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The ultimate number of lines required for large, expanding establishments cannot always be accurately forecast. This P.A.X. is ideal for such establishments, as there is virtually no limit to its extension. We recommend its use where ultimates of 300 or more lines are envisaged. The minimum equipment is 100 lines.

The most economical and usual system employs uniselector type linefinders with two-motion group and final selectors mounted together on Line Units, but a system similar to that used for public exchanges working on a uniselector-per-line basis can be supplied if specified. Typical illustrations of both types of equipment are shown.

Equipment features are described in general terms, as they are varied in detail to provide the most economical arrangement for the particular exchange.

The maximum extension line loop resistance including the telephone is 1000 ohms, but long line equipment to extend this limit can be supplied.

A 3-digit or mixed 3- and 4-digit numbering scheme is used, depending upon the number of lines required.

Full intercommunication by direct dialling, with complete secrecy except on party lines is given.

Any of the special facilities described for P.A.X. 50/400 can be provided, although not necessarily in precisely the same way. Should other facilities be required, we shall be pleased to make recommendations.

Standard types of apparatus as used in public exchanges are employed. They include heavy duty uniselectors (B.P.O. No. 2), 100-outlet two-motion selectors (B.P.O. 4000), line relays N45000 (B.P.O. 600), and relays N30000 (B.P.O. 3000) for other circuits. For systems based on uniselector-per-line working, the line circuit switches are light-duty uniselectors (B.P.O. No. 3).

Racks are of open type but can be enclosed in dust-proof covers if specially ordered.

Switching equipment is normally provided on a basis of 1 lost call in 200 at the busy hour.

The general finish for racks and apparatus covers is cream enamel. Connecting wires are p.v.c. insulated, and cables have p.v.c. insulated conductors and a cream p.v.c. sheath.

The arrangements generally conform to standard public exchange practice, separate racks being provided for uniselectors and selectors. The racks incorporate grading facilities. A combined main and intermediate distribution frame (either single or double sided) accommodates such protective devices as may be required for tie lines, and I.D.F. type terminal strips for cross-connections. Separate racks and/or wall mountings are provided as necessary for tie line and miscellaneous circuits. Racks are usually 8 ft. 6.5 in. (259 cm) high and 4 ft. 6 in. (137 cm) wide.

The equipment is composed of standard Line Units each accommodating the line and switching apparatus for two independent 50-line groups. Ten uniselector type linefinders are provided per 50-line group. Cross-connecting facilities are included on the unit. Transistor type ringing and tone generator circuits are usually provided. They are on two jack-in mountings at the bottom of the first Line Unit. On the second and subsequent units these two shelf positions can be occupied by tie line or other relay sets, capacity for 10 of which is provided at the rear of each unit.

Systems requiring a 4-digit numbering scheme, i.e. with second group selectors, have separate second selector racks each with capacity for 60 circuits.

Line Units and racks are 7 ft. 9 in. (236 em) high and 4 ft. 6 in. (137 cm) wide.

The distribution frame, equipped with connection strips, protectors and fuses as required, may be of single-sided wall/floor type or double-sided floor mounting type. The exchange side of the frame illustrated has 4 protector mountings and 4 connection strips per vertical, but in most cases the capacity is 3 of each (or 6 connection strips if protectors are not required).

Facility for extension is a feature of the system by virtue of the self-contained nature of the Line Unit, which reduces the amount of cabling work to be done at the exchange. Increased traffic demands also can quickly be met by jacking-in extra selectors.

The working voltage is 46-52V (50V nominal).

We usually recommend a single battery with charging rectifier operating on the automatic full float principle, which enables the plant to be left unattended for long periods. Orders should specify the a.c. mains voltage and frequency.

The standard provision is a line test case for fitment near the M.D.F. and a routine test set for applying functional tests to jack-in equipment, connection being by plug and jack. This test set is in a wooden cabinet mounted on top of a Uniselector Racks, Test Desk and Ringer Panel in a large P.A.X. mobile metal cabinet. employing uniselector-per-line working.

For ordinary extensions, Etelphones N1900/N1065 without push-button, or N1901/N1066 with push-button are usually recommended.

A special telephone N23868A for executives is available.

Because of the many possible variations, no general guide to the accommodation required can be given. The best possible layout from the practical and economic aspects is devised for each exchange. A clear height of 15 - 18 inches (38.1 - 45.7 cm) above the racks is necessary to allow for overhead cables.

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Last revised: October 10, 2019