This BPO approved PABX is for use where the ultimate requirements are likely to exceed 100
extension lines. It is constructed on the self-contained unit principle to minimise cable
costs and facilitate extension. The capacity is virtually unlimited. The switching scheme
employs linefinders of heavy-duty uniselector type in conjunction with 2-motion group and
final selectors. Floor-pattern cord switchboard sections as necessary are provided to deal
primarily with incoming calls from the public exchange and from private exchanges over
inter-PBX lines if required. The system may be used for a single exchange or for a main
and satellite scheme having a central group of manual positions and a common numbering
PABX 3 equipment can be supplied with standard or tropical finish; transparent p.v.c.
enclosures can be fitted to the open type-racks on request. Racks and apparatus covers are
cream enamelled, cables are sheathed in cream p.v.c. and connecting wires are p.v.c.
Exchange and inter-PBX line circuits can be arranged to meet all normal signalling
Maximum extension line loop resistance is 600 ohms excluding the telephone, but long-line
equipment to extend this limit to 2000 ohms can be supplied. Exchange-line loop resistance
may be up to 1000 ohms depending upon the limits imposed by the remote exchange equipment.
Resistance limits for inter-PBX lines vary with the type of signalling employed. For
example, the limit for a loop-disconnect auto/auto circuit terminating at a distant 50V
exchange is 1200 .
3-digit, 4-digit or mixed 3- and 4-digit.
Level 1 -Spare;
Levels 2 to 8-Extensions and inter-PBX linesvia second selectors if necessary;
Levels 9 and 0-Access to public exchange and PABX operator assistance as required.
Extension-to-extension calls are dialled direct.
Outgoing exchange calls are obtained by dialling the access digit, then the subscriber's
number (or instructing the distant operator if the exchange is manual).
calls may be set up by the PABX operator. Incoming exchange calls are received at the
Manual extensions may use auto
The line unit accommodates all the calling and switch equipment normally needed for two
independent groups of 50 extensions.
Grading facilities are provided on the group selector shelves. The unit illustrated has
group selectors on shelves A and B, linefinders, line relays, and resistors for the two
50-line groups on shelves C and D, and final selectors on E and F. Enquiry selectors can
be fitted on shelves A and B (10th position). A separate multiple can befitted on shelf F
(positions 9 and 10) for
incoming or enquiry selectors.
The group selector rack accommodates six shelves of 10 switches, which may include
enquiry, incoming first or second selectors as required. Second selectors are necessary
only for PABXs having a 4-digit numbering scheme.
The relay set rack accommodates jacked-in type relay sets for exchange line,
assistance, inter PBX, ringing, tone and pulse control, and other circuits. A
battery-driven dynamotor for generating the ringing, tone and pulse supplies is mounted at
the bottom of the first rack. Wiring for a second machine with automatic changeover is
A manual switchboard in standard steel-framed, 2-panel sections similar to
that illustrated, is supplied for exchanges up to 1600 lines. Sections can be arranged in
suite with single-panel 30' angle sections and a cable turning section. They have
removable kicking panels and rear doors. Woodwork is normally oak relieved with grey
Warerite facings. The normal arrangement is for a panel multiple. Other arrangements
are possible if more lines are required.
All extensions have a combined call-and-answer jack when lamp-per-line working is adopted,
a calling lamp having only one appearance. '0' level circuits, exchange lines and
inter-PBX lines can have ancillary lamps, for which a 6V a.c. supply is provided.
Free-line signalling can be given for exchange and inter-PBX lines. With lamp-per-line
working, indication is given of any extension giving false calling condition such as
There is capacity for 15 cord circuits per section, the relays for which are on jack-in
mountings; facilities for cord testing are included.
A hand generator, lightweight headset and a chair for the operator are supplied. Jacks can
be provided for connecting selected extensions to the exchange lines at night (see
alternative arrangement below).
This is a cordless switchboard suitable for use where night traffic is heavy. It consists
of an oak cabinet equipped with a forest-green stove-enamel control panel, four connecting
circuits, and a multiway cord with plug and socket to allow easy removal of the
switchboard when not in use.
The working voltage is normally 50V. For loads over 100A a duplicate battery float system
Below 100A loading a single battery float system is normally supplied, but a duplicate
battery arrangement can be provided for greater security or for quick recharging after
supply failure. Equipment is tailored to the requirements of particular exchanges.
With a single battery float system, the capacity of the battery supplied depends upon the
period of reserve required in case of supply failure. This period is normally 24 hours.
The 24-cell battery is 'floated' across the closely regulated output of an automatic
float-charge rectifier unit. The output of the charging unit is sufficient to cover the
busy hour load and thus the battery is maintained in a fully charged condition. Voltage
regulation of the charging unit is by transductor control and the output is smoothed to
One switchboard position with cable turning section circuits up to a total of five in each
group of 50 auto extensions. Manual extensions are accessible by direct dialling from auto
extensions or via the switchboard. Where a large number of manual extensions are required a
separate circuit is available, which does not reduce the number of auto extensions. Barred
access denies public exchange access to particular extensions completely or partially, as
Inter-PBX line working can be arranged for manual/manual, auto/manual or auto/auto
Assistance is provided by the PABX operator, for whom extensions dial the required access
Extension call lamps are provided according to the size of the exchange. Above 800 lines,
or in smaller exchanges if preferred, assistance level relay sets, each with a jack and
lamp appearance, are provided. On smaller exchanges lamp-per-line working is used.
Enquiry calls can be made by an extension engaged on an exchange call. A single depression
of a press button enables the extension to dial and secretly converse with another
extension whilst the exchange line is held. A further depression of the button returns the
original extension to the exchange line.
Operator recall is achieved by double depression of the press button on exchange calls or
single depression on manual inter-PBX calls.
Transfer is effected by the operator.
Trunk offering enables the PABX operator to enter established connections.
Free-line signalling gives automatic lamp indication of free exchange lines.
Night service is available in two versions:
- Routing of incoming exchange calls to predetermined night extensions.
- Receipt of calls at a night service switchboard where each can be routed by the
night-service operator dialling the required extension and connecting it through.
Components are of standard BPO type and proven reliability. They include
Type 3000 and Type 600 relays, Type 2 heavy-duty uniselectors and Type 2000, 100 outlet
The equipment and layout for each exchange vary with its size and particular
requirements. Switch quantities are normally calculated on a busy-hour grade of service of
one lost call in 200, excepting calls to the switchboard.
The main distribution frame may be double-sided floor mounted or single-sided wall and
floor mounted. It accommodates fuses, fuse dummies, test jacks, and also IDF-type
connection strips if necessary.
The apparatus racks are of open type, supported by inter-connecting overhead ironwork
which carries the cable. They are braced to a wall or ceiling. A clear height of 18in
(457mm) is required above the racks for cabling.
Taken from the Plessey Telephone Catalogue 1971