PABX No. 7

Extension users guide

Descriptive Leaflet - 1972

Descriptive Leaflet - 1980

Introduced in 1969

3 Internal
ISSUE 4 - DEC 1974

Private Automatic Branch Exchange No. 7
(P.A.B.X. No. 7)


This instruction describes the facilities offered by, and the equipment used for the PABX 7.  The planning of a PABX 7 installation is dealt with in F1071 and installation instructions are given in F1072.

The switching equipment consists of 4000 type two-motion selectors and Type 2 Uniselectors.  The trunking arrangements, which are shown on Diagram SA 8540, are essentially similar to those of the PABX 1.  The equipment is accommodated in two cabinets, known as Equipment's, PABX 7/1 .... and 7/2.... having capacities of 9 + 50 and 11 + 50 respectively these cannot be extended. The Equipment, PABX 7/1 .... which includes the ringing and tone supplies can itself form a complete PABX but generally the PABX 7 will be supplied only to those subscribers requiring both units.  Both equipment cabinets have doors back and front for engineering access.  Power is supplied by secondary cells float charged by a rectifier from the mains supply, this is of the type described in Q0070.  The layout of the equipment is shown on Drawings CD 2289 and CD 2290. Relay- sets of types not shown on the drawings are fitted in any of the positions allocated to miscellaneous circuits shelf jacks and the necessary wiring is provided on site when required.  A single desk-mounted cordless switchboard, known as Switchboard SA 8551 (see picture) is provided with a handset as standard, but a lightweight headset (Trimphone style) can be supplied as an alternative.  The normal capacity of the PABX is reduced by one exchange line and one automatic extension line for each auto-auto inter-PBX line or manual extension provided and by one exchange line for each manual-manual inter-PBX line provided.

PABX No. 7 Switchboard

The PABX 7 numbering scheme is as follows:-

0 - Assistance
9 - Direct access to exchange lines
8 - Night service
7 -
6 - Direct access to inter PBX routes
5 -
4 - Spare
3 - Spare
200 - 299 Auto extensions
1 - Spare

The following general facilities are provided as standard and a more detailed description is given in the relevant diagram notes.

Exchange lines Service to the public exchange is given over bothway lines.  The PABX calling condition is an earth on the B wire.  Enquiry and transfer facilities are available on all exchange calls.  Exchange line relay-sets are suitable for connection to CB and automatic public exchanges.  To operate to a CBS public exchange a Unit, Auxiliary Apparatus, CBS 536 must be fitted at the public exchange in each PABX exchange line, and Relay-sets SA 8541 at the PABX modified as detailed in F1071, par 23.  Requests for connection to other types of exchange should be referred to THQ/TD7.2.2.

Extension lines Extension line circuits are contained in jack-in relay-sets with ten circuits per base.  Individual extensions may be barred direct access to the public exchange or completely barred from connection to the exchange.  The normal extension signalling limit may be increased by the provision of long line equipment to Diagram SA 8104.  Manual extensions with individual appearances on the switchboard may be provided.  Incoming service to manual extensions is automatic and under night service conditions automatic outgoing service is provided.

Inter-PBX lines
Terminations for the following methods for signalling are available and are fitted in place of exchange line relay-sets:-

Auto-auto loop dialing (Relay-set SA 8550).

Manual-manual generator AC or BALANCED BATTERY bothway signalling (Relay set SA 8568)
Provision is not made for the extension of incoming exchange calls to the above inter-PBX circuits.  The operator is unable to extend dialed-in 0-level calls which are answered via the assistance circuit and connect extensions to auto-auto circuits.  Enquiry and transfer facilities are available on manual-manual but not auto-auto calls.

Until such time as full facility SCDC circuits are developed an auto-auto/SCDC converter can be provided.  Auto-Auto traffic and facility limitations apply see P1062.

Auto-auto or auto-manual IVF Signalling (SSAC]3) (Relay-sets 1A1/SA 10009)
It is necessary to provide Equipment Signalling No. 24/... when SSAC13 signalling is used.

Auxiliary inter-PBX lines
Auxiliary inter-PBX circuits using Relaysets SA 8109 may be associated with any extension line.  This facility is only available where the distant installation is suitable for earth signalling and dialing.  Incoming exchange line calls may be extended to auxiliary inter-PBX circuits.

Inter-PBX traffic limitations
Connecting circuits are held for the duration of dialed-in inter-PBX calls and calls from extensions to auxiliary inter-PBX circuits connected to the extension multiple.  As inter-PBX circuits usually carry heavier traffic than an extension line the total number of auto-auto and auxiliary inter-PBX circuits should normally be restricted to two per unit. This number may, however, be increased when it is known that the inter-PBX circuits will not be heavily loaded.  The GM (Traffic Divn) should be consulted when necessary.  This does not apply to IVF SSAC13 circuits.

Night service
Provision is made for level 8 night service at all installations.  DC calling devices should be fitted in locations to meet the subscriber's requirements and will normally be Bells No. 56C.  Other types of trembler bell may be used, if required, subject to the current in the circuit from the PABX equipment not exceeding 300 ma with all bell armatures held un-operated.  If bells requiring a higher current or mains-driven bells are requested they should be connected as detailed in M0060.

Direct extension night service, and designated extension night service to one extension only, may also be provided when required. These forms of night service may be associated with any extension line.  Designated extension night service may be provided in addition to level 8 night service.  Both methods allow normal extension facilities during night service.

Article from
Post Office Telecommunications Journal

Autumn 1968
By H. F. Edwards

The Post Oflice is introducing a new private automatic branch exchange - to be known as the PABX 7 - which is intended for rental by customers who need something larger than the largest PABX 1 (capacity of 10 exchange lines and 49 telephone extensions), but are not likely to need more than 20 exchange lines and 100 extensions.

Hitherto, these customers have had to buy from a contractor one or other of the approved large-type PABXs 3 or 4 which start with a 50-extension capacity but can be enlarged in stages up to several thousand extensions.

Although highly adaptable, these large systems tend to be expensive (in terms of cost per extension) in the smaller sizes and manufacturers have produced cheaper PABXs, for sale abroad, by applying the simple “marker” switching system, already used for the PABX 1, to a 100-extension capacity design.

The Post Office has adopted a design by Standard Telephones and Cables Ltd., has arranged with the company for a few modifications to be made and placed an initial order for 200 to be supplied during 1969 and 1970.

Although the PABX 7 will be suitable for the majority of customers, it has some facility limitations at present, for example, incoming exchange calls cannot be connected over inter-PBX extensions to other PBXs.  Later versions will have the full range of facilities.

There has for long been a demand from customers for a larger PABX available on rented terms, particularly from those whose businesses have outgrown the capacity of 49 extensions of the No. 1.  Consequently, almost all the first 200 PABX No. 7s are already reserved and there will be no difficulty obtaining renters for as many more as can be obtained in the next few years.  More-over, there are so many thousands of PMBX's (manually-operated branch exchanges) of this capacity still in use that an annual demand for some hundreds of No. 7s can be expected.

The PABX 7 utilises equipment of the “Strowger” type already widely used in PABXs Nos. 1-6, but more economically.  The large PABXs 3 and 4 use the rather expensive two-motion selectors for all calls made from extensions and the PABX 4 uses them also for incoming calls to extensions.

The PABX 7 (like the PABX 1), apart from momentary seizure and release in exchange calls, uses them only for extension-to-extension calls and calls dialed in from other PBXs. “Dial 9 for exchange” calls are made and incoming calls via the operator are connected by the cheaper single-motion selectors (uniselectors) used as linefinders in a marker system.

The reason for the limitation to 49 extensions of the No. 1 design was that the capacity of an ordinary uniselector is 50 outlets and one outlet had to be used for another purpose, leaving 49 for extensions. In the No. 7, two of these uniselectors are used, calls being switched by one or the other, thus giving a capacity for connection to too extensions.

On a PABX 1, extensions have two-digit numbers, in the range 21-69, and the remaining first digits are used as follows: 7 - Direct lines to another PBX; 8 - “Dial-8” night service; 9 - Public exchange; 0 - PABX operator; (1 is not used).

If the PABX 7 had used only two-digit numbers there would have been none left for the above services so a three-digit numbering system is used in which all extensions have the first digit 2 and the extension range is therefore 200-299.
The remaining first digits are used for: 7 - Direct lines to another PBX; 3, 4, 5 and 6 - Direct lines to additional PBXs; 8 - “Dial-8” night service; 9 - Public exchange; 0 - PABX operator; (1 is not used).

The PABX 7’s facilities are similar to those of the PABX I but the No. 7 has the advantage, for some users, that it can be connected over privately-rented direct lines to more than one other PBX (for extension calls only on the present design).

The capacity has so far been quoted, for simplicity, as 20 exchange lines plus 100 extensions, without any mention of the capacity for inter-PBX lines.  In fact, the figure of 20 represents the total capacity for outside lines whether to exchange or to other PBXs. Inter-PBX lines with dialling facilities also eat into the capacity for 100 extensions.  For example, a PABX 7 with three such lines would have its capacity reduced to 17 exchange lines and 97 extensions.

The design of the PABX 7 is interesting in that the first of the two cabinets accommodating the equipment comprises a  self-contained PABX of 9 outside lines and 50 extensions capacity - almost the same as that of the largest PABX 1.

It therefore offers an attractive alternative to the PABX 1 for future use.  Whereas an exhausted PABX 1 cannot be extended and must be replaced by a PABX 7 or other larger PABX, an exhausted PABX 7 first unit can be extended by adding a second unit to bring its capacity up to 20 + 100.

It is possible, therefore, that eventually PABX 7 first units may take the place of 10 + 49 PABX 1's.  For the first few years, however, while the supply is limited, all PABX 7s will be allocated to customers who need to use both units immediately.

The physical dimensions of each of the two cabinets of the PABX 7 are much the same as those of the single cabinet of the 10 + 49 PABX 1.

The first unit is actually a little smaller and lighter but the second unit is somewhat wider than the PABX 1, although no heavier. It may be possible, on redesign, to reduce the width and so minimise difficulty in handling through doorways and so on.

The PABX keyboard, about the size of a type-writer, pale grey and green and angular in the modern style, is entirely  push-button operated, except for the dial used for making outgoing calls.

The calling lamps and labels are enclosed in the push-buttons so that the keyboard is similar to the new one being introduced for the PABX 1.  Since the two boards will then be practically identical in operation, no fresh training problems should arise
when an operator goes from one to the other.

An early version of the PABX 7 has been in use since March, 1967, serving the Brighton Head Post Office.  A few others of  an early pattern are now being installed to meet the pressing needs of subscribers who have been waiting some time for them.

The first of the main order are expected to become available early in 1969.

Additional Pictures

All taken in 1970 with the switchboard pictures from the Swedish Embassy (look like an early design)







BACK Home page BT/GPO Telephones Search the Site Glossary of Telecom Terminology Quick Find All Telephone Systems

Last revised: January 18, 2022