gec.gif (1164 bytes)GEC  25 LINE

A Management Company of The General Electric Company Limited of England

25 Line Private Automatic Exchange

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

Maximum capacity

25 extension lines.

Connect circuits
Provision for 4 simultaneous conversations.

Numbering scheme
21 to 45.

Executive's priority
Selected executives can have a priority-button telephone enabling them to break into an existing conversation.

Tie lines

Up to 4.

Up to 10 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 and 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
Up to 4 by adding special attendant's cabinet.

Mode of operation

Fully automatic operation using 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.

Power unit
AC power unit: input, 100 to 120V or 200 to 250V 50 to 60 Hz single phase; output, 26V d.c., 3A.
Battery: 26V 32 AH; with float charger, input as for a.c. power unit; output, 26V d.c. 3A.

Line loop resistance
Not to exceed 250 Ohms.

Construction Complete switching unit is table or wall mounted and contained in a lift-off dust cover painted dark green. Wired-in equipment on a steel frame hinged to
give full access for maintenance.


Catalogue Number Height Width  Depth Weight
25 line switching unit PX2225 71 cm (28 in) 61 cm (24 in) 25 cm (10 in) 68 kg (150 lb)
AC mains unit SU3201 43 cm (17 in) 32 cm (13 in) 23 cm (9 in) 17 kg (38 lb)
Float charger SU4201 43 cm (17 in) 32 cm (13 in) 23 cm (9 in) 19 kg (42 lb)


Taken from a GEC-AEI brochure SLX2F - dated 1968


The G.E.C. 25 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 stair. The exchange is essentially designed for internal communication only; but by means of the G.E.C. Special Switching Bell Set, it can be connected to one line 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 exchange can be supplied with initial equipment for as few as ten lines; equipment can be added later to bring the total to 25 lines. Through circuits for conversation are established by connecting links, of which two, three or four may be fitted as desired, to permit the corresponding number of simultaneous conversations.

The exchange is contained in a dust-proof cabinet with a glass front, arranged for either table or wall mounting.

A complete system consists of the exchange unit to which the telephones are connected by their individual line-wires. Operating current can he derived from the G.E.C. mains unit specially designed for use with this exchange.

Early 1950's case  
Component overview  

of this P.A.X. which emphasises its invaluability to all businesses are:

Every extension user can dial any other; a maximum of four simultaneous conversations can he 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 the calls of certain executives arc given priority so that they can get through to the wanted extension even though that extension may be already engaged. A pushbutton is provided on the telephone instrument which is depressed when' Busy' tone is received. The executive is then immediately connected to the wanted party. The push-button 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.

These 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. The limited conference facility entails the use of one connecting link per participant, which may result in blockage of calls through the exchange until the conference has ended. The unlimited 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 is opened by each participant being individually called by the convener; each then presses the pushbutton on his instrument to give direct connexion to the conference circuit at the exchange. Please state the type required and the number of participants.

Officials absent from their offices can be called by a system of lamps or bells. A special 'Locating' number is dialled from any telephone 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 exchange by means of G.E.C. exchange line switching telephones. This system allows a maximum of 10 extensions, each with a G.E.C. Exchange Line Switching Telephone, access to up to four public exchange lines. Full details are given in leaflet PXL7.

The exchange can be arranged to give a fire alarm service that meets the particular requirement 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 is wise first to consult the local fire regulations.

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 the buildings.

A maximum of two such tie-lines can be used with this 25-line exchange. 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.

A G.E.C. Switching Telephone can be provided so that a secretary may filter calls to an executive. The executive would have a G.E.C. Extension Telephone. Calls between executive
and secretary are not routed through the
exchange. Full details are given in leaflet STL17.

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. leaflets STLI6 and STLI8. 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 250 ohms.

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 0.4 990 900
6.5 24 25 0.408 1570 1430
10 22 23 0.64 2410 2192

Each extension requires a two-core 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 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, 25 pair or 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. 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.

A 26 volt d.c. power supply is required for the operation of the exchange and may be obtained from an a.c. mains unit or storage batteries. The specially designed G.E.C. mains unit is recommended except where the mains supply is unreliable, when a storage battery may be preferable. If a storage battery is employed a float-charge system should be used. The recommended capacity for the battery is 20 ampere hours at the 10-hour rate of discharge.

Later model component view

From Catalogues PXL 2B and PXL 2D

G.E.C. 25 LINE P.A.X.

Line Circuits Link Circuits Dimensions Weight Catalogue Number
Wired Equipped Wired Equipped ins cms lb Kgs Standard Tropical
25 10 4 2 28 x 25 x12 71 x 63 x 30 145 66 PX 2100 PX 2210
25 15 4 2 28 x 25 x12 71 x 63 x 30 145 66 PX 2115 PX 2215
25 20 4 3 28 x 25 x12 71 x 63 x 30 145 66 PX 2120 PX 2220
25 25 4 3 28 x 25 x12 71 x 63 x 30 145 66 PX 2125 PX 2225


From Catalogue PXL 2E

G.E.C. 25 LINE P.A.X.

Line Circuits Link Circuits Dimensions Weight Catalogue Number
ins cms lb Kgs Standard
25 4 28 x 25 x12 71 x 63 x 30 150 68 PX 2225

From Catalogue PXL 2B
Power Supply Unit
for operation from 200/250 volt, 50 c/s single phase mains - 24/28-volt DC. Described in leaflet SUL 1A.
Catalogue Numbers: Standard SU1102- Tropical SU1202
Dimensions : 21 x 15 x 9 ins; 53 x 39 x 24 cms.
Weight: 66 lb; 30 kgs.

Power Supply Unit for operation from any alternative single phase AC mains - 24/28-volt DC.
Dimensions as above.
Catalogue Numbers: Standard SU2102- Tropical SU2202

Automatic Float Battery Charger to charge a 12-cell lead acid battery for operation from 200/250-volt 50 c/s single phase AC mains - 24/28-volt 0-3 amp. DC. Described in leaflet SUL 1A.
Catalogue Numbers: Standard SU1111- Tropical SU1211
Dimensions: 21 x 15 x 9 ins; 53 x 39 x 24 cms.
Weight: 70 lb; 32 kgs.

Automatic Float Battery Charger for operation from any alternative single phase AC mains - 24/28-volt 0-3 amp. DC.
Dimensions as above.
Catalogue Numbers: Standard SU2111- Tropical SU2211

Lead Acid Storage Battery less stand, 12-cell, nominal voltage 26, acid and accessories, capacity 30 ampere hours at 10 hour rate of discharge.
Catalogue Numbers: Standard BA1101- Tropical BA1101

From Catalogues PXL 2D and PXL 2E
Power Supply Unit
for operation from a 100/120 or 200/250 volt 50/60c/s single-phase mains supply having an output of 24/28 voltsDC.
Catalogue Number: SU3201
Dimensions: 17 x 121 x 9 ins; 43 x 32 x 23 cms.
Approx. Weight: 38 lb; 17 kgs.

Automatic Float Battery Charger for operation from a 100/120 or 200/250 volts 50/60c/s single-phase mains supply having an output of 24/28 volts, 0-3 amps DC, and suitable for charging a 12-cell, lead-acid battery.
Catalogue Number: SU4201
Dimensions: 17 x 122 x 9 ins; 43 x 32 x 23 cms.
Approx. Weight: 42 lb; 19 kgs.

12 Cell Lead-Acid Storage Battery, less stand, acid and accessories, having a capacity of 20 ampere-hours at the 10 hour rate of discharge
Catalogue Number: BA1106

Taken from the GEC Catalogue Leaflet No. PXL 02B (Date unknown but later than 1946) the GEC Catalogue Leaflet No. PXL 02D (Date unknown but later than 1959) and the GEC Catalogue Leaflet No PXL 02E (Date unknown but later than 1960)


PX 2125 - 25 line model (pictured 1953)


PX 2125 with case removed (pictured 1953)


PX 2125 with case removed and gate open (pictured 1953)


BACK Home page GEC Main Menu Search the Site Glossary of Telecom Terminology GEC Telephone Quick Find All Telephone Systems

Last revised: October 09, 2010