GEC PABX 4

PABX 4
Private Telephone System

The PABX 4 allows for:

FULL INTERCOMMUNICATION
Any extension user is connected to any other extension by dialling the appropriate number.  The exchange can have a 3 digit, mixed 3 and 4 digit or a 4 digit numbering scheme depending on the total number of extensions fitted.

OUTGOING CALLS
Direct access to the public exchange is obtained by dialling '9'.  If the public exchange is automatic, the caller then dials the required number; if the exchange is manual, the call is completed by the public exchange operator.  Selected extensions may be barred from this facility, if desired, by a simple alteration to the strapping in the automatic exchange equipment.  Similarly, calls can be dialled to other private exchanges over interswitchboard lines by dialling either one or two routing digits prior to dialling the requisite extension number on the distant exchange.

Alternatively, calls to the public exchange or interswitchboard calls can be made via the switchboard by dialling '0'.  The operator answers the call and asks for the required number.  The caller can then restore his handset while the operator sets up the required connection using a keysender.  As soon as the wanted party answers, the operator keysends the local extension number requiring the call.  Until the extension answers, appropriate supervisory signals are provided for the benefit of the operator who may re-enter the circuit if necessary.  When the extension does answer, communication is established directly between the exchange line subscriber and the extension independent of the switchboard. if all '0' level lines are engaged, busy tone is heard by the caller.

An extension speaking on an external call can recall the operator by pressing the pushbutton on the telephone and dialling '0'.

CALL WAITING INDICATION
Three switchboard lamps are provided to indicate that calls are waiting for the operator.  These are labelled calls waiting, busy, and congestion.  A typical method of operation is for the calls waiting lamp to light if two or more calls are waiting, the busy lamp to light if six or more calls are waiting and the congestion lamp to light if twelve or more calls are waiting.  The number of calls waiting at each stage before the respective lamps light can be varied to suit the requirements of individual customer demands.

INCOMING CALLS
An incoming call from the public exchange is signalled at the switchboard and is answered by the operator, who extends the caller to the wanted extension by keying the appropriate number.  When the extension answers, the switchboard connect circuit is automatically released without any further action by the operator.  The extension can later recall the operator by pressing the pushbutton on the telephone and dialling '0'.

CALL-BACK AND AUTOMATIC TRANSFER
An extension engaged on a call over an exchange or interswitchboard line may hold the call (by pressing a pushbutton on the telephone), while he calls another extension.  At the end of his conversation with the second extension, he can resume his original conversation by pressing the pushbutton again.  Alternatively, he may transfer the call to the other extension merely by replacing his handset.

If an extension calls a second extension using the recall facility and receives no reply, or the wanted extension is engaged, he can recall the operator by pressing the pushbutton twice in succession and dialling '0'.

An extension that has received a call by the transfer facility can further transfer it to another extension.  The number of times a call can be transferred is unlimited.

CONTINUOUS RING CALL GUARD
If an extension calls another extension for the purpose of transferring an external call and receives no reply, or, conversely, the wanted extension is engaged at that time, any attempt made to evade the responsibility of further dealings with the call by replacing the extension instrument handset results in the telephone instrument receiving continuous ringing current.  Ringing continues unabated until communication has been re-established with the original calling party.

The provision of this safeguard ensures that virtually no calls can be 'lost' through negligence or design and that call-back and automatic transfer can only be effected after the desired party has answered his or her telephone.

RELEASE
At the end of all extension-to-extension calls the switch train is released by the first party to replace the handset, except when the caller is an executive.  On executive and operator calls, release is provided.  If an extension line becomes looped, either through a fault or because a handset replaced correctly, a line throw-off facility becomes effective and releases the switch train for use by other callers.

NIGHT SERVICE
Night service is provided by arranging that incoming calls to the switchboard (either exchange or inter-switchboard) ring bells at selected locations.  The call can be answered at any extension by dialling '8'.  The call can then be automatically transferred to the wanted extension as described under call-back and automatic transfer.  This is the standard arrangement provided for night-service working.  Alternative arrangements can be supplied; these are described below.

RINGING AND TONES
Ringing current and service tones, e.g., dial, ringing, busy, and number unobtainable, are generated by a battery driven ring and tone machine.  Two machines are provided and in the event of failure of one, a standby is brought into service automatically.

ALARMS
Any faults arising that adversely affect the operation of the exchange give rise to audible and visible alarms on the automatic equipment and/or at the switchboard.  An alarm signal is caused by any of the following conditions.

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

EXECUTIVE RIGHT-OF-WAY
By means of this facility, the calls of certain executives are given priority, so that they can get through to a wanted extension even when that extension is already engaged.  When the executive hears busy tone, he dials a further digit '1'. This causes intrusion tone to be connected to the conversing extensions indicating that an executive is waiting.  When the conversing parties replace their handsets the required extension is automatically rung and connected to the waiting executive.

Additional Features
The PABX4 allows several additional features to be provided either at the time of installation, or at a later date.

AUTO/MANUAL EXTENSIONS (PRIORITY ANSWERING)
An extension can be connected directly to the operator's switchboard for priority assistance.  When the handset is lifted a call signal is given at the switchboard.  The operator answers, establishes the connection, then rings back the caller.  These extensions can be called direct from automatic extensions and can dial their own calls without the assistance of the operator by pressing the enquiry pushbutton before lifting the handset.

GROUP HUNTING FINAL SELECTORS
Special final selectors can be fitted to serve certain numerical groups allowing the selector to hunt over from two to ten lines in the group in the event of the first line being engaged.  A caller can thus be connected to a free line in the group without the inconvenience of having to dial other numbers.

CONFERENCE LINES
These enable a conference to take place by telephone while the participants remain at their desks.

STAFF LOCATION
Members of staff 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 digits in a personal code.

ALTERNATIVE NIGHT SERVICE ARRANGEMENTS
Two other forms of night service can be provided as alternatives to that described earlier.

1.  Selected extensions can be connected to individual exchange lines to answer incoming calls.  Should the extension be engaged on an internal call, an audible signal is given to indicate that an exchange call is waiting.

2.  All incoming exchange fine and interswitchboard line traffic can be routed to selected extensions and extended to the wanted extension by the call-back and automatic transfer.

ROUTE RESTRICTION
In STD areas facilities can be provided to bar certain public exchange codes to selected extensions, and to give local metering of main exchange calls at the PABX.  If required, local meters can also be fitted at those individual extensions accorded appropriate access facilities.

PUSHBUTTON TELEPHONES
Any of the dial telephones can be replaced with pushbutton (keyphone) instruments.

One of the main features of the GEC PABX4 is versatility.  Exchanges of virtually any size and providing a wide range of facilities can be built up from standard racks to suit almost any application.  If you would like to know more about GEC's PABX4 system, or details of a system designed to your particular requirements, please advise how many public-exchange, interswitchboard, extension lines, and the range of facilities you require.

Equipment
The PABX will work to public automatic or CB manual exchanges.  If the public exchange is of the magneto or CBS type, an auxiliary circuit is supplied to effect the necessary signalling.

The automatic switching apparatus is mounted on single-sided open racks thus providing easy access to the equipment for maintenance.  The racks are 2286 mm (7 ft 6 in) high and 1370 mm (4 ft 6 in) wide, except for the combined MA/ringer rack which is 838 mm (2 ft 9 in) wide.  A main distribution frame is provided to facilitate connections between the extensions, exchange lines, and interswitchboard lines, and the exchange.  The system comprises five different racks: line and final selector rack; group selector rack; relay set rack; operator's connect rack; and MA/ringer rack.

Easily removable metal covers protect the relays against dust and damage.  All apparatus is the same as that used in many large public exchanges throughout the world, and includes the GEC SE50 two-motion selector.  Throughout, the exchange and apparatus are designed to give ease of maintenance.

The line and final selector rack accommodates the equipment for one hundred line circuits, linefinders and allotters for these hundred extensions, and twenty final selectors.

The group selector rack accommodates sixty group selectors and the relayset rack twenty relaysets for exchange or inter-switchboard lines together with the relay sets for call-back and automatic-transfer service.

The operator's connect rack carries the relays and other apparatus associated with the operator's position as well as the keysender equipment.

The ringing machines, associated control relay sets, alarms and '0' level relay sets and other miscellaneous apparatus are situated on the combined MA/ringer rack.

The switchboard is floor mounting and has been designed to ensure straightforward and speedy operation.  All connections are established by key operation and keysending.  Calling and supervisory lamps are associated with each key.  The system can be easily extended to incorporate more operators by adding further positions as the overall capacity increases.  Positions need not be "en suite" as there is no multiple between them.  Access to cable terminations is from the front or rear thus enabling switchboards to be located against a wall if required.  Each position is 1016 mm (3 ft 4 in) high, 660 mm (2 ft 2 in) deep, and 1054 mm (3 ft 5 in) wide.

The switchboard fascia panel is hinged so that it can be easily swung forward to allow access to the wiring at the back of the keys and lamps. The desk front allows plenty of space for writing and compartments are provided below for telephone directories or other references.  A lightweight headset is provided for each operator; the position is inoperative until the plug on the operator's headset has been correctly inserted.  Thus, any person interfering with the keys when an operator is not present would not affect calls being set up on adjacent positions if the safeguard is correctly used.

The number of external routes on a standard switchboard is 30 divided between extension, public exchange, and interswitchboard lines.

The Line identification panel is normally situated alongside the switchboard, and may be either table or wall mounted.  It ensures the operator is made aware of any alarm indications from the automatic equipment and also serves to identify, and remove from service, any exchange line or interswitchboard line that is considered unserviceable for any reason.

The exchange operates on a 50v 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.

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.  This ensures that the battery connected to the mains is isolated from the exchange.

Recommended telephones include the GEC746, GEC741, GEC740, and LST4D rotary-dial instruments, and also the GEC 'keyphone' series of pushbutton telephones.

All telephones are fitted with a pushbutton to provide call-back and automatic-transfer facilities.

Line wires can be 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 600 ohms. This can be extended to 1000 ohms by the addition of auxiliary equipment.  A third wire must be connected from each telephone to a nearby earth (ground) point.  If there is no convenient earth point, the third wire must be run to the exchange

DISTRIBUTION AND PROTECTION
The simplest method of connecting the extension instruments to the PABX 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 be accidentally introduced into external fines, e.g. due to lightning.  All lines to the public exchange and all external interswitchboard lines and external extension lines must be connected to protection apparatus before being connected to the private exchange equipment.  The protection equipment is mounted on the main distribution frame.  All the exchange lines and external lines terminate on one side of the frame; cables from the private exchange terminate on the other side.  The two sides can be cross connected as desired by flexible connections.  Changes in the allocation of extension numbers, and line testing facilities, 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 is planned, and quotations for materials submitted.

1972

GEC

Last revised: 18 November, 2000