Patent GB-214331-A (GB)
Abstract: 214,331. Automatic Telephone Manufacturing Co., Ltd., and Gillings, C. Jan. 18, 1923. Automatic exchange systems employing primary and secondary line switches are provided with means comprising a chain circuit including contacts controlled by the numerical switches for marking idle secondary switches busy if all the numerical switches to which they have access are in use, whether the numerical switches have been operated or not. Useless hunting of primary switches is prevented by means of a relay in a further chain circuit which is completed when all the secondary switches to which the primary group has access are in use or otherwise incapable of extending a call. Meters may be provided for registering the number of times this relay is operated and the number of calls held up, and an audible alarm may be given if it is operated for more than a predetermined time. Line switches and first selector. In the system shown, Fig. 1, 25-point line switches are used, and first selectors are divided into subgroups of 2,5 accessible to 50 secondary switches. A call is extended in the usual manner over line switches C, D and first selector E. Line relay 16 of the switch D, however, controls an additional contact 28, and switching relay 17 an additional contact 29. The first selector E has an additional contact 37 controlled by slow release relay 31 and an additional off normal contact 48. Preventing selection of secondary switches incapable of extending the call. The contacts 37. 48 of all first selectors accessible to a group of secondary switches are arranged as shown in chain with a preferably-slow-release relay 53 individual to the group. When all the first selectors are busy, relay 53 energizes relay 54. Relay 54, which may be of the type described in Specification 176,143, grounds the release conductors of all the secondary switches in the group which are not in use. Preventing useless hunting of primary switches. Each group of primary switches has a slow-release relay 13 individual to it arranged in a chain circuit including three contacts in parallel for each secondary switch to which the group has access. Two of these contacts 28, 29, are actuated by the line and switching relays of the secondary switch, the third is controlled by the relay 53. When all the secondary switches accessible to a group of primary switches are busy, relay 13 energizes and removes ground from the wire 2, at the same time connecting busy-tone to it. If a call is made relay 56 energizes in series with the line relay 3, but is of too high a resistance to allow the latter to operate. Relay 56 completes a circuit for a meter 58 for recording the number of times a call is held up. Relay 13 also closes a circuit for a meter to record the number of-times it has been operated and also a circuit for an alarm lamp and a delayed audible alarm so that, if all the trunks are busy for more than a predete.rmined time, attention is attracted. If the number of contacts to be operated by relay 53 is great, several simultaneously-operated relays may be used. Modification in which certain selectors are common to several groups of secondary switches. As shown, Fig. 2, five selectors of which the first and last, 103, 104, are shown, are common to the two groups of secondary switches shown and possibly to others, and can only be taken into use when no others are available. When the twenty selectors 101, 102 accessible only to the left-hand group are all busy, the circuit of relay 114, which corresponds to relay 53, Fig. 1, is broken only at contact 112. When the five common selectors are busy, relay 111 is energized and completes the circuit for relay 114. A similar arrangement may be employed for operating relays such as 13, Fig. 1.