gec.gif (1164 bytes)GEC  PABX 10+49

A Management Company of The General Electric Company Limited of England

10+49 Line Private Automatic Branch Exchange

Maximum capacity
10 public exchange lines: 49 extension lines.

Connect circuits
Provision for 7 simultaneous inter-extension calls in addition to a maximum of 10 public exchange calls.

Numbering scheme
21 to 69 for extensions: 7 for tie-line access; 8 for night service; 9 for public exchange access; 0 for operator/switchboard access.

Public exchange service
Outgoing: direct access from extensions, or through operator's switchboard.  Individual extensions can be prevented from dialling public-exchange calls.
Incoming: through operator's switchboard.

Enquiry and transfer
Telephones with a pushbutton can be supplied to all or selected extensions to enable a public-exchange call to be held while the user calls another extension with an enquiry.  When the enquiry is complete, the original extension returns to the public-exchange call, or can transfer it to another extension or to the operator.

Night service
Incoming public-exchange calls can be answered from any extension by dialling '8'; alternatively, the feature can be restricted to one extension only.  Calls can then be transferred to other extensions by the enquiry and transfer method.

Trunk offer
The operator can offer a public-exchange call to an extension already engaged.

Tie lines

Normally, up to 4 on one route only.

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 and a personal code, to operate lamps or bells.

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

Manual extensions
Up to 4 telephones without dials may be wired direct to the operator's switchboard for special priority services.

Route restriction
Individual extensions may be permitted to dial local but not long-distance calls.

Mains fail protection
Public exchange lines can be automatically connected to designated extensions in the event of an AC mains supply failure.

Mode of operation

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

Tones and ringing
All-electronic tone generator provides dial tone, ring tone and current, busy tone, number unobtainable tone and operator-intrusion tone.

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

Lamp indication at switchboard.
Urgent: release alarm, ring fail and blown fuse.
Deferred: failure to replace receiver (PG condition).

Power unit
AC power unit: input, 100 to 120V or 200 to 250V 50 to 60 Hz single phase; output 50V d.c. 10A.
Battery: 50V 32 AH; with float charger, input as for AC power unit, output 50V d.c. 3A.

Line-loop resistance
Not to exceed 900 ohms.

Switching equipment: Plug-in and wired-in units on a steel framework in a floor-mounting steel cabinet (painted grey) with lift-off dust-proof panels front and rear.

Switchboard: dual-grey desk-mounting console.

Dimensions and weight


Catalogue Number

Height    Width    Depth    Weight
Switching equipment     PB2449 237cm (93in    109 cm (43 in) 46 cm (18 in)    566 kg (1250 lb)
Switchboard      13cm (5in)    38 cm (15 in)    31cm (12 in)    9 kg (19 lb)
A C mains unit     SU3204 122cm (48 in) 48 cm (19 in)    38cm(15 in)    108 kg (238 lb)
Float charger     SU4202 53cm (21in)    41cm (16in)    25 cm (10in)    35 kg (76 lb)

Taken from a GEC-AEI brochure SLB7B - dated 1968

This is the Mark 2 variant - see below for more detailed pictures


  • Automatic connexion between extensions.
  • Seven connecting links for extension-to-extension calls.
  • Dialled access to public exchange by all or selected extensions.
  • Compact desk-mounting attendant's cabinet, fitted with key-calling equipment.
  • Access to public exchange via attendant's cabinet by all extensions.
  • Incoming calls signalled on attendant's cabinet.
  • Trunk-offering facility available at attendant's cabinet.
  • Call-back and automatic-transfer facilities on all public exchange calls available to all or selected extensions.
  • Night-service working when attendant is not on duty.
  • Dust-proof automatic unit.

The G.E.C. 10 + 49-line private automatic branch exchange has a capacity for ten lines to a public exchange and forty-nine extensions. 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. 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 seven 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 housed in a dust-proof, sheet-steel cabinet, 7' 0" x 3' 6" x 1' 10" (214cms x 107cms x 56cms). The cabinet is fitted with hinged doors, front and rear, to allow easy access to the equipment for maintenance purposes. The cordless attendant's cabinet is attractively designed to stand on a desk or table, the size being 10" high, 1' 2" 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.


Any extension user is connected to any other extension by dialling the appropriate number. The extensions are normally numbered from 21 to 69.

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's 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.

An extension engaged on a call over an exchange line may hold the call (by depressing a button on his telephone), while he calls another 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 lie 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, the user may transfer the call again.

By pressing the push-button twice in succession an extension can recall the attendant by causing the associated exchange-line lamp to flash.

Four lines are connected direct to the attendant's cabinet. These are additional to the 49 automatic extensions and 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.

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.

The automatic-switching apparatus is mounted in a steel cubicle, which has hinged doors to give access to the equipment. 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 are designed to give ease of maintenance.

The compact cabinet, on which the manual equipment is mounted 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 the 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 30 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, a rheostat should be used to step 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.

Dial, busy, and ringing tones, together with ringing current, are generated by means of vibrating generators fitted in 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.

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 "K" and the Gecophone "K" described in G.E.C. Leaflets STL 13 and 14.
A telephone fitted with a push button is supplied for any extension requiring the call-back and automatic transfer facility. When ordering please state the number of telephones required with push buttons fitted.

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 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 MTL 2.

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 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 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 cables from the automatic unit terminate 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 the use of a distribution frame.
When a distribution frame is not employed, the G.E.C. protector Cat. No. PR1111, 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+49 LINE P.A.B.X.

Line Circuits Link Circuits Dimensions Weight Catalogue Number
Exchange Extension ins cms lb Kgs Standard Tropical
10 49 7 84 x 42 x 22 214 x 107 x 56 1250 570 PX 2301 PX 2401

Power Supply Unit
for operation from a 200/250-volt 50c/s single-phase mains supply, having an output of 48/52 volts DC.
Catalogue Number: SU 1206
Weight: 2381b; 105kgs
Dimensions: 48 x 19 x 14 ins; 122 x 48 x 37cms

Power Supply Unit for operation from any other single-phase mains supply, having an output of 48/52 volts DC.
Dimensions as above.
Catalogue Number: SU 2206

Automatic Float Battery Charger for operation from a 200/250-volt, 50c/s single-phase mains supply, having an output of 48/52 volts, 0-3 amp DC, and suitable for charging a 24-cell lead-acid battery.
Catalogue Number: SU 1204
Dimensions: 20 x 15 x 9 ins; 53 x 39 x 24cms
Weight: 761b; 34.6kgs

Automatic Float Battery Charger for operation from any other single-phase mains supply, having an output of 48/52 volts, 0-3 amp DC.
Dimensions as above.
Catalogue Number: SU 2204

24-Cell Lead-Acid Storage Battery less stand, acid and accessories, capacity 30 ampere-hours at 10-hour rate of discharge.
Catalogue Number: BA 1104

Taken from the GEC Catalogue Leaflet No. PXL 02B (Date unknown) and the GEC Catalogue Leaflet PXL 07 (undated but probably later than 1959)

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

10+49 PABX  Mark 2 (Drawing 701593)
Additional pictures
Taken in 1962

Front view with doors fitted Rear view with doors removed to expose wiring
Front view with doors removed Front view with doors and some covers removed
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Last revised: November 30, 2021