Continuously self-checking scanner alarm system

1,010,595. Automatic alarm systems. UNION CARBIDE CORPORATION. Dec. 27, 1961 [Jan 3, 1961], No. 46181/61. Heading G4N. [Also in Division H3] Relates to an alarm system, particularly a temperature alarm, including a rotary scanning switch 10 which monitors in turn the outputs of a number of devices, e.g. thermocouples 16, connected to the switch contacts 14, and a thyratron 20 which fires when any such output exceeds a certain level to operate an alarm or control relay 24. According to the invention a test signal is applied to one contact 14 so as to fire the thyratron once during each (or alternate) cycle(s) of the scanning switch, the thyratron being extinguished in this case before the relay 24 has time to operate, provided there is no fault in the system. The temperatures corresponding to the various thermocouples 16 are indicated in known manner on a C.R. oscilloscope 19. The synchronizing signal which initiates the horizontal trace of the tube 19 is of opposite polarity to the thermocouple outputs and is applied from circuit 28 over the first contact 14 of the scanner 10, via the scope "vertical" amplifier 18 to trigger a multivibrator MV1. This in turn triggers a multivibrator MV2 which over line 41 feeds a test pulse to the next contact 14 of the scanner. This pulse when scanned is amplified at 18 and fires the thyratron 20. A further multivibrator MV3 is fired as MV2 reverts to normal to cut off a series switching tube 30 which extinguishes the thyratron before the alarm relay 24 has time to operate. Under these normal conditions a further multivibrator MV4 is triggered as the thyratron fires to hold-out a relay 50 which, however, relapse in the event of a fault in the test " fail-safe " circuitry. The multivibrators are all mono-stable. MV1 may remain in its " set " condition for just over one scanning cycle of switch 10 so that test pulses are produced only in alternate cycles, Fig. 2 (not shown). This is to prevent the test pulse from precluding operation of the alarm relay 24 by the raised output of a thermocoupled scanned just prior to the test pulse.