GEC 100 LINE EXTENSIBLE PRIVATE AUTOMATIC BRANCH EXCHANGE
- 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
- Night-service working when operator is not on duty.
- Easily extended by the addition of standard racks and manual-board
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
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
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
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
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
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 AND TONES
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.
LINE TEST SET
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
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
|Size of Copper Conductors
||Maximum Permissible Length of
||AWG (B & S)
DISTRIBUTION AND PROTECTION
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.
|Line and selector rack
||102 x 54 x 14
||260 x 137 x 36
|Relay-set rack and M.A.R.
||102 x 33 x 14
||260 x 84 x 36
||56 x 26 x 33
||142 x 68 x 85.6
|EXAMPLES OF EQUIPMENT REQUIRED
FOR TYPICAL EXCHANGE
||Line and Selector Racks
||Combined MAR RSR
N.B. Where special facilities are required, an extra R.S.R. may be
required to mount the equipment.
|TYPICAL EXAMPLES OF
POWER PLANT REQUIRED
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
Taken in 1960
Line and selector rack drawing 701028
combined with MAR & SAR Drawing 701034