GB964752A - Electrical protection system - Google Patents

Abstract

964,752. Automatic alarm systems. AMERICAN DISTRICT TELEGRAPH CO. Oct. 26, 1962, No. 40554/62. Heading G4N. Two different types of alarm systems are used in a fire or burglar alarm device, an alarm indication being given to a central station from the device in response only to simultaneous alarm signals from the two systems. Ultrasonic and microwave alarm systems 10, 19 are employed in Fig. 1, relays UA, UB, and MC, MB being shown in the non-alarm, energized position, while relay MA is in its non-alarm, de-energized position. For an alarm signal to be sent over lines 17, 18 must be disconnected, i.e. contacts UB3, UB4 and MC3, MC4 must open simultaneously, which requires the simultaneous energization of alarm systems 10, 19. Either of relays MA, MB of system 15 are operated under alarm conditions depending on whether there is an increase or decrease in the anode current of tube 20, respectively, the operation of either relay deenergizing relay MC. On closure of MC5, MC6, a resistor 27 will be connected in shunt with part of the potentiometer 26 of system 10 which sets the A.C. firing point of thyratron 11, resulting in a decrease in the alarm signal voltage necessary to cut off thyratron 11 and therefore increasing the sensitivity of system 10. A similar increase in sensitivity in system 19 occurs when relay UB drops out, since the engagement of UB6, UB7 disconnects shunt resistor 21 of relay coil MA and connects resistance 22 in shunt with relay coil MB. Since relays MA, MB operate on increasing and decreasing current, respectively, they are both made more sensitive. In order to prevent one system going into, and out of, alarm before the other, relays UB, MC are slugged by capacitors 16, 24. The increased sensitivity may be eliminated after a predetermined time under non-alarm conditions by thermal contacts 28, 29 (e.g. bimetallic thermostats) after de-energization of relays MC, UB due to the flow of A.C. current therethrough. Contact 29 is ganged with contact 30. A circuit is described for holding the alarm relays in their alarm position even though the rest of the circuit has reverted to the "no-alarm" condition (Fig. 2, not shown). A device incorporating a "capacity fence" and a photo-electric alarm system is also described (Fig. 3, not shown) A supervisory signal V 1 is derived in Fig. 4 at 112 to give an indication of failure of a part of the ultrasonic alarm system 110 or of loss of carrier signal. Under normal conditions the output alarm voltages V 2 , V 3 of the ultrasonic and microwave systems, respectively, are zero or very low, and the condition of alarm relay 118 will be determined only by V 1 , which will drop to a very low value and cause release of relay 118 only under fault conditions, due to a drop in the emitter current of T 1 , T 2 . Under alarm conditions, V 2 , V 3 tend to lower the emitter current of T 1 , T 2 , respectively, but increase that of T 2 , T 1 , respectively, so that the two alarm systems must have simultaneous outputs and B, C be at negative potential at the same time for the common emitter current of T 1 , T 2 to drop sufficiently to release relay 118. Arrangements for connection to Fig. 4 for automatically increasing the sensitivity of one alarm system when there is an output from the other is described (Figs. 7 and 9, not shown), the sensitivity being increased with increases in the alarm signal. Specification 704, 779 is referred to.