Patent GB-808445-A (GB)
Title: Electrical network for detecting a condition, such as fire or overheat due to various causes
IPC: G08B17/06
Filed: 1956-07-17
Abstract: 808,445. Radiation-sensitive electric systems. SPECIALTIES DEVELOPMENT CORPORATION. July 17, 1956 [Oct. 27, 1955], No. 22057/56. Class 40 (3). [Also in Group XXXVIII] A self-re-setting bridge network responsive to a change of condition, e.g. heat, light or humidity or operable by a mechanical force, is described for operation as a fire or overheat detector in a jet aircraft (see Group XXXVIII). At normal temperatures the network is unbalanced and achieves balance just before an alarm temperature of 700‹ F. is reached by a steady rise of temperature. At and above such an alarm temperature the network is unbalanced in the opposite direction to give an alarm. The network also includes, a time constant circuit sensitive to rapid rates of temperature rise above 530‹ F., and a polarized relay sensitive to overheat above 600‹ F. The network comprises a D.C. source, a heat-sensitive variable resistor 12 which is of very high resistance at normal temperatures but of low resistance at alarm temperatures, further resistors 13-16 bridge-connected as shown, a polarized relay 17 forming part of a time constant circuit 39-41 sensitive to rapid rates of temperature rise above 530‹ F. connected to the voltage dividing resistor 14, the relay 17 without the time constant circuit being also sensitive to temperatures above 700‹ F., and a polarized relay 18 sensitive to overheat above 600‹ F. connected to the voltage dividing resistor 15. The relay 17 when energized either by the time constant circuit or by the bridge moves a switch arm 22 which energizes a relay 23 operating switches 30, 33, the switch 30 energizing a warning lamp 36 and the contact 34 providing a holding circuit. The relay 18 operates a switch arm 22a and a relay 24 operating similar switches 33a, 45, and a warning lamp 47, but when the relay 23 operates the circuit of the lamp 47 is broken via the switch 30. Diodes 28, 28a are provided to effect quick return of their respective relays and the relay 17 is slugged by the condenser 42 against the effects of flame flash. Protecting varistors 37, 38 and limiting resistors 26, 26a, 35, 35a are also provided. The settings of the voltage dividers 14, 15 and the values of the bridge resistors are such that the relay 17 operates at a fire temperature of 700‹ F. and the relay 18 at an overheat temperature of 600‹ F. The time constant circuit 39-41 which also incorporates the relay 17 discriminates between slow and fierce fires by separately operating the relay 17 when there is a rapid rate of temperature rise above 530‹ F., thus a fire condition followed by overheating and so on may operate an on-off sequence of the lights 36, 47 to indicate the condition prevailing. In Fig. 5 the graph a-l indicates the variation of temperature during the flight of a jet aircraft from warm-up and take-off to landing and cooling off. If a peak A representing a rapid rate of temperature rise appears during the flight then the system operates as follows. The time constant circuit operates the relay 17 above 530‹ F. because the rise of temperature is rapid and the relay 23 is energized to light the lamp 36. If the rate of rise decreases then the relays 17, 23 are deenergized by the time circuit, the switch 30 returns to make with contact 31 and the lamp 36 is extinguished. However, should the temperature still rise, but at a slower rate, then the relays 18, 24 and the lamp 47 are energized at 600‹ F. to indicate overheat. A further slow rise to 700‹ F. causes the bridge circuit to operate the relay 17, thus extinguishing lamp 47 and lighting lamp 36.