Patent GB-519211-A (GB)
Title: Improvements in or relating to electrical signalling systems
IPC: G08B25/04
Filed: 1938-09-17
Abstract: 519,211. Fire alarm systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., JONES, E. W., and IRELAND, E. B. Sept. 17, 1938, No. 27182. [Class 40 (i)] In a closed circuit system in which the loop is intermittently opened and intermittent connections to earth are produced when a box is pulled, a test is first made for a permanent earth which gives an alarm, but if the loop does not open satisfactorily when there is no earth fault, the receiving circuits are automatically changed from loop working to respond to the intermittent-earth. In a modification the device responsive to the intermittent earth produces interruptions in the loop at the central station. Normal operation. When a box is pulled, off-normal springs N1, N2 operate, impulse springs IMPI, 2 remain closed for an initial test period and ground over rectifier MRA is fed to the loop. At the central station high resistance earth test relay E pulls up followed by ER which locks and shortcircuits E whereupon EX operates followed by EB, EC, ED. Relay EC reverses the polarity of the earth test and EX falls back at about the same time as ED operates. Relay'ET does not pull up to the ground over rectifier MRA, and its circuit is improved by the shunting of T, RA, RS, RB during the test. Relays EC, ED are released by the relapse of EX which is thus reconnected to the circuit, and relay EWS pulls up and locks during the release of ED so that EF does not have time to operate and E remains short-circuited. Relays T, RA, RS all respond to the impulses which are also punched on a tape at PR and gonged at GG. Relay RA repeats the impulses to P which operates an indicator over lead 11, and RS operates GR during the impulse series to disconnect the test relays E. EX, ET. Short-circuited box, and succession working. The operations proceed as for a normal box until the first break impulse occurs whereupon the non-release of RS causes SC to operate, lock, and change the loop to an earth return by connecting both legs over battery BA to the gong, puncher, and receiving relays to ground. Relay SD operates the short-circuited-box lamp SBL. On termination of the call P and G hold and slow-to-operate relay DA pulls up to release SC and restore the loop whereupon the receiving relays operate and P, DA fall back, SCA which was operated by SC falling back slowly to prevent the operation of EW at this time. If a call is originated at a normal box when a short-circuited box is running, the initial test ground thereat is of sufficient duration to hold the receiving relays long enough to release GR whereupon SE pulls up to release SC and restore loop working to the normal box and the short-circuited box falls out. During impulsing by the normal box, GR pulls up again and releases SE so that when the normal box has completed its call the short-circuited box resumes. Line break conditions. These differ slightly from the short-circuited box working described in the preceding paragraph and have inferior possibilities as regards succession working. In the case of a break, EW would operate at the point referred to in the preceding paragraph to change the circuits to full earth working conditions. A call operates SA or SB. (not shown), dependent on which side of the break the call occurs, to disconnect the other leg. Permanent earth. Relay ET operates in the test circuit when the battery polarity is reversed so that ETR operates and locks in series with EC so that EX and EWS do not pull up as in normal conditions. Consequently, EF pulls up and lights lamp EAL relays ETR and EC being released and E re-operated to maintain ER, EF and the lamp EAL. Lamp BEL is flashed in case of a permanent rectifier earth at the box. Telephone facilities in short-circuited box conditions. Momentary earth over a rectifier is applied manually in the case of a fire or police call or automatically as described in Specification 471,759 from an ambulance telephone box in the same loop. Short-circuited box conditions are set up at the central station as before, but as no code is received, RA is released and remains so: P is steadily operated, G and DA pull up, and SC releases to operate EW which brings in full earth working. This provides microphone current over the two legs of the loop in parallel, the box telephone circuit being as described in the above-mentioned Specification. The operation of SD disconnects one winding of the central station telephone repeating coil RPC which is connected up over key KTS to prevent speech currents from being balanced out if the calling box is in the middle of the loop. The telephone call lamp TCL is lit by TA which does not remain operated on an alarm call, since relay CO (not shown) operating on receipt of a code releases it. Modification. Fig. 3. On a normal call, RA, RB, T, PR and GG respond to the code as before, and sluggish relay PA pulls up to disconnect test relay E which, however, operates in case of a short-circuited box. ER and S then operate in turn, the circuit of delayed alarm relay EA is made, and the first relay M of a counting chain M ... S pulls up. In response to the code which is assumed to be of three digits, E alone responds to the impulses and ER relapses on the interdigital pauses with a result that the counting chain is saturated by the full code. On commencement of the second of the usual three code transmissions, SX operates and locks to bring about short-circuited-box working, and E now repeats the impulses to EP which repeats them in the loop thereby operating the normal code responsive equipment. In case of an earth fault, the delayed alarm relay EA pulls up followed by EF which lights lamp EFL. Specification 328,665, [Class 40 (i)], also is referred to.