GB1178957A - Improvements in and relating to Electrical Protective Systems - Google Patents

Abstract

1,178,957. Automatic alarm systems. G. A. G. IVE. 29 April, 1968, No. 20320/68. Heading G4N. In a protective system Comprising a plurality of contacts, operated by a burglar or fire, connected in a single electrical loop, the system is arranged to have two modes of operation, in one of which the system is insensitive to operation of some of the contacts and in the other of which the system is sensitive to operation of all of the contacts. Such a system is particularly suited to day and night monitoring of a building when only some of the contacts are required to be monitored during the day. In one embodiment, Fig. 2, normally closed contacts C1-C5, e.g. on doors and windows, and normally open contacts M1, M2, e.g. in a floor mat, are connected in series and parallel respectively, into a single electrical loop including a D.C. source B and normally energized relay R1. The two modes of operation of the circuit are provided by switching the direction of current flow (by means not illustrated) and connecting unidirectional current conducting devices, e.g. diodes D1, D2, D5, D4 or electrolytic capacitors, in parallel with those normally open contacts C1, C2, C5 and normally closed contacts M2 which require monitoring only in one mode, i.e. at night. The diodes or capacitors are so connected that, for one direction of current flow, the current through the relay is not materially affected by operation of any of these contacts, while in the other direction of current flow, it is de-energized by such operation to energize an alarm circuit. In an extension of the above circuit, Fig. 3 (not shown), a second normally de-energized relay (R2), is connected in series with the battery and further normally open contacts (F1)-(F4), each connected in series with a resistor (r2)-(r5), are connected in parallel with the relay (R1), the relays (R1), (R2) and resistors (r2)-(r5) being so chosen that, if one of the switches (F1)-(F4) is closed, relay (R2) is energized, to energize one alarm circuit and if one of the switches (M1), (M2) is closed, relay (R1) is de-energized opening contacts (R1/R2) to prevent energization of relay (R1), to energize a second alarm circuit. In the example described, the contacts (F1)-(F4) are five alarm contacts, whilst the remaining contacts are intruder alarm contacts. In a second embodiment (not illustrated by drawings) those normally closed contacts which require only part-time monitoring are shunted by resistors such that at one energizing voltage lever the normally energized relay is not deenergized by their operation but at a lower voltage level the relay is de-energized to energize an alarm circuit. In a third embodiment, Fig. 5 (not shown), the normally closed contacts (C1, C3) are shunted by capacitors (C10, C11) and the circuit is energized during the day by A.C. and at night by D.C. In a fourth embodiment, Fig. 6 (not shown), the relay is made insensitive to the operation of those contacts in the form of single-pole, changeover switches (C1, C3) by connecting a capacitor (C12) across it. At night the capacitor is removed from the circuit by opening of switch (53) so that operation of one such contact causes relay R1 to be de-energized temporarily to permanently open the circuit by opening relay contact (R1/1).