Patent GB-361558-A (GB)
Title: Improvements in or relating to systems of electric remote control or supervision
IPC: H02J13/00
Filed: 1930-10-30
Abstract: 361,558. Distant control systems. COVENTRY AUTOMATIC TELEPHONES, Ltd. and SMART, N. C., of General Electric Co., Stoke, Coventry. Oct. 30, 1930, No. 32596. [Class 40 (i).] In means for preventing signals from two or more substations being sent simultaneously over common line wires to a receiving station, of the kind in which a substation has a guarding device normally connected to the line, but removed therefrom when the substation engages the line to signal to the receiving station, the substation having once engaged the line temporarily replaces its guarding device on the line wires. The times between the engaging of the line and subsequent retesting are different for different substations. Fig. 1 represents a series of graphs for three substations, the horizontal scale representing time and the vertical scale an engaged condition of the line. Station X seizes the line at 20 for a period 20-22 which is longer than the period 20-21, the time of operation of a guard device. If neither of the stations Y, Z has seized the line during the period 20-21, their guard devices are operated at 21 and remain operated until the line is freed by station X. This station replaces its guard device on the line for a period 22-23, and finally, if the line is free, re-engages the line at 23 and prepares a signal to the receiving stations. Two or more substations may engage the line in the time 20-21. To prevent such stations from retesting simultaneously, the minimum difference between any two pre-retesting periods is substantially equal to the retesting period of any substation plus a time equal to the operation of a guard device. If p = the operation time of the guard device and q a margin of safety the minimum safe pre-retesting period is p + q. The minimum safe time between the end of a retesting period 23 and the commencement 24 of the retesting period of the succeeding station also is p + q. The minimum pre-retesting period of station X = 20-22 = p + q. The minimum preretesting period of station Y = 20-24 = 3 (p + q). If there -are n stations the minimum pre-retesting period of the n<th> station is (2n-1) (p + q). Assuming for example that stations X and Y engage the line within the period 20-21. When X retests the station Y will operate X's guard device and prevent station X from again engaging the line. Station Y on retesting finds the line free and engages it. If more than two stations engage the line within the time 20-21, the station which retests last is the one which signals first. Figs. 3 and 4 show, respectively, a receiving and a transmitting ration. Assuming that a circuit-breaker with contacts cb1, cb2 trips, cb1 operates relay ST, and cb2 marks a contact on bank rt2 of switch RT. Contact st2 operates relay LA, st3 removes a guard relay G from the line wires 50, 51 and applies earth through contact t1, whereby the guard relays at all other stations, and L at the receiving station, operate, and st4 commences the operation of dashpot relay D. Contact la1 operates relay LB. At other substations the guard relays through their contacts such as gl cut relays such as ST out of circuit. At the receiving station contact b1 operates relay B which prepares a circuit for LX. At the end of the pre-retesting period of the substation, relay D operates relay T, whereby through t1 relay G is replaced across the line wires, and L is deenergized ; l1 operates LX, which at ln1 prepares a circuit for LY. Relay T due to the movement of relay D releases so that relay L is energized, Contact l1 holds relay B and allows relay LY to operate in series with LX. After a further interval relay D operates relay S which effects the operation of relays AS, BS, CS. The interaction of these relays causes alternately the release and actuation of relays LA, L, and the stepping of switches RT and RS until the wiper st2 reaches the contact marked by cb2. Relay SP then operates, sp1 operating, relay P, sp2 preventing further impulsing and sp3, locking SP through st5. P and PR are relays of the type described in Specification 354,847, [Group XXXV], in which the armatures remain locked mechanically after attraction by electromagnets ; p3 releases ST which drops slowly. At the receiving station, since relay L is now energized, contact l1 releases relay C, previously operated' upon the falling back of contact l1, c1 operates relay PR, the contact pr1 of which changes the indication given by lamps V, W. On the release of ST, st2 releases LA, st3 releases L and all the G relays of other substations, st4 releases relay D, st5 releases SP, and st6 steps the switch RT to home position; l1 releases B, whereby the contact b2 releases LX and LY and the contact b3 causes RS to step to home position. Instead of direct current being employed for engaging the line, carrier or voicefrequency currents might be used. The invention may be applied to systems such as described in Specifications 201,153 and 268,515, [both in Class 40 (i), Electric signalling systems &c.], in which signals from one station to another are checked back before transmission. The invention may also be applied to a system employing a plurality of telephone subscribers connected in parallel to one set of line wires.