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 plan.

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. insulated.

Exchange and inter-PBX line circuits can be arranged to meet all normal signalling conditions.

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 .

Numbering
3-digit, 4-digit or mixed 3- and 4-digit.

Level allocations:
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.

Main Facilities
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). Alternatively, calls may be set up by the PABX operator. Incoming exchange calls are received at the switchboard.
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 provided.

Switchboards

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 misplaced hand-set.

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).

Night-service switchboard
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.

Power Equipment
The working voltage is normally 50V. For loads over 100A a duplicate battery float system is used.

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 CCITT standards.

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 required.

Inter-PBX line working can be arranged for manual/manual, auto/manual or auto/auto circuits.
Assistance is provided by the PABX operator, for whom extensions dial the required access digit.
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:

1. Routing of incoming exchange calls to predetermined night extensions.
2. 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.

Equipment

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.