Patent GB-360412-A (GB)
Title: Improvements in fire alarm systems
IPC: G08B29/16
Filed: 1930-07-30
Abstract: 360,412. Fire-alarm systems. TELEFONAKTIEBOLAGET L. M. ERICSSON, 33, Kungsgatan, Stockholm. July 30, 1930, No. 23020. Convention date, Aug. 1, 1929. [Class 40 (i).] In a fire-alarm system of the type in which fire and fault alarm apparatus at a central position are controlled over protective conductor loops each of which comprises two line wires which are adapted to be opened by thermostatic switches in the event of a fire to control the fire alarm and also facilitate the indication of a fault if one of the wires is interrupted, means are provided to cause operation of the fault alarm in the event of a short-circuit across the line branches. As shown in the Figure, protective loops O<1>, O<2>, containing thermostatic switches T<1> .. T<5>, each of which has contacts in both line wires of the loop, are provided at a central position with relays R<1>, R<2> adapted to control fire and fault alarms BK, FK. Normally the relays R<1>, R<2> are energized over the loop circuits, but in the event of fire, both line branches will be opened to de-energize both relays and operate the fire alarm BK over a circuit including key BT and contacts 1, 2 of the relays. Simultaneously the fault alarm FK is operated by the relay FR over the circuit including section alarm lamp FL, key FT, and contacts 5 or 6 of relays R<1>, R2. The fire alarm may be rendered inoperative by key BT which when in operative position indicates the section of the fire. The fault alarm may be nullified by key FT which when operated lights lamp DL and operates relay CR. When the protective loop is restored to normal condition, relays R<1>, R<2> are re-energized and the lamp DL is extinguished and relay CR released to cause illumination of lamp CL which indicates normal condition of the loop circuit. The keys FT, BT may then be restored to normal. When a fault alone occurs on one or other of the line branches, one or other relay R<1>, R2 releases to effect operation of the fault alarm over one or other contact 5, 6 in a manner similar to that previously described. In the event of a shortcircuit across the line branches, e.g. at P, a fuse S in one line is blown to prevent the short-circuiting of relay R<1> which remains operated over the other line. De-energization of relay R<2> effects operation of the fault alarm. In a modification corresponding ends of the line branches are supplied at the same polarity the relays being inserted at opposite ends. Furthermore, instead of a fuse S, a resistance may be employed. In a modification the fire and fault signals are controlled by separate relays. Each relay has two windings connected in the respective line branches the windings of the fire alarm relay acting additively to effect normal energization of the relay while the windings of the fault relay act differentially to cause normal de-energization. Upon recurrence of a fault the fault relay operates the alarm in a manner similar to that previously described. In the event of a fire, both windings of the fire relay are open-circuited to cause operation of the fire alarm. A short-circuit in the loop circuit causes the blowing of a fuse to open the short-circuit of one winding of the fault relay and thereby causing operation of the fault alarm. In a further modification, the independent windings of the relays are inserted in opposite ends of the line branches and one of the windings of the fire relay may serve the purpose of the fuse S or equivalent resistance.