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The G.E.C. 15-line private automatic telephone exchange provides rapid and secret intercommunication by the simple dialling of two digits. It is invaluable in large and small businesses to provide automatic inter-connexion between members of the staff. The exchange is essentially designed for internal communication only, but by means of the G.E.C. exchange line switching telephone, it can be connected to up to four lines from a public exchange. It employs the same apparatus that has been standardised for large public exchanges by many Overseas Governments and the British Post Office.

The numbering scheme is 2-9 and 01-07. Through circuits for conversation are established by connecting links, of which there are two permitting two simultaneous conversations.

The exchange, which is arranged for either table or wall mounting, is-contained in a dust-proof steel cabinet. The standard cabinet is finished furniture green, but other colours can be supplied to special order. A complete system consists of the exchange unit which includes a power unit, the telephones, the connexions from the telephones to the exchange and possibly a distribution frame and protection apparatus.

Internal view
Internal view with gate open


Every extension user can dial any other; a maximum of two simultaneous conversations can be held.

The operation of the system is fully automatic and needs no manual supervision.

Absolute secrecy of each conversation is ensured by the use of circuits and apparatus designed to eliminate overhearing.

By means of this facility certain executives may exercise priority and get through to the wanted extension even though that extension may be already engaged. A pushbutton, provided on the telephone instrument, is depressed when busy tone is received. The executive is then immediately connected to the wanted party. Alternatively, the pushbutton can be dispensed with if required, by means of permanent strapping on terminals at the exchange, whereby the executive never receives busy tone and is always immediately connected to the wanted party.

The use of an internal service is not necessarily confined to any one building; two G.E.C. P.A.X. units in buildings several miles apart can be linked by a tie-line, to permit intercommunication between extensions in both buildings. Access to the tie-line is by dialling 00.

Please state the type of P.A.X. into which the tie-lines must work and the line loop resistance of the tie-line itself. Should an existing P.A.X. be of other than G.E.C. manufacture, its circuit drawings should be provided with an enquiry.



These enable conferences to take place by whilst the participants remain at their desks.

The conference facility provides for conferences with no limit to the number of participants as no links are employed; each participant's telephone is connected to the exchange by an additional line pair and is fitted with a pushbutton. The conference by each participant being individually called by the convener; each then to pushbutton on his instrument to give direct connexion to the conference circuit at the exchange. Please state the of participants required.

Officials absent from their offices can be called by a system of lamps or bells. A special 'locating' number is dialled from phone followed by digits in a personal code. Please state whether a visible or audible call is required, how many codes are needed, and the number of calling stations likely to be used.

Although the exchange is intended for internal intercommunication in a private network, it can be associated with up to four lines to the main public telephone by means of G.E.C. exchange line switching telephones. Extensions having these telephones can make and receive public exchange calls. These calls can be transferred between extensions with the switching telephones.

Please state the type of public exchange into which the telephones must work and the line loop resistance of the exchange line. This information should be obtained from the local telephone Administration.

The exchange can be arranged to give a fire alarm service that meets the particular requirements of any organisation. For instance, by dialling a "Fire" number from any telephone, alarm signals can be made to sound at required points. Please state the facilities required, the calling system to be adopted and number of fire points. It wise first to consult the local fire regulations

A secretary can be provided with a G.E.C. secretary's switching telephone whereby she can filter all incoming calls to the executive, who is provided with a G.E.C. extension telephone. See leaflet STLl7.

Direct connexion between executive and secretary is provided without the call being routed through the P.A.X.

Any automatic telephone having a dial pulse speed of between 7 and 14 pulses per second, with a ratio of 2 to 1 break to make, is suitable for use with this exchange.

Recommended telephones are the "G.E.C. 706" telephone and the "G.E.C. 711" telephone described in G.E.C. leaflet Nos. STL16 and STL18.

G.E.C. Loudspeaking Telephones have been designed especially for this P.A.X., and are described in leaflet STL15. The user has complete freedom of movement about the room whilst making the call.

For extensions having executive right-of-way, the telephone instruments are fitted with a special pushbutton.

Any insulated twisted pair, or open-wire conductor for external lines, may be used. Lines should be suitably protected if exposed to the risk of damp or mechanical damage. Line-loop resistance (the resistance of two wires in series) should not exceed 800 ohms. The following table gives examples of maximum lengths for various line wire conductors.

The following table gives examples of maximum lengths for various line-wire conductors:-

Size of Copper Conductors Maximum Permissible Length of Cable Pair
lb/mile AWG (B & S) SWG mm Diam. Yards Metres
4 26 27 0.4 3168 2880
6.5 24 25 0.508 5024 4576
10 22 23 0.64 7712 7014

Each extension requires a two-wire connexion to the exchange unit, unless the extension is provided with executive right-of-way facilities, when the requirement is a three- wire connexion to the exchange, with an earth (ground) connexion from the telephone.

The P.A.X. includes its own 50-volt d.c. power supply unit which will operate from a 100, 110, 230, 240 or 250 volt, 50 or 60 c/s a.c. mains supply.

The simplest method of connecting the telephone instruments to the exchange is to use a separate line wire pair for each telephone. A saving in the cost of line wires, however, may often be effected by running a multi-core cable, 12-pair, from the exchange to a junction-box so situated that only short lengths of separate line wires are then required to connect the telephones to the junction-box.

Distribution frames on which the exchange lines are terminated, and to which the telephones are wired, are not normally used with this exchange, but can be supplied if required. The use of a distribution frame simplifies alterations to numbering schemes and line testing, and also provides protection against high voltages and heavy currents that may accidentally be introduced from extraneous sources.

As the distribution scheme must vary according to the geographical layout encountered, please state whether or not cable and junction-boxes are to be supplied; if they are, a dimensioned sketch of the disposition of the exchange unit and telephones should be included.

Provided the line wires connecting each telephone to the exchange are all run inside a building, no form of protection against lightning is required. However, should any line wires be run outside the building, line-protection apparatus must be employed on each such line. Please state the number of lines to be protected; suitable protection apparatus will then be provided.


G.E.C. 15 LINE P.A.X.

Line Circuits Link Circuits Dimensions Weight Catalogue Number
ins cms lb Kgs
15 2 22.5 x 21 x 10 57.2 x 53.4 x 25 130 59 PX 1215

Taken from the GEC Catalogue Leaflet No. PXL 08 (Date unknown but later than 1959)


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Last revised: November 29, 2021