plessey1.jpg (6468 bytes)PLESSEY
PAX 10 & PAX 15

10 and 15 line
PAX Units

Designed with emphasis on low cost and simplicity, these newly developed 10 and 15 line private automatic exchanges provide secret communication between extensions, with intrusion facilities available to selected extensions.

Both exchanges are ideally suited for use in small manufacturing units, depots, warehouses, offices, small businesses, and similar establishments where they can function adequately as the sole means of internal communication.

Alternatively, either can be gainfully employed as an auxiliary system in larger organisations to relieve the internal traffic load on a busy private branch exchange, so enabling the PBX operator to handle the important external calls with greater ease and efficiency.

High lights general to both designs:

  • Powered by integral transformer/rectifier-types available cover all normal mains voltages and frequencies.
  • Mains derived ringing and tones-eliminating need for expensive conventional equipment.
  • Low fault liability-robust and reliable components.
  • Easy maintenance-straight forward circuit design.
  • Wall mounting-small space requirements.
  • Total enclosure.
  • High-grade finish.

10 line PAX

This unit, equipped with one connecting circuit, provides secret communication between any two of up to 10 extensions at any one time. Calls are made by dialling single-digit codes, the extensions being numbered 1 to 0. One extension may have an intrusion facility enabling the user to intervene in an established connection. This is accomplished by momentary operation of the press button provided on the particular telephone, and signalled by an intrusion tone to line. The engaged parties clear down on request, and the wanted extension is then dialled.

pax10.jpg (28596 bytes) pax10x.jpg (16431 bytes)
PAX 10 cover off PAX 10 cover on

15 line PAX
Two connecting circuits permit two simultaneous secret conversations between any two pairs from up to 15 extensions.  Calls are made by dialling one or two-digit codes, extensions being numbered 1 to 9 and 01 to 06. Up to three extensions may be provided with intrusion facilities.

When a dialled extension is engaged, busy tone is transmitted.  If the caller has the intrusion facility, he can break into the engaged connection by dialling digit 1.  When the engaged extension users replace their handsets on request, the wanted extension is re-called automatically.

pax15.jpg (24800 bytes)

PAX 15 cover off

Equipment Features
The exchange equipment's are housed in mat elephant-grey all metal cabinets fitted with quick-release covers.

The switching apparatus is of British Post Office standard type and consists of strip relays Type 12 (common yoke), major relays Type 3000, and two types of single-motion selector; miniature (10 line unit) and heavy-duty type (15 line unit).

The apparatus is mounted on a hinged-gate framework, giving convenient access to rear wiring and the back plate miscellaneous equipment.  This includes the line terminal block and ringing-and-tone equipment, comprising mains transformer (with input, output and d.c. fusing), rectifier, and capacitor/resistor network.

The back plate incorporates wall mounting brackets and protected cable entry holes for the passage of line cables and mains supply leads.  Connection to the mains supply can be permanent or by flexible lead and plug.

Wiring is p.v.c. insulated throughout.

Ringing and Tones
Dial, busy and intrusion tones are mains derived via a 28V transformer tapping and capacitor/resistor network. Ringing is at
mains frequency and transformer tapped at 56V (nominal).

The working voltage of both exchange units is 45 to 55V d.c. (50V nominal), derived from the standard built-in transformer which is designed for connection to 50Hz mains supplies within the range of 200 to 250V.  Alternatively, transformers for operation on 60Hz, or within the range of 100 to 150V, can be supplied.

Extension Telephones
Two-wire Plesseyphones specifically developed for PAX working are recommended for general extension use.  For the extension with the intrusion facility on the 10 line unit, a 3-wire version with press button is available.  Both types are supplied in silver-grey, lava green, hemp beige, ivory and black.  Alternatively, other types of standard automatic telephones with and without press button can be used, provided each incorporates an a.c. ringer and has a dial adjusted to transmit 10 pps with a 2:1 break/make ratio.

Line Limits
The loop resistance of extension lines can be up to 1000 for the 10 line unit; for the 15 line unit 550 ohms maximum is permissible.  Using 6 lb/mile (0.5mm) cable, these values correspond respectively to distances between the PAX and telephone of approximately 625yd (593m) and 3585yd (3262m).

10-line unit

12in x 8.75 in x 7.25 in (305 x 221 x 186mm).

15-line unit
12in x 24in x 8.25 in (305 x 610 x 209mm).

One 10-line unit 14 lb (6.4 kg) net.
4 units (multiple export pack)
112 lb (50.8 kg).

One15-line unit 48 lb(21.8 kg) net.
4 units (multiple export pack)
260 lb (117.9 kg).

Equipment item   Code No.
10-line exchange unit   N24451A
15-line exchange unit   N24608A
2-wire telephone   N2020C
3-wire telephone with press button (10-line PAX only)   N2021C

Taken from the Ericsson catalogue, edition No. 49

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Last revised: October 10, 2019