gec.gif (1164 bytes)GEC PABX 2+23

GEC 2 + 23 Line
Private Automatic Branch Exchange

  • No switchboard operator required

  • Automatic connexion between extensions

  • Four connecting links for extension to extension calls

  • Dialled access to public exchange

  • Incoming calls transferable among extensions

  • Call-back and automatic transfer facilities available on all exchange calls

  • Executive right-of-way facilities available to all or selected extensions

  • Dust-proof automatic unit

The G.E.C. 2 + 23 line private automatic branch exchange has a capacity for two lines to a public exchange and twenty three extensions.  It has no switchboard or attendant.  One extension calls another by dialling two digits.  An extension obtains direct access to the public exchange by dialling 9.  This direct access is given to selected extensions and withheld - if desired from others.

An incoming call from the public exchange is answered by any of five pre-determined extensions, who may then transfer the call to the wanted party.  Up to four extension-to-extension conversations can be held simultaneously.  When an extension makes a call to the public exchange, the connecting link in the PABX is in use only whilst the connexion to the exchange line is being established. The link is then released for use by the next caller.

The P.A.B.X will work to a public automatic exchange and C.B. manual exchanges. If the public exchange is of the magneto or C.B.S. type, an auxiliary unit is supplied to effect the necessary signalling.

The PABX is mounted in a sheet-steel dust-proof cabinet having lift-off doors front and rear to give access to the equipment.  The size of the cabinet is 6' 3" x 2' 0" x 1' 2" (191 x 61 x 36 cms). The exchange employs the same apparatus as has been standardised for large public exchanges by many Overseas Governments and the British Post Office.

The complete P.A.B.X. system consists of the automatic unit, telephones, bells to indicate incoming calls from the public exchange, line wires from the unit to the telephones, power supply equipment and protection apparatus for all lines that run outside buildings.


Any extension user is connected to any other extension by dialling the appropriate number.  The extensions are normally numbered from 20 to 43, omitting number 40.

Direct access to the public exchange is obtained by dialling the single digit '9'.  If the public exchange is automatic, the caller then dials the number of the required subscriber; if the exchange is manual, the call is completed by the public-exchange operator.  Certain extensions may be barred from this facility, if desired, by a simple adjustment to the strapping in the automatic unit.

A call from the public exchange is taken by any one of five Answering Extensions, who are pre-determined and can transfer the call to any extension by depressing a pushbutton on their telephone and dialling the wanted number. An incoming call is signalled on separate bells, which are located at points convenient to the Answering Extensions.  These bells are additional to those incorporated in the telephone instruments. Should busy tone be received after the wanted number has been dialled, then the Answering Extension dials 1 to interrupt the established call.

An extension engaged on a call over an exchange line may hold the call (by depressing a button on his telephone), whilst he calls any other extension. At the end of conversation with the second extension, he resumes his conversation on the exchange line by depressing the button again. Alternatively, if he wishes, he may transfer the exchange line to the other extension merely by replacing his own handset.
Any telephone may receive a transferred call ; if it is fitted with a push-button, then the user may transfer the call again.

An executive may break in should the extension he requires be already engaged. When he hears busy tone, he may dial a further digit "1" to interrupt the established call. A warning tone is given to indicate to the conversing extensions that a third party is on the line. Any extension may be provided with this facility by a simple strap adjustment in the automatic unit.

Night service is provided by arranging for one or more of the Answering Extensions to answer incoming calls at night.

The exchange operates on a 50-volt DC power supply. Where the mains supply is AC, the DC supply may be obtained from a mains unit or from storage batteries. A G.E.C. mains unit is recommended except where the mains supply is unreliable, when a storage battery is necessary to maintain continuity of service. When a 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. Further information regarding the mains-supply units and float chargers is given in G.E.C. Leaflet SUL 1A.

Where the mains supply is DC, the exchange operates from batteries, and a rheostat steps the mains voltage down to the value required to charge the batteries. With this arrangement a charge-discharge system must be used so that the battery connected to the mains is isolated from the exchange.

Should the mains supply be interrupted, operation of the P.A.B.X. ceases when it is operated from a mains supply unit, but the result is not isolation from the public system because the two exchange lines become automatically connected to any two pre-determined extensions, from which calls on the public system may be made or received.

When the P.A.B.X. is operated from a battery, the operation of the exchange is unaffected by failure of the mains power supply, unless the duration of the failure is such that the battery becomes discharged. This period is normally at least 24 hours.

Dial, busy, and ringing tones, together with ringing current, are generated by means of vibrating generators fitted inside the automatic unit.

Should a fuse in the exchange unit blow, an audible alarm signal is given.

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 "G.E.C. 1000" Telephone, the Muraphone, and the Gecophone described in G.E.C. Leaflets STL 13 and STL 14.

A telephone fitted with a push-button for each of the Answering Extensions and for any other extension requiring the call-back and automatic transfer facility.

A secretary may be provided with a G.E.C. Switching Telephone so that all incoming calls to an executive may be filtered. The executive would have a G.E.C. Extension Telephone. Calls between the executive and secretary are not routed through the P.A.B.X.
Full details of this service are given in G.E.C. Leaflet MTL 2.

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 900 ohms. A third wire must be connected from the telephones of Answering Extensions, and extensions having the call-back and automatic transfer facility, to a nearby earth (ground) point. If there is no convenient earth point, the third wire must be run 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 3560 3240
6.5 24 25 0.408 5650 5150
10 22 23 0.64 8680 7860

The simplest method of connecting the extension instruments to the P.A.B.X. unit 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.B.X. unit to a distribution box so situated that only short lengths of line wires are required to connect each telephone instrument to the distribution box.

The exchange equipment must be protected against high voltages and heavy currents, which may accidentally be introduced into external lines from extraneous sources. Both the lines to the public exchange and all external extension lines must be connected to protection apparatus before being extended to the exchange unit. For this purpose, the G.E.C. protector, Cat. No. PR 1111, containing fuses and electrodes is recommended.

Protectors for the exchange lines are often supplied by the public telephone exchange administration.


G.E.C. 2 + 23 LINE P.A.B.X.

Line Circuits Link Circuits Dimensions Weight Catalogue Number
Exchange Extension in cms lb Kgs Standard Tropical
2 23 4 75 x 24 x 14 191 x 61 x 36 374 166 PB2101 PB2201


Power Supply Unit for operation from a 200/250 volt 50/60c/s single-phase mains supply, having an output of 48/52-volts D.C.
Catalogue Numbers - Standard SU1109 & Tropical SU1209
Dimensions: 20 x 15.5 x 9-ins (53 x 39 x 24cms)
Weight: 62lb (28.3kgs)

Power Supply Unit for operation from any alternative single-phase mains supply, having an output of 48/52-volts DC.
Dimensions: as above.
Catalogue Numbers - Standard SU2109 & Tropical SU2209

Automatic Float Battery Charger for operation from a 200/250 volt 50/60c/s single-phase mains supply, having an output of 48/52 volts, 0-2amp DC, and suitable for
charging a 24 cell lead-acid battery.
Catalogue Numbers - Standard SU1110 & Tropical SU1210
Dimensions: 20 x 15 x 9ins (53 x 39 x 24cms)
Weight : 76lb (34.6kgs)

Automatic Float Battery Charger for operation from any alternative mains supply, having an output of 48/52-volts 0-2 amp DC.
Dimensions:  as above.
Catalogue Numbers - Standard SU2110 & Tropical SU2210

24-Cell Lead-Acid Storage Battery less stand, acid, and accessories, capacity 20 ampere-hours at the ten hour rate of discharge.
Catalogue Numbers - Standard BA1103 & Tropical BA1103


Taken from the GEC Catalogue Leaflet No. PXL 03A (Date unknown)

An advert for this model can be found in GEC Current Comments, Volume 5, Number 1, 1950

Current Comments article Volume 5, No. 2, 1950

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

Last revised: November 26, 2021