Patent GB-706198-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04M11/04
Filed: 1951-03-12
Abstract: 706,198. Manual exchange systems ; substation circuits. SKOGSBERG, E. L. March 12, 1951, No. 5945/51. Class 40 (4) In an interconnection or exchange system, particularly one used for fire alarm or police purposes, in which battery feed may be over the two-wire line, or over one or both of the wires and earth, the substation microphone is connected to earth via a blocking element such as a rectifier or relay contact which blocks the circuit to earth in response to the normal voltage condition of the line loop but unblocks it in response to a voltage condition imparted to the line loop in the case of a line fault such as an open circuit or short circuit of or between the line wires respectively. Normal operation.-The normal current in the line loop and r1 and r2 (Fig. 1) operates the two - step relay R3 to its first step, but R4, being differential, stays down. Contacts (not shown) of R3 and R4 control lamps to indicate calls and line faults. In the normal condition no lamps are lit. An open circuit in line L releases R3, an earth fault operates R4, and a short-circuit operates R3 to its second step, the appropriate lamp being lit in each case. Relay R3 also operates to its second step to the calling condition, so lighting of the call/short circuit lamp is assumed to indicate a call. If, however, no caller is on the line, the operator tests for the fault condition. Normal call.-A privileged user, e.g., a policeman, opens the call box by means of a key so that K2 is not operated. Taking down of the handset signals the operator, who inserts the plug of the normal cord circuit (Fig. 2) into the jack Sj (Fig. 1), whereupon R2 (Fig. 1) and R5 (Fig. 2) pull up. R7 and R8 are connected up in place of R3 and R4, R7 being fully operated as a result. Calling. lamp L3 is thus lit, and supervisory lamp L1 is extinguished by the operation of R9. The call is completed in the usual manner. A substation may be rung in the usual way or by intermittent flashing of lamp La (Fig. 1). In the former case the operator's key is put into position 02 and key 03 is operated; in the latter case the operator's key is put into position O1 whereupon R6 is repeatedly operated by impulse sender Im and in turn operates' R1 (Fig. 1) which extinguishes lamp La each time it pulls up.  Emergency call.-A person wishing to call the fire brigade opens the call box without a key, thus closing K2 (Fig. 1) to operate R4 at once. When the' operator plugs in, R8 pulls up and operates R9 which is held by R7 when the handset is removed, thus preventing the lighting of L1. Faults occurring during a call result in the lighting of L2 or L4 (Fig. 2).  Calls on faulty line.-As soon as the operator realises that a fault exists she plugs in the cord circuit of Fig. 3, whereupon R1 (Fig. 1) operates connecting thesubset apparatus between the line wires and earth. R7 operates and L2 is lighted indicating that the line is serviceable. The initiation of a call then brings up R8 so that L3 is lighted, R7 releases (short-circuit) and L2 is extinguished. Throwing of the key to position 02 allows the operator to speak with the caller. Filters F serve to eliminate earth current noise. The substation is called by throwing the key to position O1, the repeated operation of R6 releasing R1 to flash lamp La. Automatic change-over of talking circuit on the occurrence of fault conditions.-As shown in Fig. 4, two substations may share a line, but the operation will be described for substation Al only. The means at the exchange for answering calls is not shown. Relay RIO is polarized and is normally held in its intermediate position by bias winding III. On the initiation of a call relay RIO is energized in series with the subset Ta and rectifier n3, and closes its upper contacts to light L3. An open circuit condition causes R10 to close its lower contacts to bring up R12, whereas a short circuit condition energises R11 to bring up R12. In either case, R12 connects the a and b wires together, disconnects the positive pole of the battery from ground, and connects the negative pole to ground. A calling condition then operates R10 over a and b in parallel (or one of them if the other is open circuit) to ground via n2. Speech also takes place over the same circuit.