GB1206954A - Electrical testing apparatus - Google Patents

Abstract

1,206,954. Automatic alarm systems. D. D. BOWEN. 13 March, 1968 [16 March, 1967], No. 12341/67. Heading G4N. [Also in Divisions G1, H2 and H3] In an alarm system of the type which produces a given output in response to a change in an input exceeding a predetermined minimum duration, test pulses are repeatedly transmitted through at least part of the circuit, the test pulses being of a shorter duration than said predetermined minimum and are arranged to discharge a capacitor having a continuous charging circuit so that in event of a fault, the failure of the test pulses causes the capacitor to charge and provides a separate indication of faulty operation. As shown in Fig. 4, an alarm-operating relay 6 is arranged to be energized by the opening of contacts 21 which turns off transistor TR21, reverses the state of bistable transistors TR22, TR23, turns off TR7, and turns on TR8. A circuit 23 prevents operation by spurious short-duration pulses. The output of a multivibrator TR13, TR14 is differentiated at C1 and provides short-duration test pulses at the collector of TR15. These are injected into the base circuit of TR21, but are too short to operate relay 6 and are used to turn on TR16 which prevents capacitor C3 charging sufficiently to turn on TR17 and light the fault indicator lamp 2. " Fleeting " alarm circuit.-The bi-stable circuit comprising TR22, TR23 acts as a store to maintain the alarm condition after the cessation of any signal from contacts 22 of long enough duration to be passed by circuit 23. The alarm is reset by restoring the state of bi-stable circuit TR22, TR23 but if contacts 21 are still open (to indicate an abnormal condition) relay 6 will be re-energized and will continue to provide the alarm. The test pulses are also fed via a diode D1 to the bi-stable circuit TR22, TR23 so as to reset it immediately after it has been set by each test pulse. On receipt of a genuine " fleeting " alarm signal, TR7 is turned off and converts the multivibrator TR13, TR14 into a monostable circuit so as to inhibit the test pulses until the bi-stable circuit TR22, TR23 is reset manually. Modifications.-Fig. 3 (not shown) relates to a similar arrangement using a further multivibrator (TR9, TR10) operating flashing indicator lamps. The test pulse generator 7 and monitoring circuit 8 may be coupled sequentially by means of a uniselector to a plurality of alarm circuits.