gec.gif (1164 bytes)GEC  100 LINE


  • Automatic connexion between extensions.
  • Fifteen connecting links for extension-to-extension calls for each hundred line unit.
  • Dialled access to public exchange by all or selected extensions.
  • Attractive floor-mounting manual board.
  • Access to public exchange via manual board by all extensions.
  • Incoming calls signalled on manual board.
  • Trunk offering facility available at manual board.
  • Call-back facilities on public exchange calls available to all or selected extensions.
  • Night-service working when operator is not on duty.
  • Easily extended by the addition of standard racks and manual-board sections.

The G.E.C. 100-line extensible P.A.B.X. is built up of a number of standard units.   These units comprise:-

  • A line and selector rack.
  • A combined miscellaneous-apparatus and relay-set rack.
  • A two-panel manual-board position.

By assembling these units in varying quantities, an exchange having from fifty to six hundred lines can readily be built up, and extensions easily carried out.   This construction differs from that generally used in larger exchanges where one rack is devoted to each type of switch.   The new construction has the following advantages:-

  • The exchange occupies a minimum of floor space.
  • Less cabling is required between racks consequently the cost and installation time are reduced.
  • Extensions to the exchange are easily installed.
  • Standard racks are used for all exchanges, hence local stocks can be held.

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 with-held, if desired, from others.   All extensions can gain access to public exchange, via the P.A.B.X. operator, by dialling "0".

An incoming call from - the public exchange is signalled on a manual switchboard, and extended to the wanted party by the operator.   Up to fifteen extension-to-extension conversations can be held simultaneously for each hundred extension lines.   When an extension makes a call to the public exchange via the manual board, the connecting link in the P.A.B.X. is in use only while the connexion to the board is being established.   The link is then released for use by another 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 circuit is supplied to effect the necessary signalling,

The automatic exchange apparatus is normally mounted on open racks, thus providing easy access to the equipment for maintenance.   The racks are 8ft 6.5ins high (261 cms) and 4ft 6ins (l37 cms) wide (line rack), or 2ft 9ins (84 cms) wide (M.A.R. - miscellaneous apparatus rack).   The switchboard is attractively designed, and stands on the floor.   The size of each position is 4ft 8ins high x 2ft 2.5ins wide x 2ft 9.5ins deep (142 cms x 68 cms x 85.6 cms).

A complete P.A.B.X. system consists of the automatic equipment, manual board, telephones, line wires from the exchange to the telephones, power-supply equipment, a main distribution frame, and protection equipment for all lines that run outside buildings.


Any extension user is connected to any other extension by dialling the appropriate number.   The exchange has a three-digit numbering scheme; normally a one hundred line exchange would be numbered 200 to 299, a two hundred line exchange 200 to 399, and so on.

Direct-access to the public exchange is obtained by dialling a single digit, say, "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 manual switchboard, direct access to which is obtained by dialling a single digit, say, "0".   Each extension has an individual appearance at the manual board, thus giving positive indication of the calling extension to the operator.   The operator answers the call by plugging an answering cord into the calling extension's jack.   This releases the common automatic apparatus in the exchange for use by other callers, and leaves the extension connected direct to the manual board.   The operator then completes the required connexion.   All extensions may obtain calls to the public exchange via the manual board.   In this way unauthorized calls to the public exchange may be barred by the operator.

Where required, direct calls to the public exchange can be routed from level "0" and calls to the manual board from level "9".

An incoming call from the public exchange is signalled on the manual board by a glowing lamp, and is answered by the operator, who extends the caller to the wanted extension by plugging into the extension jack.   If the extension is engaged on an extension-to-extension call, the operator can use a trunk-offering cord to break in to the established connexion and offer the incoming exchange call.

Night-service working is established by connecting the exchange lines to selected extension lines by cords.   These extensions now operate as ordinary independent exchange lines for the duration of the night-service period.

The automatic-switching apparatus is mounted on single-sided open-racks.   Two types of rack are supplied:-

  • Line and selector rack.
  • Combined miscellaneous-apparatus rack and relay-set 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 are designed to give ease of maintenance.   The racks can be fitted in a dust-proof, sheet-metal cabinet, if specified en order.

The line and selector rack mounts the equipment for the line circuits, line finder and allotter circuits, and group and final selectors for a hundred extension lines and fifteen connecting links.

The relay set and miscellaneous apparatus rack houses the auto-manual exchange-line relay sets, the ringing and alarm equipment and any additional equipment required for special facilities.   Relay sets for up to twenty exchange lines can be mounted on one rack.   Where a large amount of equipment is required for special services, a second rack is supplied.

The switchboard mounts on the floor, and is made of medium-oak or teak, with a grey wearite keyboard and lamp panel.   The switchboard is built in sections and can easily be extended as required.   Each section or position, has fifteen cord circuits, a trunk-offering cord circuit and the usual position circuits, viz operator's telephone circuit, cord test circuit, pilot circuit, and fuse alarm circuit.   A lightweight headset is supplied for each operator.

The apparatus associated with these circuits is mounted in the rear of the switchboard, and is easily accessible for maintenance.   The cord-circuit relays are mounted on jack-in plates, and the cords themselves can easily be changed if they become frayed, as they are connected to quick release terminals

The exchange operates on a 50-volt DC power supply.   This is in the form of a lead-acid battery floating across the terminals of a constant-potential rectifier unit operating from the AC mains supply.   The table below shows the recommended battery capacities for different sizes of exchange.

When the mains supply is DC a motor-generator or rotary transformer converts the mains supply to the required voltage.   With this arrangement a charge-discharge system must be used so that the battery connected to the mains is isolated from the exchange.

Ringing current, and dial, ringing, busy, and NU tones are generated by a permanent-magnet inductor tone ringing machine.   This compact machine mounts on a standard jack-in plate.   In the event of a fault on the machine a standby machine can be jacked-in in place of the faulty machine, thus automatic change-over equipment is not required.   The ringer, which operates on the exchange battery, has an output of five watts.   This is adequate for 200 extension lines, and a separate machine is supplied for every two hundred extension lines connected to the exchange.

Any faults arising that adversely affect the operation of the exchange give rise to audible and visible alarms on the racks and at the manual board.   An alarm signal is caused by any of the following conditions:-

  • Permanent loop on an extension line.
  • Blown fuse.
  • Selector failing to restore to normal.
  • Ringing-supply failure.
  • Failure of mains supply to the charging equipment.
  • Failure of allotter.

A test set can be supplied, when required, to test the extension lines and instruments.   The test set is mounted near the M.D.F. and the tests that can be carried out include:-

  • Voltmeter tests of line condition, line resistance, insulation resistance, fault earth or other potentials on line.
  • Dial speed test.
  • Speaking tests.

In addition, an extension can be rung from the test set, or a howler tone connected if an extension has not replaced his handset.

Any automatic telephone having a dial impulse speed of between 7 and 14 impulses per second, with a ratio of 2 : 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. Catalogue Leaflets Nos. STL.13 and STL.16.

A telephone fitted with a push button is supplied for any extension requiring the call-back facility.   When ordering please state the number of telephones required with a push button fitted.

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 telephone of extensions having the call-back 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 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. 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 main distribution frame 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, e.g., due to lightning.   Both the lines to the public exchange and all external open-wire extension lines must be connected to protection apparatus before being connected to the P.A.B.X. unit.   The protection equipment is mounted on the main distribution frame.   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 the use of a distribution frame.

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.


An extension engaged on a call over an exchange line may hold the call (by depressing a pushbutton 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 main exchange line by depressing the pushbutton again.   This facility can be given to all or selected extensions ; all extension telephones having the facility must be fitted with a pushbutton.

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 No. MTL.2.

These enable a conference to take place by telephone whilst the participants remain at their desks.   The conference is called by the convener who rings each participant individually ; the operator then connects a cord from the jack of the appropriate extension to a conference circuit jack.   By momentarily operating the ring key, the operator connects a short pulse of ringing current to the line, so calling the extension to the conference.

A "speak" key is fitted to allow the operator to talk to an extension in conference, but a tone is connected to the circuit when the "speak" key is operated to warn the participants against unauthorised listening-in.

Officials absent from their normal place of employment can be called by a system of bells or lamps.   An extension is connected to the staff location equipment by dialling a special code number ; he then dials the code of the person required.   When ordering, please state whether a visible or audible display is required, how many codes are needed, and the number of calling stations likely to be used.

Tie lines can be provided between the P.A.B.X. and other private telephone exchanges, either in adjacent buildings, or in buildings up to several miles away to permit intercommunication between extensions in all buildings.

Please state the type of P.A.X. into which the tie lines must work and the line loop resistance of the tie lines themselves.   Should an existing exchange be of other than G.E.C. manufacture, its circuit drawings should be provided with an enquiry.

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

Equipment Dimensions Weight Catalogue Number
in cms lb Kgs Standard Tropical
Line and selector rack 102 x 54 x 14 260 x 137 x 36 1920 883 PB3100 PB3200
Relay-set rack and M.A.R. 102 x 33 x 14 260 x 84 x 36 960 442 PB3100 PB3200
Manual-board section 56 x 26 x 33 142 x 68 x 85.6 350 161 PB3100 PB3200


Exchange Extension Lines Line and Selector Racks Combined MAR RSR RSR Manual-board Positions
10 100 1 1 - 1
20 100 1 1 - 2
20 200 2 1 - 2
30 200 2 1 1 3
30 300 3 2 - 3
40 300 3 2 - 4
40 400 4 2 - 4
50 400 4 2 1 5
50 500 5 3 - 5
60 500 5 3 - 6
60 600 6 3 - 6

 N.B. Where special facilities are required, an extra R.S.R. may be required to mount the equipment.

Extension Lines (Ultimate) Charging Current (amp.) Battery Capacity (ampere-hours)
100 0-5 30
200 0-10 50
300 0-10 75
400 0-15 100
500 0-20 140
600 0-25 180

N.B. Where an exchange is to be extended at a later date, it is advisable to install power plant capable of supplying the ultimate exchange.  The battery capacities quoted will operate the exchange for a period of 16 hours in the event of  a failure in the mains supply.

Taken from the GEC Catalogue Leaflet No. PXL 06A (Date unknown but later than 1959)

Taken in 1960

Line and selector rack drawing 701028 combined with MAR & SAR Drawing 701034



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Last revised: November 30, 2021