gec.gif (1164 bytes)GEC  50 LINE EXTENSIBLE PAX

A Management Company of The General Electric Company Limited of England

50 Line Extensible Private Automatic Exchange

The features referred to on this leaflet are fully described in a separate publication.

The exchange is built up from self-contained 50-line extension-switching units. An exchange can consist initially of a single switching unit and extended later up to an economic maximum of 400 extensions. When more than two of these units are required, group-selector units are included to ensure full interconnexion between the switching units. Two types of group selector are offered: the first uses uniselectors (limited expansion) and is recommended where it is known that the capacity will not exceed 200 lines; the second (full expansion) uses 2-motion selectors and is used when the capacity may exceed 200 lines.


50 extension lines extensible to an economic maximum of 400 lines, in multiples of 50 lines.

Connect circuits
Each 50 line switching unit provides for up to six simultaneous conversations.

Numbering scheme
Ultimate capacity not more than 100 lines - in the range 00 to 99. Ultimate capacity more than 100 lines - 3 figure numbers.

Executive's priority
Selected executives can break into an existing conversation : either the executive operates a pushbutton on the telephone when he hears engaged tone, or the break in can be effected automatically.

Tie lines

Tie line facilities are available to suit individual requirements.

Up to 10 designated extensions can be connected together via a special circuit.

Fire alarm
Various procedures are available to suit individual requirements and local fire regulations.

Staff location
Staff absent from their offices may be called by dialling a special locating number followed by a personal code, to operate lamps or bells.

Secretary service
A secretarial switching telephone enables secretaries to filter calls to an executive.

Public exchange lines
Public-exchange service can be provided after the addition of a special attendant's cabinet.

Mode of operation

Fully automatic using 2-motion selectors, uniselectors and relays. All conversations secret from each other.

Tones and ringing
Electronic tone generators provide dial tone, ring tone and current, and busy tone.

Required dial pulse characteristics: 7 to 14 i.p.s., 2:1 break to make ratio.

Audible indication of blown fuse; visual indication of failure to replace receiver (PG condition).

Power Unit
AC power unit (for use on 100-line exchanges, or less): input 100 to 120V 60 Hz single phase or 200 to 250V 50 Hz single phase; output, 50V d.c. 10A.
Battery: 50V 32 AH (up to 100 lines) with float charger, output 50V DC 3A; 50V 48 AH
(up to 200 lines) with float charger, output 50V DC 5A. Float charger input as for AC power unit. Above 200 lines, individual recommendations are made.

Line loop resistance
Not to exceed 900 Ohms.

Wired-in and plug-in equipment mounted on a steel framework and contained in steel cabinets (painted dark green) with lift-off dust-proof covers front and rear.

Dimensions and weight


Catalogue Number Height Width  Depth Weight
50-line switching unit PX3650 183 cm (72 in) 66 cm (26 in) 48 cm (19 in) 227 kg (500 lb)
Group selector unit PX7203 183 cm (72 in) 66 cm (26 in) 48 cm (19 in) 227 kg (500 lb)
Group selector unit PX7404 183 cm (72 in) 66 cm (26 in) 48 cm (19 in) 227 kg (500 lb)
AC mains unit SU3204 127 cm (48 in) 48 cm (19 in) 38 cm (15 in) 108 kg (238 lb)
3-A Float charger SU4202 53 cm (21 in) 41 cm (16 in) 25 cm (10 in) 35 kg (76 lb)
5-A Float charger SU4203 53 cm (21 in) 41 cm (16 in) 25 cm (10 in) 48 kg (105 lb)


Taken from a GEC-AEI brochure SLX4F - dated 1969


The G.E.C. 50 line extensible private automatic telephone exchange provides rapid and secret intercommunication by the simple dialling of two or three digits. It is invaluable in large and small businesses to provide automatic interconnexion between members of the staff. The exchange is essentially designed for internal communication only, but, by means of the G.E.C. P.A.X. Attendant's Cabinet. it can be connected to up to five 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, and incorporates the famous G.E.C. SE50 two-motion selector.

The first unit installed may be partially equipped, and may later be extended to 50 lines. Thereafter additional 50 line units may be added to bring the total to 400 lines or more. When three or more such units are installed, group selector units are provided to retain full availability for all extensions. The tables give examples of the requirements for exchanges of various sizes.

Through circuits for conversation arc established by connecting links of which three, four, five or six per unit of 50 lines may be fitted as desired, to permit the corresponding number of simultaneous conversations.

The exchange is contained in floor-mounting, dust-proof, sheet-steel cabinets fitted with lift-of doors front and rear to give easy access to the apparatus for maintenance. The cabinets are finished in dark green.

A complete P.A.X. system consists of the automatic unit of units, telephones, line wires from the unit to the telephones, power-supply equipment, protection apparatus for all lines that run outside buildings, and an attendant's cabinet when required.

of this P.A.X., which emphasise its invaluebility to all businesses, are:-

Every extension user can dial any other: a maximum of six simultaneous conversations can be held per 50 lines.

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.

Its means of this facility, the calls of certain executives are given priority, so that they can get through to the wanted extension even though the extension may be already engaged. A pushbutton is provided on the telephone instrument and is depressed when " busy" tone is received. The executive is then immediately connected to the wanted party. 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.

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

Two types of conference facility are available: one for a number of participants limited by the number of links available. and the other without such limit. Limited conference facilities entail the use of one connecting link per participant which may result in blockage of calls through the exchange until The conference has ended. Unlimited conference facilities provide for conferences with no limit to the number of participants as no links are employed; each participant's telephone is connected to an auxiliary unit by an additional line pair and is fitted with a push-button. The conference is opened by each participant being individually called by the convener; each then presses the push-button on his instrument to give direct connexion to the conference circuit at the exchange.
Please state the type of conference facility required. the number of participants and conveners.

Officials absent from their offices can be called by a system of lamps or bells. A special "Locating" number is dialled front any telephone followed by digits in a personal code. Please state whether a visual or audible call is required. how many codes arc needed, and the number of calling stations likely to be used.

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

Although the exchange is intended for internal intercommunication in a private network, it can be associated with up to five lines to the main public telephone exchange by means of a P.A.X. Attendant's Cabinet. The extension user having this cabinet on his or her deck acts as an operator, when necessary, to make and receive calls over the lines to the local public exchange, and to switch the lines through the P.A.X. to any extension.

Please state the number of exchange lines required, the type of public exchange to which the private system must work, and the line loop resistance of the exchange lines. This information should be obtained from the local telephone Administration.

The use of an internal service is not necessarily confined to any one building ; two or more G.E.C. P.A.X. units in buildings several miles apart can be linked by tie lines to permit intercommunication between extensions in all buildings.
Three such tie-lines car, be used with each 50-line extensible exchange unit. Please state the type of P.A.X. into which he tie-line must work and the line loop resistance of the tie-lines themselves. Should an existing P.A.X. be of other than G.E.C. manufacture its circuit drawings should be provided with an enquiry.

A secretary may be provided with a G.E.C. Switching Telephone whereby she can filter all incoming calls to the executive, who would have a G.E.C. Extension Telephone. Calls between the executive and secretary are not routed through the P.A.X. Full details of this service are given in G.E.C. Catalogue Leaflet No. MTL 2.

Any automatic telephone, having a dial impulse speed of between 7 and 14 impulses per second with a ratio of 2 to 1 break to make, is suitable for use with this exchange.
Recommended telephones are the "New Gecophone", the "G.E.C. 1000" Telephone and the G.E.C. Muraphone "K", described in leaflets STL 16 and 13.

G.E.C. Loudspeaking Telephones, which enable the user complete freedom of movement about the office whilst making the call, have been specially designed for use with this P.A.X.
For extensions having executive right-of-way or unlimited conference facilities, the telephone instruments are fitted with a special push-button. Please state on orders the number of telephones required fitted with push-buttons.

Any insulated twin conductor, suitably protected if exposed to risk of damp or mechanical damage, may be used to connect the telephone to the exchange, provided the line loop resistance does not exceed 1000 ohms. A third wire must be connected from the telephones of extensions having executive right-of-way to the exchange unit.

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 3960 360
6.5 24 25 0.408 6280 5720
10 22 23 0.64 9640 8768

The simplest method of connecting the telephone instruments to the exchange is to use separate line wires for each telephone. A saving in line wires and space, with a gain in neatness, may often be effected by running a multi-core cable from the P.A.X. unit to a junction box or boxes so situated that only short lengths of line wires are then required to connect the telephones to the junction box.

For the larger installations, distribution frames, on which the exchange lines are terminated and to which the telephones are wired, will be supplied if required. The use of a distribution frame simplifies alterations to numbering schemes, and line testing, and also mounts apparatus to give protection against high voltages or heavy currents that may accidentally be introduced from extraneous sources on external lines.

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.

A 50 volt DC power supply is required for the operation of this exchange and may be obtained from an AC mains unit or storage batteries. The exchange generates its own ringing and tone supplies from the DC input. The specially designed G.E.C. mains unit is recommended except w1lere the mains supply is unreliable or when three or more units are installed. In such cases the use of a storage battery is strongly recommended. The recommended capacity of the battery is 30 ampere-hours for installations up to 100 lines; 40 ampere-hours for above 100 lines and below 200 lines; above 200 lines the capacity of the battery is dependent on many variables and is best decided at the time of enquiry. Further information regarding the mains supply units and float chargers is given in G.E.C. Catalogue Leaflet SUL 1.



Line Circuits Link Circuits Dimensions Weight Catalogue Number
Wired Equipped Wired Equipped ins cms lb Kgs
50 30 6 4 71 x 25 x 19 181 x 64 x 57 500 227 PX 3630
50 40 6 4 71 x 25 x 19 181 x 64 x 57 500 227 PX 3640
50 50 6 5 71 x 25 x 19 181 x 64 x 57 500 227 PX 3650


A group selector unit is required on exchanges having more than 100 lines. Where the exchange will not exceed 200 lines, a uniselector type of group selector can be used.

Line Circuits Dimensions Weight Catalogue Number
Wired Equipped ins cms lb Kgs
24 10 71 x 25 x19 181 x 64 x 57 500 227 PX 7201
24 15 71 x 25 x19 181 x 64 x 57 500 227 PX 7202
24 20 71 x 25 x19 181 x 64 x 57 500 227 PX 7203


A group selector unit employing SE50 type selectors is required when the exchange has more than 200 lines, or when it is anticipated that the exchange will ultimately be extended to have over 200 lines. One SE50 group selector unit is required for every 100 lines.

Line Circuits Dimensions Weight Catalogue Number
Wired Equipped ins cms lb Kgs
12 5 71 x 25 x 19 181 x 64 x 57 500 227 PX 7404
12 10 71 x 25 x 19 181 x 64 x 57 500 227 PX 7405

Typical Exchanges
A typical 200 line exchange requires either
4 line units and 1 uniselector group selector unit or
4 line units and 2 SE50 group selector units

A typical 300 line exchange requires
6 line units and 3 SE50 group selector units

Similarly for every additional 100 lines, 2 additional line units and 1 SE50 group selector unit are required.

Provided the line wires connecting each telephone to the exchange are all run inside a building, no form of protection against lightning is required. Should, however, 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. Protection apparatus for lines to the public exchange is usually supplied by the public telephone exchange Administration.

Power Supply Unit for operation from a 100/110 volt or 200/250 volt 50c/s or 60c/s single-phase mains supply having an output of 48/52 volts DC. Suitable for operating one or two P.A.X. units.
Catalogue Number: SU3204
Dimensions: 48 x 19 x 14 ins; 122 x 48 x 37cms
Weight: 238lb; 108kg

Automatic Float Battery Charger for operation from a 100/110 volt or 200/250-volt, 50c/s or 60c/s single-phase mains supply having an output of 48/52 volts, 0-3 amps DC, and suitable for charging a 24-cell lead-acid battery.
Catalogue Number: SU4202
Dimensions : 20 x 15 x 9 ins; 53 x 39 x 24cms
Weight: 76lb; 34.5kg

Automatic Float Battery Charger for operation from 100/1 10-volt or 200/250-volt, 50c/s or 60c/s single-phase mains supply, having an output of 48/52 volts, 0-5 amps DC, and suitable for charging 24 cell lead-acid battery.
Catalogue Number: SU4203
Dimensions: 20 x l5 x 9 ins; 53 x 39 x 24cms
Weight: 105lb; 48kg

24 Cell, Lead-Acid Storage Battery less stand, acid and accessories, capacity 30 ampere-hours at the 10-hour rate of discharge.
Catalogue Number: BA1104

24 Cell Lead-Acid Storage Battery as above, but of 40 ampere-hours capacity.
Catalogue Number: BA1107

Taken from the GEC Catalogue Leaflet No. PXL 04D (Date unknown but later than 1959) and the GEC Catalogue Leaflet No. PXL 04E (Date unknown but later than 1968)


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Last revised: November 30, 2021