Extension users guide
Introduced in 1969
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
6 - Direct access to inter PBX routes
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
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
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.
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.
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.
Post Office Telecommunications Journal
By H. F. Edwards
THIS IS THE NEW PABX7
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
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
- 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
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
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
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
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
All taken in 1970 with the switchboard
pictures from the Swedish Embassy (look like an early design)