gec.gif (1164 bytes)GEC  PABX 5+20

A Management Company of The General Electric Company Limited of England

5 + 20 line Private Automatic Branch Exchange

Maximum capacity

5 public-exchange lines; 20 extension lines.

Connect circuits
Provision for 4 simultaneous inter-extension calls in addition to a maximum of 5 public-exchange calls.

Numbering scheme
30 to 49 for extensions; 1 or 2 for tie lines.

Public exchange service
Outgoing: direct access from extensions after operating a pushbutton switch on the telephone.  Individual extensions can be prevented from dialling public-exchange calls.

Incoming: No operator's switchboard is required; a call is answered at one of up to 8 designated extensions and routed to the required extension by the enquiry and transfer method.  The call can be offered to an extension already engaged.

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.

Executive's priority
Selected executives can break into an existing conversation by dialling an extra digit '1'.

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

Tie lines

1 or 2 on one route only.

Designated extensions can be connected together via a special circuit.

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.

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

Mode of operation

Fully automatic operation using uniselectors and relays.  All conversations are secret from each other.

Tones and ringing
Mains-supply derived dial tone, ringing tone, ringing current and executive intrusion tone; electronically derived busy, number-unobtainable and link throw of tones.  When the exchange operates from a battery power supply, all tones and ringing current are provided by an all-electronic tone generator.

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

Audible indication of blown fuse.

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

Line-loop resistance
Not to exceed 1000 ohms.

Wired-in units on a framework, hinged for easy maintenance, and contained in a steel cabinet (painted dark green) with hinged doors.

Dimensions and weight

Description Catalogue Number Height    Width    Depth    Weight
Switching equipment PB2204 138cm (54in) 161cm (64in) 36cm (15in) 303kg (667lb)
Float charger SU4202 53cm (21in) 39cm (16in) 24cm (9in) 35kg (76lb)

Catalogue Leaflet printed in 1968

5+20 line Private Automatic Branch Exchange

The G.E.C. 5+20 line private automatic branch exchange has a capacity for five lines to a public exchange and twenty extensions.  It has no switchboard or attendant.  One extension calls another by dialling two digits.  An extension obtains direct access to the public exchange (either automatic or C.B. manual) by depressing a pushbutton on the telephone instrument.  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 eight 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 not used.  Thus, calls can be made to the public exchange even when all links are in use.

The PABX is mounted in a sheet-steel dust-proof cabinet having hinged lift-off doors at the front to give access to the equipment.  The equipment panels are mounted on hinge-posts enabling each panel to swing forward to improve accessability during maintenance.  The illustration below shows one of the exchange line circuit panels partly open.  The size of the cabinet is 4' 6" x 5' 3" x 1' 2".  The exchange employs the same apparatus as has been standardised for large public exchanges by many Overseas Governments and the British Post Office.

Cabinet with all covers removed and one exchange line circuit panel partly open


  • No switchboard operator required. Automatic connexion between extensions.
  • Up to four connecting links for extension-to-extension calls.
  • Pushbutton access to public exchange for all or selected extensions.
  • Incoming calls transferable among extensions.
  • Call-back and automatic transfer facilities available on all exchange calls.
  • Executive right-of-way facilities available to selected extensions.


Any extension user is connected to any other extension by dialling the appropriate number. The extensions are numbered from 30 to 49.

Direct access to the public exchange is obtained by depressing the pushbutton on the telephone instrument. 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.

Equipment can be supplied to prevent certain extensions having direct access to the public exchange dialling their own trunk calls (see Additional Features).

A call from the public exchange is taken by any one of eight answering extensions, who are predetermined and can transfer the call to any extension by depressing a pushbutton on their telephone and dialling the wanted number. If required an extension can be prevented from receiving an exchange call by a simple adjustment to the strapping. Should an answering extension attempt to transfer an exchange call to an extension that is barred these calls, the answering extension itself will be re-rung. 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. If all link circuits are engaged when an answering extension attempts to transfer a call he can interrupt one of the conversations by holding the pushbutton depressed for an extended period. This causes a warning tone to be sent to the two conversing extensions who should then restore their handset. If they do not, the link is force released after a short interval to allow the main exchange call to be connected. 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 the pushbutton 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 pushbutton again. Alternatively, if he wishes, he may transfer the exchange line to the other extension merely by replacing his own handset, provided the second extension is allowed to receive transferred calls. A call may be re-transferred as often as required between extensions.

An executive may break in should the extension he requires be already engaged. When he hears busy tone, he dials a further digit '1' to interrupt the established call.
Two methods of operation are possible, private and non-private, the required one being selected by a simple adjustment to the strapping in the automatic unit.
When the 'private' operation is used, an intrusion tone is heard by both parties, but the executive who wishes to contact one of them cannot hear the conversation in progress. Upon hearing the intrusion tone both parties should replace their handsets, whereupon the required extension will be rung without any further operation by the executive. When the 'non-private' operation is used, an intrusion tone is heard by both parties and the executive can immediately enter the conversation. He can, in fact, carry out the whole of his required conversation without the original call being cleared. However, if the executive expects to carry on a long conversation it is advisable that the original call is cleared ; the wanted extension will then be called automatically as described above.

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

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

When the exchange power supply is obtained from the mains rectifier unit, dial and ringing tones, together with ringing current, are derived from the mains unit and busy tone from a transistored generator.

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


Recommended telephones are the loudspeaking telephone, the ' G.E.C. 706' telephone, and the ` G.E.C. 711 'telephone described in G.E.C. leaflets STL15, STL16 and STL18 or equivalent.
A telephone fitted with a pushbutton is supplied for each of the answering extensions and for any other extension requiring the call-back and automatic transfer facility. When ordering, please state the number of telephones required with pushbutton fitted.
G.E.C. Loudspeaking Telephones, which enable the user complete freedom of movement about the office whilst making a call, are fully described in Catalogue Leaflet No. STL15.

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 1000 ohms.

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 3960 3600
6.5 24 25 0.408 6280 5720
10 22 23 0.64 9640 8768

The simplest method of connecting the extension instrument to the PABX 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 PABX 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. PR1111, containing fuses and electrodes is recommended. Protectors for the exchange lines are often supplied by the public telephone exchange Administration.
On receipt of a dimensional sketch of the premises, with indication of the location of each telephone, the most economical distribution scheme will be planned and quotations for material submitted.


The standard PABX is wired to allow several additional features to be provided either at the time of installation or at a later date. The apparatus required to provide these additional features is mounted external to the main automatic unit. The additional features include:

Up to two tie lines can be provided between the PABX and another private telephone exchange, either in an adjacent building or in a building several miles away, to permit intercommunication between extensions in both buildings.
Please state the type of private exchange 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.

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 the 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.

If necessary, additional equipment can be supplied to prevent those extensions having direct access to the public exchange from dialling their own trunk calls. Please give details of those digits and combinations of digits that are to be barred.

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 PABX. This service is fully described in Catalogue Leaflet STL17.

When the public exchange is other than an automatic or C.B. Manual type, e.g. magneto or C.B.S., an auxiliary unit is required to effect the necessary signalling. Please state the type of public exchange.

These enable conferences to take place by telephone while the participants remain at their desks. The conference is opened by each participant being individually called by the convener ; each then dials a special digit to gain access to the conference circuit. Immediately the connexion is established, the link circuit is released. No extra line wires or pushbuttons are required. The executive right-of-way facility cannot be used to gain connexion to an extension engaged on a conference call. In these circumstances the executive hears a special tone to indicate that the extension is in conference. Please state the number of participants required.

Should the mains supply be unreliable, a storage battery and float charger can be supplied to ensure continuity of service. This replaces the built-in power unit. In addition, a transistored ringing generator must be fitted to supply the ringing current and tones, etc.

Similarly, should the mains supply be d.c. the exchange must be operated from batteries with a rheostat to step 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.


G.E.C. 5+20 LINE P.A.B.X.

Line Circuits Link Circuits Dimensions Weight Catalogue
Exchange Extension ins cms lb Kgs
5 20 4 54 x 63 x 14 137.5 x 161 x 36.2 667 302.5 PB2204

For normal use a power unit is built into the exchange. This operates from a 100/120 or 200/250-volt 50/60 c/s single-phase mains supply. Where a battery and float charger is required the following is recommended:-

Automatic Float Battery Charger for operation from a 100/120 or 200/250 volt 50/60 c/s single-phase mains supply having an output of 44/55 volts, 0-3 amp d.c., and suitable for charging a 24-cell lead acid battery.

Dimensions: 20 x 15 x 9in (53 x 39 x 24 cms)

Weight: 761b (34.6 kg.)

Catalogue Number: SU 4202

Taken from the Catalogue leaflet No. PXL012A


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Last revised: October 07, 2010