Patent GB-328665-A (GB)
Title: Improvements in fire alarm systems
IPC: G08B29/06
Filed: 1929-01-04
Abstract: 328,665. Reid, F. Jan. 4. 1929. Fire, police, and like alarm systems. - In order. to ensure reliable non-interference working of the alarm boxes in a " closed circuit " fire or like alarm system and to facilitate testing in''open circuit " systems, a current rectifier is connected in the earth or metallic, return lead of each alarm box. Closed circuit working. As shown in Fig. 1, numerical code alarm boxes A, B, 0 are connected in a loop circuit to central station registers 1, 2. Normally, current flows round the loop but upon operation of any alarm e.g. box A, lever 9 is operated in accordance with the code disc 8 to intermittently open the loop circuit and thereby effect registration of the corresponding signal. If another box is already in operation coils 10, 11 remain deenergized and effect locking of the lever 9 to prevent signalling in accordance with known principles. In the event of an open-circuit fault in the loop the central station switch 13 is reversed to earth one pole of the battery and connect the other pole to the two undamaged portions of the loop in parallel to facilitate signalling over either of those portions and earth return. In order to eliminate interference by the short-circuiting during signalling of an operated alarm e.g. box B, by alarms A, C through the earth connections of the boxes, rectifiers 14, 15, 16 are included in the earth leads and are connected in the same direction. With this arrangement, current flow between the boxes A, C through earth to short-circuit the box B is prevented. In addition to the coils 10, 11 a third coil may be provided connected in the earth circuit of the box to ensure greater magnetic force on the armature 12 (which co-operates with the lever 9) when earth working of the system is employed. The third coil overcomes the objections that normally, under earth working conditions, only one coil 10, 11 is effective and that in the event that the break in the loop is repaired the cores 10, 11 act in opposition and thus prevent normal operation of the box. Open, circuit working. As shown in Fig. 3, an alarm box 1a is connected to the central station CS over a single line with earth return although a metallic return may be provided. A rectifier 4<a> is connected in the earth circuit and the arrangement is such that normally no current can flow from the station battery 6<a> over the alarm indicator 7<a> and line but upon operation of the alarm box to reverse the connections of the rectifiers, the indicator 7<a> is operated and a relay 8a is energized in the return circuit. Relay 8<a> operating contacts 9<a>, 10<a> to reverse battery connections becomes de-energized and restores the initial conditions. Relay 8<a> thus vibrates to send impulses through the indicator 7<a> which being slow releasing holds up to operate a signal in the circuit of contact 11<a>. In the event of an earth fault, on the line, relay 8<a> is continuously energized and operation of the alarm signal is prevented. Upon receipt of an alarm, plug 12<a> is inserted in the jack 13<a>and the connections of battery to line are reversed. Upon restoration of the alarm box indicator 14<a> is operated. Telephone apparatus may be switched into circuit by reversing the key 16<a>. The provision of a rectifier at the alarm box facilitates testing since the line may be tested merely by reversing the station battery. A plurality of lines may be tested automatically and in succession by a master clock which at predetermined intervals initiates the test, a step-by-step operated cam controlling switches associating a testing relay with the lines in turn. Alternatively the test switches may be controlled by a motor. Fig. 4 shows testing equipment comprising contacts 21 - - 25 which when reversed cut out the station battery 6<a> and alarm indicator 7<a> and switch in a testing battery 20 connected to a differential relay 18 one winding of which is connected to the line and the other to earth through a rectifier 19. In the event of a line fault relay 18 is energized to operate the fault indicator 27. When the alarm indicator is operated contact 39 is opened to prevent actuation of the fault indicator. Each line is provided with a set of contacts 21 - - 25 the sets being operated in succession during the test.