Patent GB-314512-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q3/00
Filed: 1929-02-18
Abstract: 314,512. Associated Telephone & Telegraph Go., (Assignees of Voss, J. H., and Willis, B. D.). June 29, 1928, [Convention date]. Automatic exchange and metering systems.- -A primary line switch PLS of plunger type has a line relay 24 which grounds the holding wire 23 to busy the line to connectors at once and to send the master switch on when the line switch plunges. Operation of the plunger is delayed, to give time for the line switch to realign itself if a subscriber initiates a call immediately after releasing one, until the operation of a relay DR the high resistance winding of which is in the operating circuit and which shunts this winding by a low resistance winding which is normally short-circuited to make the relay slow to pull up. The operating circuit also includes a chain of contacts controlled by the line relays of the preceding switches in the group to prevent two switches from plunging simultaneously and where two groups of line switches have access to common trunks such as 7, Fig. 2, a relav 13 in the operating circuit of one group controls contacts in that of the other to prevent simultaneous plunging by a switch in both groups. The secondary switch SLS is similar to the switch PLS except that it has not a separate cut-off armature and that the holding winding receives current as soon as the switch is seized. The secondary switches are marked busy by a relay 65 when all their outlets are busy. If one of them is seized before the busy mark has become effective, its line relay 41, which normally holds the connection pending the return of ground from the group selector, is unable to do so, and the primary switch is released and plunges again when the master switch finds a new trunk. The 4th plunger springs 30, 46 are not needed for control and can be used for single metering under the control of an electro-polarized relay 32 in the line circuit or for multiple metering from apparatus ahead, relay 32 being retained or omitted as indicated by the alternative connections shown in dotted lines. Grading. Relays 15, 16 busy the common trunks 7 (which lead to secondary line switches) to their respective master switches PMS1, PMS2, while there are idle individual trunks 6, 8 (leading direct to group selectors). Master switches. When a primary line switch plunges, or when the last outlet of the secondary group on which it is standing is taken into use, SR pulls up, opens the line-switch operating circuit and energizes LR which unlocks the master switch and energizes OR which shortcircuits its upper winding to make itself sluggish and opens a further point in the operating circuit. When the master switch, which is driven by a spring, finds an available trunk, SR falls releasing LR which locks the master switch and OR which after a short-interval restores the operating circuit to its normal condition. When the masterswitch reaches the end of its bank, springs LS change over and if the last trunk is busy, TR pulls up, locks up mechanically, holds LR and energizes the restoring solenoid 71. When the master-switch reaches the beginning of the bank the springs of TR are unlatched. The secondary master-switch is similarly controlled except that when all its outlets are busy, ground is removed from 57<1> and the master-switch does not hunt. Alarm arrangements; statistical metering. A meter M4 registers the number of times all the outlets of a secondary switch have been busy. A meter M3 counts the calls through a group of primary switches and a meter M5 those through a group of secondary switches. A relay 58, which is of the kind described in Specification 156,076 and only closes its contacts after its circuit has been made for a predetermined time, is energized in parallel with LR and operates a meter M2 when hunting is prolonged. Alternatively the control of 58 is given to OR jointly with the line relays as indicated by the connections shown in dotted lines and M2 then records calls attempted during an all trunks busy period. Circuit 75 is grounded by SR. TR or LR when the masterswitch operates, or by the relapse of CR on the operation of a line relay, and if this ground persists a delayed alarm on 51 is operated. In the secondary master-switch the alarm wire is connected to a delayed action relay 61 similar to relay 58. 61 energizes a sluggish relay 62 which energizes 65 to mark the group busy and makes the delayed alarm circuit.