] 30653/70 Heading G1N In an ionisation fire alarm, an alarm is provided both when the potential at the junction of the measuring ionisation chamber 1 and a resistance element such as a reference chamber 8 exceeds an upper threshold and falls below a lower threshold, i.e. both when the potential drop across the measuring chamber 1 increases above a first pre-set value due to fire detection or fall in insulation resistance of the reference chamber and when the potential drop across chamber 1 falls below a second preset value due to a fall in the insulation resistance of chamber 1. As shown, Fig. 4, a pair of complementary F.E.T.'s 10, 11 have their gates connected to the junction, one transistor firing when the junction potential exceeds the upper threshold and the other transistor firing when said potential falls below the lower threshold to fire a SCS 17 via its two control electrodes 17a, 17b to energize the alarm relay 18. Other arrangements utilizing a pair of complementary F. E. T.'s are described with reference to Figs.3, 5 and 6 (not shown). In alternatives, a pair of similar F. E. T.'s are utilized, Fig. 7 (not shown), a single F.E.T. is utilized, Fig. 8 (not shown), a cold cathode tube having two discharge paths is utilized, Fig. 2 (not shown), and a cold cathode tube is connected across both the chambers 1 and 8, the grid being connected to the junction, and a discharge tube is connected across the chamber 8, Fig. 1 (not shown). As shown in Fig. 9, the F.E.T.'s 44 and 45 control a transistor 46 thereby providing a selfholding circuit. In a modification, Fig. 10 (not shown), a transistor (53) has its collector emitter path connected across the resistor 50, the base of the transistor (53) being connected to a voltage divider connected across the resistor 47 and the drain electrode of the F.E.T. 44. Instead of providing the same alarm signal for fire detection and insulation resistance breakdown, separate signals can be provided, Figs. 11-13 (not shown), Fig. 11 corresponding to Fig. 4, Fig. 12 corresponding to Fig. 9, and Fig.13 corresponding to Fig. 10. The fire alarm signal may be a D.C. signal, and the insulation resistance breakdown alarm signal may be an A. C. signal or a repeatedly interrupted D. C. signal, said signal being transmitted to a central station either over the power supply lines, in coded form, or by radio means.