Patent GB-936302-A (GB)
Title: Fire alarm or warning system
IPC: G08B17/06
Filed: 1960-01-14
Abstract: 936,302. Fire alarms. GRINNELL CORPORATION. Jan. 14, 1960 [Jan. 20, 1959], No. 1363/60. Class 118. A fire detection system comprises a Whetstone bridge including temperature-sensitive resistance in two of its arms which provides separate indications when (1) the rise of temperature is relatively small corresponding to a small fire or the beginning of a larger one, (2) the rise of temperature is large corresponding to a large fire, (3) the rise of temperature is very large corresponding to an electrical fault in the system such as an open circuit. (1) The bridge becomes slightly unbalanced and the resulting change in potential between B1, B2 will oppose the normally existing small steady potential and the armature 80 of a galvanometer relay 78 engages contact 81 when the resultant potential across B1, B2 has reached a certain level. The coil of a normally closed relay 30 is thus short-circuited, causing armature 28 to engage contact 32 so that current no longer flows through lead 40 and the coil of normally energized relay 58 so releasing the relay and operating an alarm (not shown). The supervisory current through lead 10 will increase and will be sufficient to initiate the " warming up " of a time delay relay 26 which forms part of the circuit for energizing a solenoid 88 controlling a sprinkler system. (2) The unbalance potential will be sufficient to operate a further galvanometer relay 82 as well as relay 78. When the contacts of relay 82 and, after the elapse of the time delay, of relay 26 are closed, the resistor 36 will be taken out of circuit and the increased current due to this now flowing through the coils of relays 42, 50 will operate these relays to actuate sprinkler system solenoid 88 via armature 48 and contact 44 and an alarm (not shown) connected to terminals 54, 56. Both relay 26 and 82 must therefore be closed before the sprinkler system is turned on. (3) The unbalance potential will become very great and operating of the sprinkler system will be prevented by the energization of a relay 86 connected across B1, B2 and is arranged so that it will respond only to a large unbalance potential even if this is of the same polarity as the steady normal potential across B1, B2 (indicating a drop in temperature when relays 78, 82 do not respond). Current passes through the coil of a relay 16 to move armature 14 from contact 24 to 18 so that some of the current to the coil of relay 58 will be by-passed through lead 35 and the alarm connected to relay 58 energized. Relay 86 will respond in less than the '"warm up time of relay 26 thereby preventing operation of the sprinkler system. Armature 14 will also connect 20, 22 to short out the relays 78, 82 to prevent damage to them due to a large out of balance current. Supervision.-If one of the leads 72, 76 or 29 is open-circuited the loss in current in relay 30 will divert the supervisory current to lead 33 and the loss in current in relay 58 will operate an alarm, similarly if B1, B2 are opencircuited the loss of the steady biasing potential will cause relay 78 to close, thereby shortcircuiting relay 30 and operating an alarm. Similarly, if lead 10 or 40 is open-circuited relay 58 will be released to operate the alarm. A second embodiment in which relays 86 and 16 are replaced by one double wound relay is also described (Fig. 3, not shown).