PAGE No. 37

P.A.B.X. No. 3
July 1955

P.A.B.X. No. 3 is the largest of three types of private automatic branch exchange equipments recently standardized by the British Post Office. It is for use where the ultimate number of extension lines will be not less than 50 and may be increased, as circumstances demand, to any desired capacity.

Before the 1939-45 war, the British Post Office had no agreed standard types of Private Automatic Branch Exchanges (P.A.B.X.). Thus a customer whose requirements included access to the public telephone network could approach any approved manufacturer of telephone equipment and request that a P.A.B.X. be 'tailor-made' to suit his particular needs. The manufacturer would then prepare circuits and equipment drawings, submit them to the Post Office and obtain approval. Manufacture having been completed, the equipment would be installed and handed over to the Post Office for maintenance. This procedure was unsatisfactory to all parties.

To the manufacturer, because the differing individual requirements involved excessive engineering and draughting.

To the Post Office, because the varied types of P.A.B.X. presented ever-increasing maintenance problems.

To the customer, because long engineering time imposed delivery restrictions which he could not fully appreciate.

Fig. 1
Trunking Arrangements for P.A.B.X. No. 3

The British Post Office decided that standard P.A.B.X.'s should be designed, and that the task should be assigned to the British Telephone Technical Development Committee. Development of the following three types of P.A.B.X. was undertaken:-

  1. P.A.B.X. No. 1, having capacity for 10 exchange lines and 49 automatic extension lines, and employing a cordless manual switchboard.
  2. P.A.B.X. No. 2, having capacity for 10 exchange lines and 49 automatic extension lines, and employing a cord switchboard to accommodate also an additional 30 manual extension lines.
  3. P.A.B.X. No. 3, for any requirement exceeding 10 exchange and 49 extension lines.
Fig. 2
Typical One-position Switchboard with Cable Turning Section

While the standardization of P.A.B.X. Nos. 1 and 2 was a relatively simple matter, more work was involved in standardizing P.A.B.X. No. 3. It was decided to concentrate on a system (Fig. 1) using a cord type manual board, 50-point linefinders of the uniselector type, and 100 outlet two-motion selectors. These decisions culminated in the development of standard circuits ; but, whereas complete standardization of the equipment units of P.A.B.X. Nos. 1 and 2 followed, the P.A.B.X. No. 3 apparatus layout was left to the discretion of each manufacturer, though with certain provisos, chief among which are the following:-

  1. Components shall be of established P.O. standard types. This determines, among other things, the precise layout and size of each jack-in selector and relay set.
  2. Shelves, etc. shall be of standard type. This ensures that racks are of standard widths.
  3. The number of linefinders and group selectors for each 50'line group shall be limited to a maximum of 10.
  4. The manual switchboard shall be of the type used for the P.O. No. 1A private manual branch exchange.

Another difference between P.A.B.X. No. 3 and the two smaller equipments is that the latter were to be made for sale to the Post Office, who would rent them to the public, whereas it was agreed that P.A.B.X. No. 3 should, in the main, be sold direct to the public, the Post Office being responsible for maintenance as soon as installation was completed.

Partial standardization having been thus effected, it lay with each manufacturer's designers to evolve the most advantageous and economic lay-out possible within the provisions of (1) to (4) above.

In our own case, the main objectives sought were maximum advantage to the customer and convenience of maintenance. The realization of these objectives was in turn dependent upon the fulfilment of the following conditions:-

  1. the equipment must be readily and economically extensible.
  2. it must, in order to conserve floor space, have maximum compactness, consistent with accessibility for maintenance.

It will be shown that these conditions have been satisfied, as far as the automatic equipment is concerned.

The obligation to provide a switchboard of regulation pattern did not preclude the possibility of improving its appearance. Some improvement seemed desirable, because the orthodox standard colour combination of mahogany woodwork, black apparatus and red fibre was deemed too sombre for the modern office and not conducive to efficient operating. Brighter colour tones were therefore obtained through the use of light oak timber with green 'Wareite' (plastic) facings on the keyshelf and on the spacing strips in the jack field ; the operator's chair was also finished in green and the kicking panel covered with light brown linoleum. A typical one-position board with cable turning section is shown in Fig. 2.

In the case of the automatic equipment, since the size of each relay set and selector was pre-determined, the desired compactness could only be achieved by disposing them on racks to the best advantage.

The agreed maximum provision of 10 group selectors per 50 lines prompted the use of a standard ten-selector shelf and, consequently, a rack 4' 6' (137.2 cm) wide.

It was essential that the line circuits, being connected to the final selector multiple, should be mounted on a rack with final selectors; but before a layout could be devised, it was necessary to decide how many selectors would be required for any chosen size of line group.

The agreed grade of service was 1 lost call in 200 calls; thus 10 group selectors (for a 50 line group) could carry 3.96 traffic units, and 20 selectors (representing a group of 100 lines) could carry 7.92 traffic units. The latter could be interpreted as the maximum traffic to be carried by one group of 100 outlet final selectors. For this traffic 18 final selectors would be needed. This seemed a too liberal provision, but experience has since proved it justified where considerable numbers of tie lines terminating on special incoming group selectors have added to the traffic from the ordinary group selectors. If in such cases the capacity for final selectors had been less than 18 per group, it would have been necessary either to provide auxiliary final selectors on a separate unit (thereby increasing the amount of floor space required) or to maintain the ratio of line circuits to final selectors by reducing the number of line circuits per unit ; this again would have necessitated more units and floor space.

The decision to cater for 18 final selectors having been made, it remained to decide which of the three available standard selector shelves, i.e. 5, 7 or 10 way, could best be used. A 10 way shelf was preferred, as this was of the same width as the latest standard combined uniselector and line relay shelf. Its width was also the same as that of the group selector shelf, and the possibility of designing one unit to accommodate the line circuits, group and final selectors for 100 lines was envisaged.

The only other equipment having a direct quantitative relation to the 100 line group was ringing, tone and pulse equipment ; but, since the basis of provision for some of this was 1 per 100 lines and for the rest 1 per 200 lines, it seemed uneconomic to fit the equipment on alternative units, leaving vacant space on the others. A better solution was to mount the equipment on the relay set rack, where ringing and tone services would be needed for the exchange and inter-switchboard line relay sets.

The enquiry facility, a special feature of P.A.B.X. No. 3, necessitates the provision of a separate group selector which is multipled with the ordinary group selectors. It was decided that if the full complement of ten group selectors was not needed the enquiry selector could occupy one of the unequipped group selector positions. Failing this, the spare positions resulting from fitting 18 final selectors on two 10 way shelves could be used. This arrangement permits the provision of two enquiry selectors per 100 lines, should heavy exchange traffic require it.

The line unit, originally expected to be 8' 6' (304.8 cm) high, was finally made 7' 9' (236.2 cm) high and 4' 6' (137.2 cm) wide. Relay set racks of the same height but in two widths, 4' 6' (137.2 cm) and 2' 9' (83.8 cm), were designed, and the production of racks and shelves for stock was commenced. It was decided not to fit the shelves on the racks until the requirements for each particular P.A.B.X. were known ; this would make for easier storage and quicker delivery.

(a) Intercommunication between Extensions
All connections between local extensions can be completed automatically by dialling. Calling extensions are switched by a uniselector line-finder to a first group selector, which receives the first digit. Subsequent digits operate a final selector and direct the call to the required extension. A 'forced release' condition, operating after a delay of 30 to 60 seconds if dialled pulses are not received, is provided on the group selector, so that line faults do not degrade the service.

The progress of calls is indicated by tones in the following manner:-

  1. Dial tone at a frequency of 20 cycles per second is returned to the caller when the handset is lifted.
  2. When a free extension number is dialled, ringing tone, pulsed 0.4 seconds on/0.4 seconds off/0.4 seconds on, at 2 seconds intervals, is returned to the caller.
  3. When a spare digit or unalloted extension is dialled, number unobtainable tone at a frequency of 400 cycles per second is returned to the caller.
  4. Engaged outlets or spare extension numbers are arranged to extend engaged tone at a frequency of 400 cycles per second, interrupted for 0.8 seconds at 0.8 second intervals, to the calling extension.
  5. Ringing current at a frequency of 20 cycles per second, pulsed similarly to ringing tone, is applied to the bell of a called extension.

Special provision is made for 5 specified extensions in each group of 50 to be used as manual extensions with or without access from the final selector levels.

Extension-to-extension calls are normally released by the first party to replace the receiver, but it can be arranged that only the originating caller controls the release of the call.

(b) Exchange Line Calls
The exchange line circuit permits both-way working to the main exchange, with joint access on the outgoing side from both the manual switchboard and the selector level 9. Through dialling from an extension to the main exchange is possible on calls set up via the manual board, and, on these calls, through clearing and follow-on call trap facilities are provided. Disconnect clearing is given on outgoing calls, and the circuit is arranged to maintain the P.A.B.X. equipment in the engaged condition until the main exchange connection has released.

Incoming calls from the main exchange to the P.A.B.X. light a calling lamp on the manual board, calls then being routed by the operator. On multi position switchboards the calling lamps can be repeated as required over the several positions.

(c) Calls via Inter-switchboard Lines
Several different circuits are available to provide communication with various types of distant switchboards on a manual-to-manual, auto-to-manual or auto-to-auto basis. A discriminating facility can be used when appropriate to prevent inter-switchboard lines from being connected to exchange lines, and free line signals can be given on lines with joint
manual access. The enquiry facility is not provided on inter-switchboard tielines.

(d) Extension to Manual Board Calls
Extensions or incoming tie lines are given access to the manual switchboard when '0' is dialled. The method of operation can be either of the following:-

  1. Lamp per Line Working
    This is the normal provision up to a maximum of 800 lines. Each extension and dialling-in inter switchboard line jack on the switchboard has an associated lamp which glows when '0' is dialled on the particular line.
  2. '0' Level Working
    In this arrangement, made essential by limitations of the multiple when 800 lines are exceeded, '0' level calls are directed to special relay sets, each of which has an associated jack and calling lamp on the manual switchboard.

(e) Enquiry Circuit
An extension engaged on an exchange line call can, by pressing a button on the instrument, dial any other extension over the enquiry circuit. During the enquiry, the exchange line is held and isolated. The extension can re-establish connection with the exchange line by again pressing the instrument button.

Should all enquiry circuits be engaged, operator re-call conditions are automatically set up, assistance being thus made available.

(f) Operator Recall
An extension engaged on a call via the manual board can re-call the operator by pressing a button on the extension instrument. When the call is connected to an exchange line, the button must be pressed twice to set up the operator re-call flash. If the call is by direct access from selector levels, the exchange line calling lamp flashes, instead of the supervisory lamp.

(g) Transfer of Exchange Line Calls
Transfer of these calls can only be effected by recalling the operator by a double operation of the instrument button. Verbal instructions for rerouting the call can then be given.

(h) Night Service
The manual switchboard can be fitted with additional jacks which enable selected extensions to be left connected to exchange lines by means of cords after normal business hours.

Alternatively a night service switchboard, intended for operation by a night watchman or similar person, can be provided. The switchboard has 4 connecting circuits and is used in conjunction with a key on the main switchboard. This key is operated after normal hours to divert four exchange lines and four '0' level circuits to the night switchboard, so that calls over these circuits can be connected by the night operator.

(j) Switching
The linefinders connecting the group selectors to calling extensions are non-homing type uniselectors. Each linefinder is permanently wired to a particular 100-outlet group selector, the circuit of which is of orthodox type.

A special start circuit determines the sequence in which the selectors are taken into use, and the same circuit initiates the hunting of the linefinder. If the calling line is not found within two seconds, the start condition is transferred to a subsequent group selector and linefinder.

Normal post springs, operating on level 9, are fitted to provide for the return of number unobtainable tone to any barred extension attempting to make a direct call to the main exchange.

The standard final selector has 100 outlets ; its circuit is conventional but includes an arrangement for changing the normal connections for 'first party release' to 'calling party release' by a simple wiring modification on the shelf jack.

Should group hunting final selectors be required, they can be fitted in place of the normal type without modification to the mounting arrangements.

(k) Ringing and Tone Generation
Tones and ringing current are generated in relay sets which incorporate vibrating relays working in conjunction with suitable inductors and transformers. The circuit does not operate continuously; it is brought into use by a start signal. An alarm, extended to the manual switchboard, is given if the ringing current fails. A relay set associated with the ringing and tones relay set, and accommodating interacting relays and a uniselector, supplies the necessary pulses.

Continuous ringing current for the manual switchboard is generated separately in a special relay set which also houses the alarm equipment. Routine test facilities are provided on the ringing, tone and pulse circuits, and all feeds are wired via connection strips at the rear of the shelf for ease of identification and distribution of the various services.

(l) Numbering Scheme
For exchanges of 500 lines, or less, a 3-digit numbering scheme is used. If more than 500 lines are required, second selectors are introduced and a 4-digit numbering scheme is adopted. The allocation of first selector levels is as follows:-

Level 1 Spare
Levels 2 to 8 Extension and inter-switchboard lines via second selectors if necessary.
Level 9 Access to main exchange.
Level 0 P.A.B.X. operator (assistance calls).

The major items of equipment are the main distribution frame, line unit, group selector unit when required, relay set rack, manual switchboard and power plant.

The British Post Office supplies the power plant for exchanges under its jurisdiction.

The main frame accommodates line fuses, protectors incorporating electrodes, heat coils and test springs, and I.D.F. type connection strips if necessary; I.D.F. facilities are, however, rarely warranted, because the inter-unit connections are quite flexible, furthermore, facilities for any jumpering likely to be needed are provided on the units. For the smaller exchanges a single-sided wall type M.D.F. usually suffices. Otherwise the double-sided type is used. In either case the frame is of standard construction.

Fig. 3
Typical 100-Line Auto Unit
Fig. 4
Rear View of Auto Unit with Cables in Position

A front view of a typical partially equipped line unit is shown in Fig. 3. That the fullest possible use is made of the available space without sacrifice of neatness and accessibility for maintenance is made clear in Fig. 4, which is a rear view of line units with cables in position.

The two lower shelves are occupied by the jack-in type group selectors. Above these are two shelves, each carrying line relays and linefinders for 50 lines. The uppermost shelves accommodate final selectors. In Fig. 3 an enquiry selector is in the tenth position on shelf B, but when the full complement of group selectors is required the enquiry selectors are mounted at the end of the top shelf.

Units which are partially equipped are fully wired, to simplify the installation of additional equipment.

Grading facilities are provided at the rear of the group selector shelves, and the bank outlets are connected to the succeeding rank of selectors by jumpers which can be easily re-routed whenever required. Inter unit tie cables are run horizontally across the adjoining uprights of the respective units.

Fig. 5
Typical Relay Set Racks

Second group selectors, when required, are not associated with any particular line group and may therefore conveniently be separate from the line unit. The group selector rack is of standard construction and of the same dimensions as the line unit. It will accommodate six shelves of ten selectors, which may comprise incoming first selectors, second selectors or enquiry selectors, according to requirements.

In Fig. 5 are shown two relay set racks, each 33' (83.8 cm) wide, equipped with jack-in relay sets for exchange line, inter-switchboard line, long line, lamp lighting and miscellaneous circuits. The particular requirements for each exchange determine the number and type of circuits to be accommodated, and, consequently, the type of rack - wide or narrow - supplied.

The manual switchboard section (Fig. 2)has two panels and is approximately 56' (142.2 cm) high, 26' (66 cm) wide and 33.5' (8.25 cm) deep. When multipling is necessary, 4-panel repetition is usually arranged, any 30' angle sections in the suite having a single multiple panel.

All extensions have a combined call and answer jack; when lamp-per-line working is adopted, the calling lamp can have only one appearance, but '0' level, exchange and inter-switchboard lines may have ancillary 6-volt lamp appearances. Provision is made for free line signalling when desired on exchange and inter-switchboard circuits.

It has been stated that the sections are similar to those of P.M.B.X. No. 1A, but there is this slight difference, that the P.A.B.X. No. 3 section has capacity for 15 cord circuits.

The Company's choice of the self contained line unit design for P.A.B.X. No. 3 was based on the recognition of the definite quantitative relationship between the line relay groups, the linefinders, group selectors and final selectors. The adoption of this design has effected economies in floor space, in the amount of cable required and, therefore, in the volume of installation work, while the relatively small size of the units facilitates handling and transport.

Visit a P.A.B.X. No. 3


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Last revised: October 14, 2019