GB1019407A - Electrical protection systems - Google Patents

Abstract

1,019,407. Selective signalling. AMERICAN DISTRICT TELEGRAPH CO. Sept. 15, 1964 [Sept. 18, 1963], No. 37707/64. Heading G4H. An alarm system of the type in which alarms are signalled to a central station either by a rise or by a fall in a normal current is characterized in that while such changes are immediately indicated at the central station they are properly registered only if they are of sufficient duration. In this way, spurious alarms are rejected. As described, Fig. 1, a current of 15 ma. normally flows in a line 12 connecting a protected area with the central station. This current flows in series through two relay windings B and G and is sufficient to maintain relay B energized but not sufficient to energize relay G. If the current in the line 12 falls to below 9 ma., this being caused by a break in the circuit at the protected area and being referred to as a " break " alarm, then relay B drops out. This causes, in parallel B1, a " break alarm " register relay BA to pick-up and prepare to hold over BA3, and a warning lamp HL to be energized. Pick-up of BA breaks the energizing circuit BA2 of a timing relay T, which, if the break alarm is of sufficient duration-e.g. 2 seconds-eventually falls out and completes the holding circuit for BA over T3. The " break " alarm is now registered, and a " break alarm " indicating lamp BL is energized. When the current in line 12 returns to normal, relay B picks-up, lamp HL is extinguished, and the circuit can then be reset by manually actuating ganged switches SA and SB. If the current in line 12 rises to above 21 ma. as a result of some part of the circuit being grounded, this being referred to as a " ground " alarm, then relay G picks-up. Its contact G1 breaks the energizing circuit for the timing relay, and if the " ground " alarm is of sufficient duration, e.g. ¢ sec., relay T falls out and its contact T2 energizes a second winding G<SP>1</SP> on the relay G thereby holding it, thus registering the " ground " alarm and energizing a " ground " alarm lamp GL. The delay associated with relay G is shorter than that associated with relay B because only one capacitor 35 of the pair of capacitors 34, 35, is involved. The circuit also responds, and indicates-in a distinctive manner-a " double drop " signal, that is an alarm signal comprising a break signal of short duration (e.g. 30 msec.) immediately followed by a ground signal. When a " double drop " signal occurs first relay B falls out and relay BA picks-up, and then, simultaneously, relay B is re-energized and relay G picks up. However, the re-energizing of relay B does not cause relay BA to fall out because since relay G has also picked up relay BA remains energized over contact G2 and diode 39. The final result is that if the " ground " alarm component is of sufficient duration both lamps GL and BL are energized. The circuit can also accept " police calls " from the protected premises, that is, a succession of " break " signals at about 3 per second, caused by the actuation of a hidden alarm device on the protected premises. A transistorized form of the circuit is also described (Fig. 2, not shown).