GEC 10 + 99 LINE PRIVATE AUTOMATIC BRANCH
The G.E.C. 10 + 99-line private automatic branch exchange has a capacity for
10 lines to the public exchange and ninety-nine extensions. 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 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 ten
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 mounted on an open rack, thus providing easy
access to the equipment for maintenance. The overall size of the unit is 8' 6' x
4' 6" x l' 2" (260cms x 138cms x 36cms). If required, the unit can be enclosed
in a dust-proof cabinet. The cordless attendant's cabinet is attractively
designed to stand on a desk or table, its size being 9" high, 1' 24" 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.
|Equipment - with covers on
||Equipment - rear view
Any extension user is connected to any other extension by dialling the
appropriate number. The extensions are normally numbered from 200 to 258 and 260
to 299. (Line circuit number 259 is used to provide call-back facilities).
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
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 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
If the extension agrees to accept the exchange call, the attendant can park
the waiting call on the extension line. The connexion is still not complete for
speech, but as soon as the wanted party replaces his handset, the telephone bell
rings and on re-lifting the handset the connexion is completed.
Two distinctive ringing sequences are used so that an extension user can tell
whether an incoming call is from the main exchange or from another extension.
CALL-BACK AND AUTOMATIC TRANSFER
An extension engaged on a call over an exchange line may hold the call (by
depressing a button 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 exchange line by depressing the button again. Alternatively,
if he 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
pushbutton, the user may transfer the call again.
By pressing the pushbutton twice in succession, an extension can recall
attendant by causing the associated exchange line lamp to flash.
The attendant is also recalled if the call-back circuit is already in use when
the pushbutton is first operated.
Should an extension using the call-back facility find that the wanted
extension is engaged, the attendant can be recalled by depressing the button
three times in succession. The attendant can then contact the wanted line by
using the trunk-offering facility.
An executive may break in should the extension he requires be already
engaged. When he hears busy tone, he may dial a further digit "1" to interrupt
the established call. A warning tone is given to indicate to the conversing
extensions that a third party is on the line. Up to ten extensions may be
provided with this facility by a simple strap adjustment in the automatic unit.
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.
On normal calls between extensions the first party to replace his handset
releases the automatic equipment. When the executive right-of-way facility has
been used, the equipment releases when the executive replaces his handset,
whether this is first or last.
RINGING AND TONES
Dial and ringing tones together with ringing current are generated by means
of vibrating generators, and NU (number unobtainable), busy and warning tones by
a valve oscillator, fitted on 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.
The standard P.A.B.X. unit and attendant's cabinet are wired to allow several
additional features to be provided either at the time of installation of the
exchange, or at a later date. The apparatus required to provide these additional
features is mounted external to the main automatic rack. When required, an
auxiliary rack can be supplied to mount alongside the main equipment. The
additional features include
Four lines are connected direct to the attendant's cabinet. These 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
Facilities can be provided to allow a conference to take place by telephone,
the participants remaining at their own desks. There is no limit to the number
of participants, as no links circuits are employed in the exchange. Each
participant's telephone is connected to an auxiliary unit by an additional line
pair and is fitted with a pushbutton. A conference is opened by each participant
being individually called by the convener; each then presses the pushbutton on
his instrument to give direct connexion to the conference circuit at the
Please state the number of participants on orders or enquiries.
Officials absent from their normal locations can be called by a visual or
audible system of codes. A special "Locating" number is dialled by a caller
receiving no reply, followed by the personal code digits of the called party.
The code is then displayed or sounded throughout the building. The wanted party
dials a special "Answering" number from any extension, and is automatically
connected to the caller. Please state whether a visual or audible signal is
required, how many codes are needed, and the number of calling stations likely
to be used.
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 STL 17.
The automatic switching apparatus is mounted on a single-sided open 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 is designed to give ease of maintenance. The
rack can be fitted in a dust-proof, sheet-metal cabinet if specified on orders.
Lift-off panels ensure that the equipment is still readily accessible.
compact cabinet 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 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 40 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, the exchange operates from batteries, and a
rheostat steps 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.
Any automatic telephone having a dial-pulse speed of between 7 and 14 pulses
per second, with a ratio of 2 to 1 break to make, is suitable for use with this
Recommended telephones are the "New Gecophone", the "G.E.C. 1000" Telephone and
the G.E.C. Muraphone "K", described in G.E.C. Leaflets STL 16 and STL 13.
A telephone fitted with a pushbutton is supplied for any extension requiring the
call-back and automatic-transfer facility. When ordering please state the number
of telephones required with pushbuttons fitted.
Any 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
|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.
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
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 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 use of a distribution frame.
When a distribution frame is not employed, the G.E.C. protector, Cat. No. PR121
1, 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+99 LINE P.A.B.X.
||102 x 54 x 14
||260 x 137 x 36
Power Supply Unit for operation from a 100/110 volt or 200/250 volt 50c/s
or 60c/s single-phase mains supply having an output of 48/52 volts DC. Suitable
for operating one or two P.A.X. units.
Catalogue Number: SU3204
Dimensions : 48 x 19 x 14 ins; 122 x 48 x 37cms
Weight: 238lb; 108kg
Automatic Float Battery Charger for operation from 100/1 10-volt or
200/250-volt, 50c/s or 60c/s single-phase mains supply, having an output of
48/52 volts, 0-5 amps DC, and suitable for charging 24 cell lead-acid battery.
Catalogue Number: SU4203
Dimensions: 20 x l5 x 9 ins; 53 x 39 x 24cms
Weight: 105lb; 48kg
24 Cell Lead-Acid Storage Battery as above, but of 40 ampere-hours
Catalogue Number: BA1107
Taken from the GEC Catalogue Leaflet No. PXL 05B (Date unknown but probably later than