gec.gif (1164 bytes)GEC PABX 10+99


The G.E.C. 10 + 99-line private automatic branch exchange has a capacity for 10 lines to the public exchange and ninety-nine extensions. One extension calls another by dialling three 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. All extensions can gain access to the public exchange via the P.A.B.X. attendant by dialling "0".

An incoming call from the public exchange is signalled on an attendant's cabinet, and extended to the wanted party by the attendant. Up to ten extension-to-extension conversations can be held simultaneously. When an extension makes a call to the public exchange, the connecting link in the P.A.B.X. is in use only while 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 public automatic 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 automatic exchange unit is mounted on an open rack, thus providing easy access to the equipment for maintenance. The overall size of the unit is 8' 6' x 4' 6" x l' 2" (260cms x 138cms x 36cms). If required, the unit can be enclosed in a dust-proof cabinet. The cordless attendant's cabinet is attractively designed to stand on a desk or table, its size being 9" high, 1' 24" wide and 10" deep (24cms x 36cms x 25cms).

The complete P.A.B.X. system consists of the automatic unit, attendant's cabinet, telephones, magneto extension bells when required for night-service working, line wires from the unit to the telephones, power supply equipment, and protection apparatus for all lines that run outside buildings. A distribution frame on which all lines are terminated and all protection apparatus mounted is recommended for use with this exchange.

Equipment - with covers on Equipment - rear view

Any extension user is connected to any other extension by dialling the appropriate number. The extensions are normally numbered from 200 to 258 and 260 to 299. (Line circuit number 259 is used to provide call-back facilities).

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.

Alternatively, calls to the public exchange can be made via the attendant's cabinet, access to which is obtained by dialling the single digit "0". On dialling "0" an extension is connected to one of the two "0" level calling lamps on the attendant's cabinet, the lamp flashing to indicate the call. The attendant answers the call by operating a lever-type key associated with the calling lamp. The extension may now replace his handset, and is recalled by the attendant when the call matures. All extensions may obtain calls to the public exchange via the attendant, and equipment is provided for two simultaneous "0" level calls. If both the "0" level lines to the attendant's cabinet are in use, a third party dialling "0" may hold-on; he would then be connected to the attendant automatically when one of the lines became free.

An incoming call from the public exchange causes an associated lamp on the attendant's cabinet to flash. The attendant answers the call by operating the key corresponding to the flashing lamp, and extends the caller to the required extension by key calling the extension number on a strip of digit keys of the plunger type. If the called extension is free, the exchange-line lamp on the cabinet glows continuously until the call is answered, when the lamp is extinguished. If the called extension is engaged on an internal call, the exchange-line lamp flashes (at a different frequency to the calling signal) and the attendant can operate the trunk-offering key and offer the incoming exchange call.

If the extension agrees to accept the exchange call, the attendant can park the waiting call on the extension line. The connexion is still not complete for speech, but as soon as the wanted party replaces his handset, the telephone bell rings and on re-lifting the handset the connexion is completed.

Two distinctive ringing sequences are used so that an extension user can tell whether an incoming call is from the main exchange or from another extension.

An extension engaged on a call over an exchange line may hold the call (by depressing a button on his telephone), while he calls any other extension. At the end of the 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 pushbutton, the user may transfer the call again.

By pressing the pushbutton twice in succession, an extension can recall attendant by causing the associated exchange line lamp to flash.
The attendant is also recalled if the call-back circuit is already in use when the pushbutton is first operated.

Should an extension using the call-back facility find that the wanted extension is engaged, the attendant can be recalled by depressing the button three times in succession. The attendant can then contact the wanted line by using the trunk-offering facility.

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. Up to ten extensions may be provided with this facility by a simple strap adjustment in the automatic unit.

Night-service working is established by the operation of the night-service key on the attendant's cabinet.

Two methods of operation may be used, the required one being selected by the simple adjustment of the strapping on a terminal block.
In the first scheme one particular extension is selected to answer all the incoming calls at night. An incoming call rings this extension's telephone bell, and to answer the call, the extension lifts the handset and dials "8".

In the alternative scheme, an incoming call at night rings separate bells at different selected places. The call is answered at any extension by lifting the handset and dialling "8". For this arrangement, the number of bells required should be specified when ordering.

In either system, the call is transferred to a further extension, if necessary, by means of the call-back and automatic-transfer facility.

On normal calls between extensions the first party to replace his handset releases the automatic equipment. When the executive right-of-way facility has been used, the equipment releases when the executive replaces his handset, whether this is first or last.

Dial and ringing tones together with ringing current are generated by means of vibrating generators, and NU (number unobtainable), busy and warning tones by a valve oscillator, fitted on the automatic unit.

Two alarm lamps are fitted on the attendant's cabinet, one for deferred alarms and the other for urgent alarms. The deferred-alarm lamp lights to indicate a P.G. (permanent glow) condition within 30 to 60 seconds of the condition being set up. The urgent-alarm lamp lights to indicate one of the following: release alarm, ringing-fail alarm or fuse alarm. An audible alarm with cut-off key is associated with the alarm lamps.


The standard P.A.B.X. unit and attendant's cabinet are wired to allow several additional features to be provided either at the time of installation of the exchange, or at a later date. The apparatus required to provide these additional features is mounted external to the main automatic rack. When required, an auxiliary rack can be supplied to mount alongside the main equipment. The additional features include

Four lines are connected direct to the attendant's cabinet. These are provided for those extensions who desire the attendant to establish their calls, whether internal or external. The telephones of these extensions are not fitted with a dial, and when the caller lifts his handset, a lamp flashes on the cabinet. The attendant answers, establishes the connexion and then rings back to the caller.

Facilities can be provided to allow a conference to take place by telephone, the participants remaining at their own desks. There is no limit to the number of participants, as no links circuits are employed in the exchange. Each participant's telephone is connected to an auxiliary unit by an additional line pair and is fitted with a pushbutton. A 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 number of participants on orders or enquiries.

Officials absent from their normal locations can be called by a visual or audible system of codes. A special "Locating" number is dialled by a caller receiving no reply, followed by the personal code digits of the called party. The code is then displayed or sounded throughout the building. The wanted party dials a special "Answering" number from any extension, and is automatically connected to the caller. Please state whether a visual or audible signal is required, how many codes are needed, and the number of calling stations likely to be used.

A secretary may be provided with a G.E.C. Switching Telephone so that all 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 the G.E.C. Leaflet STL 17.


The automatic switching apparatus is mounted on a single-sided open rack. Easily-removable metal covers protect the relays against dust and damage. All the apparatus is the same as that standardised for large public exchanges in many parts of the world, and includes the G.E.C. SE50 two-motion selector. Throughout, the unit and apparatus is designed to give ease of maintenance. The rack can be fitted in a dust-proof, sheet-metal cabinet if specified on orders. Lift-off panels ensure that the equipment is still readily accessible.

The compact cabinet is constructed of matt-polished hardwood, and the hinged keyshelf and rear wall open up to give easy access to the wiring and components.

Calls to the cabinet are signalled on any of sixteen calling lamps of which ten are associated with lines from the public exchange, four with the P.A.B.X. manual extensions, and two with calls from the automatic extensions. The call is answered by the simple operation of the key associated with the flashing lamp.

A strip of plunger-type digit keys provides key-calling facilities to allow calls to be rapidly connected to the wanted extensions. The attendant's cabinet is also equipped with a dial for use on calls to the public exchange, the dial being switched out of circuit when not in use.

Other keys on the cabinet provide for supervisory facilities, night service, and the extension of alarms from the automatic unit.

The exchange operates on a 50-volt DC power supply. When 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 40 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 1.

When the mains supply is DC, the exchange operates from batteries, and a rheostat steps the mains voltage down to the voltage 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.

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 "New Gecophone", the "G.E.C. 1000" Telephone and the G.E.C. Muraphone "K", described in G.E.C. Leaflets STL 16 and STL 13.
A telephone fitted with a pushbutton is supplied for any extension requiring the call-back and automatic-transfer facility. When ordering please state the number of telephones required with pushbuttons fitted.

Any insulated twin conductor, suitably protected if exposed to risk of damp or mechanical damage, may be used to connect the telephones to the exchange provided the line loop resistance does not exceed 900 ohms. A third wire must be connected from the telephones of extensions having the call-back and automatic transfer facility to a nearby earth 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 7862

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 wire and space, with a gain in neatness, may often be achieved by running a multi-core cable from the P.A.B.X. unit to a distribution box so situated that only short lengths of wire 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. Both the lines to the public exchange and all external open-wire extension lines must be connected to protection apparatus before being extended to the P.A.B.X. unit. The use of a distribution frame, which mounts all the protection apparatus, is recommended for this exchange. All the exchange lines and extension lines terminate on one side of the frame, and a cable from the automatic unit terminates on the other side. The two sides can be cross-connected as desired. Changes in the allocation of extension numbers, and line testing, are simplified by use of a distribution frame.
When a distribution frame is not employed, the G.E.C. protector, Cat. No. PR121 1, containing fuses and electrodes, is recommended for connexion to the exchange lines and external extension lines. Protectors for the exchange lines are often supplied by the public exchange administration.

On receipt of a dimensioned sketch of the premises, with indication of the location of each telephone, the most economical distribution scheme will be planned and quotations for materials submitted.


G.E.C. 10+99 LINE P.A.B.X.

Line Circuits Link Circuits Dimensions Weight Catalogue Number
Exchange Extension ins cms lb Kgs Standard Tropical
10 99 10 102 x 54 x 14 260 x 137 x 36 1500 685 PX 2351 PX 2451


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 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 as above, but of 40 ampere-hours capacity.
Catalogue Number: BA1107

Taken from the GEC Catalogue Leaflet No. PXL 05B (Date unknown but probably later than 1959)


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Last revised: November 26, 2021