GEC-AEI TELECOMMUNICATIONS LIMITED
A Management Company of The General Electric Company Limited of England
5 + 20 line Private Automatic Branch Exchange
5 public-exchange lines; 20 extension lines.
Provision for 4 simultaneous inter-extension calls in
addition to a maximum of 5 public-exchange calls.
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
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.
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.
1 or 2 on one route only.
Designated extensions can be connected together via a
Staff absent from their offices may be called by
dialling a special locating number and a personal code, to operate lamps or
A secretarial switching telephone enables secretaries
to filter calls to an executive.
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.
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.
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.
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
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
- No switchboard operator required. Automatic connexion between
- 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
- 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.
CALL-BACK AND AUTOMATIC TRANSFER
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.
EXECUTIVE RIGHT-OF-WAY (PRIORITY)
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
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.
MAINS SUPPLY FAILURE
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
RINGING AND TONES
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
||AWG (B & S)
DISTRIBUTION AND PROTECTION
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
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.
OTHER PUBLIC EXCHANGES
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.
BATTERY POWER SUPPLY
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
G.E.C. 5+20 LINE P.A.B.X.
||54 x 63 x 14
||137.5 x 161 x 36.2
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