Patent GB-605015-A (GB)
Title: Improvements in or relating to electrical alarm signalling systems
IPC: G08B17/06
Filed: 1945-12-12
Abstract: 605,015. Fire-alarm systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., and PREIST, T. P. Dec. 12, 1945, No. 33572. [Class 40 (i)] In a fire alarm or similar electrical system, of the radial closed circuit type and A.C. energized through metal rectifiers at the signalling points, each control relay has two opposed windings connected in parallel, one of which is series connected with a capacitor to produce a null response with A.C. energization. In the embodiment shown, the spring-loaded plunger SLP at the call point MCP normally separates contacts C1, C2 and resistor YA is thus connected, through metal rectifier MRA, in series with the relays H, L, secondary winding TRB of transformer TR, the loop lines A, B and lines 10, 11 to which other loops are multipled. Relay L is more sensitive than relay H, its contacts are normally closed, and both relays have two mutually opposed parallel connected windings of which one is in series with a capacitor, that of relay H with condenser QA and that of relay L with QB. Winding TRC supplies power to the alarm circuit subject to control exercised by relays H, L. A call, i.e. the breaking of the glass at the call point, causes the forward movement of the plunger SLP, due to its associated spring, the closing of contacts C1, C2 and the short-circuiting of resistor YA. The consequent increase of line current pulses renders relay H operative, in addition to normally-operated relay L, and the following circuit changes occur: (1) contacts H1 close and complete a circuit through secondary winding TRC, contacts L1 and KR1 to light fire call lamp FCL at the central station; (2) contacts H2 close and a circuit is made through winding TRC contacts L2, H2, KR3 to engage relay FB causing closure of contacts FB1, FB2, associated with lines 16, 17 and 18, 19 respectively, and thus effect the operation of a fire bell together with the transmission of a fire call to the public fire alarm system. To " wipe out " a call at the central station, a key individual to the calling station is moved, the contacts KR1, KR3 are broken and contacts KR2, KR4 are made, thereby extinguishing lamp FCL and silencing the bells energized from relay FB. On the restoration of normal conditions at point MCP, the relay H is returned to its inoperative state, the contacts H1, H2 and circuits are made through contacts KR2, KR4, thus lighting lamp LCL and ringing bell FRB and indicating, that the key may be returned to its original position, whereupon lamp LCL is.extinguished and the bells FRB silenced. In the case of a fire call from the intermediate call point ICP, an exactly similar sequence of events is caused and the same fire call lamp FCL is lit. The faults catered for by the system are: (1) Failure of supply. The double wound relay MF energized through oppositely connected rectifiers MRE, MRF causes normally open contacts MF1, MF2 to close on supply failure, thereby ringing-bell MFB and lighting lamp MFL from battery PB. This indication is also given upon a failure in the winding TRC. (2) Break in lines A, B. Both relays H, L are de-energized, the contacts L1, L2 close, the lamp LFL lights and the bell FRB rings. (3) Short circuiting. If the loop A, B is short-circuited between the nearest station and the central station, A.C. current will flow through both windings of relays H, L which will then be de-energized and fault alarms will be given as in (2). (4) Earthing. The system remains operative with one or other of lines A, B earthed, but simultaneous earths simulate a short circuit with an alarm indication as in (2). Earth testing relay ET comprises two windings energized through metal rectifiers MRC, MRD and connected to a common earth point. Closure of key ETK1, with the existence of an earth in line B effects closure of contacts ET1, ET2 and the consequent lighting of lamp ELL and energization of bell FRB. Similar indications of an earth on line A are given by operation of key ETK2. The fault may also be traced by the removal of the links LA, LB.