266,774. Automatic Telephone Manufacturing Co., Ltd., (Automatic Electric Co.). Oct. 10, 1925. Addition to 179,930. Automatic exchange systems.-Relates to systems of the kind described in the parent Specification in which when a trunk is seized by a switch, busy ground is fed forward through a winding of a relay which is short-circuited by the ground fed back from the next switch, so that if the seized trunk or the switch ahead is faulty, the preliminary earth remains to render the trunk non-selectable and an alarm is actuated. According to the invention, means are provided at a numerical switch for preventing it from locking up on a non-defective trunk and the alarm from being given if the calling party hangs up before the release relay at the switch ahead has time to feed back the guarding earth. At the group selector S, Fig. 1, when the switching relay 56 energizes, guarding earth is fed forward through a slow-to-operate auxiliary guard relay 55, which is normally shortcircuited bv the earth returned over the third wire before it has time to pull up, but if the call is abandoned before this, energizes and bridges the outgoing speaking wires to ensure the return of the earth and the release of the connection. At the primary line switch PLS, the preliminary guarding earth is fed forward through a holding winding of the line relay 5 which is connected in series with the holding winding 7 of the line switch. The building up of the current in the pull-down winding 6 causes an induced reversal of current through the holding winding of the line relay 5 tending to cause its relapse. The invention also comprises means for preventing or rendering ineffective this induced reversed current. The secondary line switch SLS has a common two-step test and switching relay 30 which is partly operated by the preliminary guarding earth fed forward from the primary line switch to complete a testing circuit, and is fully operated to switch through the talking conductors when a free trunk is found. Primary line switch PLS, Fig. 1. The energization of the line relay 5 over its bottom winding closes circuits for the pull-down and holding windings 6, 7. These circuits include the lowresistance upper winding of the relay 5 and the winding of a supervisory relay 18 of the masterswitch MS, so that the relay 5 remains energized after the calling line has been disconnected from it and extended to the secondary switch SLS. The ground connection through the low-resistance windings of relays 18, 5 to the spring 20 marks the line busy and holds the switch PLS locked until ground is sent back over spring 20 from the succeeding switches. Providing that the trunks to the secondary switch SLS and to the group selector S, and the line and release relays of the selector, are sound, direct ground is quickly returned over the spring 20 to short-circuit the relays 5, 18, but if a defective trunk or relay is encountered these relays remain energized, the line relay 5 holding the switch PLS, and the supervisory relay 18 causing the operation of a time alarm. the relay 5 at armature 15 bridges the calling line, so that if the subscriber replaces his receiver before the selector S has had time to feed back guarding earth (assuming the trunks are not defective), the line relay 5 will not become permanently locked up since the bridge ensures the continuation of the operations and the return of the direct earth. Modified primary line switches. The effect of the induced current through the holding winding of the line relay 5 previously referred to, is prevented in the switch PLS by increasing the turns of the winding 6 and decreasing those of the winding 7 while retaining their proper operating characteristics. In the modification illustrated in Fig. 2, the difficulty is overcome by providing the line relay 205 with three windings. The pulldown winding 206 energizes in series with the bottom winding and the holding winding 207 in series with the two lower windings. The induced reversed current is ineffective since the lower winding carries a relatively large current in series with the pull-down winding 206. The extra winding also positively ensures a relapse of the fine relay when direct earth is fed back over the test wire, since at this time the bottom winding is short-circuited and a reversed current flows in the middle winding. In another modification, the circuit of the pull-down winding is completed over working contacts of the cut-off armature which is not attracted until the current in the holding winding has attained practically its full value. According to a further modification, the holding winding is energized over the calling loop in series with the initial energizing circuit, including both windings of the line relay, the response of which closes the circuit of the pull-down winding. The initial current over the holding winding and the line relay renders the induced reversed current ineffective. In a still further modification, the pull-down winding is energized in series with the calling loop and the line relay, and attracts the cut-off armature which closes springs to short-circuit one of the windings of the line relay. This renders the line relay slow-to-release and unaffected by the momentary induced reversed current. Secondary line switch SLS. When the plunger arm of the primary line switch PLS is depressed. the preliminary earth on the spring 20 energizes relay 30, the armatures of which are so adjusted that the armature 33 is attracted first and connects up the test wiper 42. If the trunk upon which the wipers are resting is busy, the test contact is grounded and before the remaining armatures of the relay 30 are attracted, its upper winding is short-circuited and its magnetic pull is reduced to a value only sufficient to hold the armature 33. At the same time the magnet 31 is energized to advance the wiper on to the next trunk which is tested in like manner. When an idle trunk is reached, the magnet 31 is not energized and the relay 30 is permitted to operate fully to extend the calling line to the group selector S. Group selectors. The line relay 51 closes a series circuit for the vertical magnet 59, the release relay 52 and the change over relay 53, but only the relay 52 pulls up in this circuit and directly grounds the release conductor. When the first digit is sent, at each relapse of the line relay, the release relay 52 is short-circuited to render it slow-to-release, and a series circuit is closed for the change-over relay 53 and the vertical magnet 59. The relay 53 holds up during the impulses and energizes the stepping relay 54, which locks over armature 66 and prepares the circuit of the rotary magnet 57. At the end of the digit, the relay 53 relapses and thereafter the magnet 57 and relay 54 interact in known manner until an idle trunk is engaged when the switching relay 56 pulls up. An additional relay 55 is provided which is normally short-circuited, but on the energization of the switching relay 56 is included in the circuit of this latter relay. The relay 55 is sufficiently slow-to-pull-up to give the line and release relays of the switch ahead time to operate before it responds, so that if the seized trunk is sound the returned holding ground on the test wiper 82 short-circuits it again before it has had time to pull up. If a defect in the trunk or succeeding selector prevents the return of this ground, the relay 55 pulls up after an interval, locks up in series with the switching relay 56 to hold the selector, bridges the incoming talking conductors at armature 69, and closes an alarm circuit for a lamp L and supervisory relay 91. If the calling party replaces his receiver during the interval between the response of the switching relay 56 and the return of ground from the switch ahead, in the case when the trunk is not defective, the guard relay 55 will respond before the relapse of the release relay 52. but the bridge at armature 69 will cause the operations at the switch ahead to proceed until the returned ground over wiper 82 again short-circuits the relay 55 which thereupon opens the bridge and causes the release of all the switches. In this case the alarm circuit will be closed for a negligible period only. All trunks busy signal. If all the trunks in any level of the selector S are busy, the wipers are advanced to an eleventh position in which a shaft cam 79 operates springs 75 to connect busy tone from a machine B to the lower conductor through the lower winding of line relay 51, and a shaft cam 80 opens the springs 78 to prevent the energization of the switching relay 56. Specifications 210,826 and 224,255 are also referred to.