Patent GB-471759-A (GB)
Title: Improvements in or relating to electrical signalling systems
IPC: H04M11/04
Filed: 1936-03-09
Abstract: 471,759. Fire-alarm systems. AUTOMATIC ELECTRIC CO., Ltd., and BROWN, W.J. March 9, 1936, No. 7118. [Class 40 (i)] In a fire-alarm system of the closed-circuit non-interference type employing street boxes capable of transmitting a code signal characteristic of the locality, telephone facilities are provided for permitting the transmission of calls of a different type, e.g. ambulance calls. The system is such that interference between telephone calls and fire alarm calls is prevented. The Figure shows a system of street boxes SS1 ... SS4 connected to a central station the circuits of the box SS2 being shown in detail. Fire calls. When the handle of the box is pulled contacts N1, N2 are operated to insert the non-interference magnets L1, L2 into the loop and earth the loop so as to permit signal transmission in the event of a fault on the loop. Simultaneously the code wheel is released and after a short test period, the code is transmitted and a gong G and tape recorder RE at the central station are operated. Relays LA LB, T also are operated and cause the number of the box to be displayed. In the event of a break in the loop, relays LA, LB, T release and a relay EW (not shown) operates contacts ew1, ew2 to connect the legs of the loop in parallel and earth the central station battery BA through the gong G and recorder RE. Under these conditions calls from the box are transmitted from earth over rectifier MR and the unbroken leg of the loop. Fire-alarm telephone facilities. When it is desired to make a telephone call from the fire alarm box, the door is opened and door contacts FADC are opened to remove a short-circuit across a sounder S, morse and telephone keys SK, TK. Depression of the telephone call key TK inserts resistance YC in the loop and effects release of marginal relay T at the central station to operate a telephone call alarm. The station operator thereupon operates contacts k1, k2 to switch in the telephone equipment. Communication with the street box may then be effected, the street box equipment being plugged into the jack TJ. Ambulance calls. The ambulance call equipment is mounted in a separate compartment having a door which is opened automatically when a glass is broken to set in motion clockwork mechanism for operating contacts Al, A2, A3 which ensure the transmission of a call under normal conditions, earthed and broken loop, or broken loop respectively. Door contacts ADC are opened to insert the equipment on the loop. Under normal conditions, sounder S is operated and the ambulance telephone hand set is connected in the loop at s1. In the event of a broken loop, current for the telephone equipment is supplied from the station calling over the unbroken leg to earth over rectifier MR. If an earth exists at the break in the loop, sounder S is operated to connect the telephone equipment in the unbroken leg, the earth fault being used to supply the necessary telephone current. The arrangements are such that an ambulance call does not interfere with the transmission of a fire call from another box. When another box is operating, the sounder S in the box from which an ambulance call is initiated is intermittently operated to prevent permanent earthing of the loop and consequential shortcircuiting of the box which is transmitting a fire call. Testing. A key ST is provided to facilitate testing of the box mechanism without transmitting a fire call.