GB807033A - Electrical fail-safe systems - Google Patents

Abstract

807,033. Photo-electric flame detection. SCULLY SIGNAL CO. July 31, 1956 [May 8, 1956, No. 23642/56. Class 40 (3). A fail-safe system for monitoring fluctuating radiation comprises means to produce an electric signal fluctating in correspondence to the radiation, means to recover the fluctuations from the signal and indicating means operable in the event of failure to recover the fluctuations for a time longer than the largest fluctuation. Infra-red light within 5-25 cycles per second from a gas burner is detected by a lead sulphide photo-cell and the signal amplified by a known high input impedance electronic monitor amplifier (not shown). Attached to the output terminals, 10, 12 of the system is an electromagnetic relay RY1 adapted to respond to fluctuations within the range of 5-25 cycles per second and shunted by a holding capacitor of the same reference C10 as the holding capacitor of the relay RY. A suppression circuit for maintaining the relay RY energized so long as the fluctuations are reproduced by synchronous energization and de-energization of the relay RY1 includes a capacitor C12 which alternately stores and discharges D.C. energy to feed the slow release load relay RY in synchronism with the recovery of the fluctuations in the signal. With the relay RY1 energized (not shown) armature 94 operates to pivot switches 90, 92 so that current from an A.C. source L1, L2 may be rectified at S to charge the capacitor C12 with a D.C. potential. With relay RY1 deenergized (as shown) the capacitor C12 discharges. In the event of failure to continue the current after the capacitor has discharged relay RY de-energizes activating a warning or other device. Known count-down apparatus may be used so that the relay is energized once every group of fluctuations. An alternative apparatus, Fig. 1 (not shown), comprises a step-up transformer tuned to the frequency band of the radiations to be detected, the signals from which are rectified and passed through a load relay which is shunted by a holding capacitor adapted to delay discharge of the relay for a time longer than the longest period of no-signal. Other sources of energy with inherent fluctuations, light of other wavelengths, sound, &c., may be detected by suitable means. Specifications 793,511 and 793,512 are referred to.