Patent GB-261880-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q3/00
Filed: 1925-09-28
Abstract: 261,880. Automatic Telephone Manufacturing Co., Ltd., and Ostline, J. E. Sept. 28, 1925. Automatic exchange systems; metering.-Relates to means for preventing the response of the cut-off relay of a rotary line switch of the homing type when a connector seizes the associated line during the time that the line switch is returning to normal. In known systems in which the private normal is earthed through the cut-off relay at this time, this is usually effected by making the resistance of the connector test relay high compared with that of the line switch cut-off relay, but according to the invention a like result is obtained by two methods which involve no special provision in the connector. According to one method, an application of which is shown in Fig. 1, the cut-off relay is not connected to the private normal, which is directly earthed during the whole time the line switch is in an off-normal condition, whilst in another method, illustrated in Fig. 2, one end of the cut-off relay is connected to the private normal but the other end is disconnected from both battery and earth while the line switch is returning to normal. In the arrangement shown in Fig. 1, the private normal PN is connected to the homing wiper H, which in every position but the first engages two metal plates HP, LP insulated from one another : the upper homing plate HP is connected to the rotary magnet DM over back contacts of the line and cut-off relays L, K, and the lower guarding plate LP is earthed. The first contact 1 is strapped to the contact 5 engaged by the test wiper P in its home position. When the line relay L picks up, the private normal PN is earthed over wiper H, and a driving circuit is closed for the magnet DM over the strapped contacts 1, 5. After moving from the home position, the switch continues its advance in the usual manner under the control of its test wiper P until an idle line is seized, when the cut-off relay K energizes in series with the driving magnet, applies a preliminary guarding potential to the test wiper P, locks to ground from the switch ahead on wiper P, and prepares a circuit for the meter M<1> which is completed when metering ground is returned over wiper M. The switch is restored over a homing circuit including the bridged plates LP, HP, on the out-off relay relapsing consequent upon the removal of earth from the test wiper P. The private normal PN is grounded over the plate LP all the time the switch is in an offnormal position. A modification is described which is very similar to that shown in Fig. 1, with the exception that the metering circuit does not include a back contact of the line relay, thereby preventing an attendant from energizing the meter by a manual operation of the line relay when the cut-off relay is energized. Other modifications are referred to in which off-normal springs are provided, or the homing wiper is divided into two separate elements instead of the plates HP, LP. In the arrangement shown in Fig. 2, the bank contacts are not modified and the switch operates in general in the usual manner. When the switch is homing over circuit DM, 7, CP, the upper end of the winding of the cut-off relay K is entirely disconnected at contacts 9, 8, 1. and no circuit can be completed until wiper H engages its home contact 1. If, therefore, a connector engages the associated line during the homing operation, it remains upon it (without sending a busy signal) and switches through when the line switch reaches normal. One side of the meter is normally connected to earth when the cut-off relay K is inert, false operation of the meter being thus prevented.