PABX No. 25/50

TYPE 25/50

The 25/50 PABX is suitable for use in a medium sized organization as it has an initial capacity of 24 extensions and it can be extended to cater for 49 extensions. The PABX can be operated from the mains supply via an eliminator or battery and charger.
If the equipment is powered by an eliminator provision is made to automatically switch the exchange lines to predetermined extensions in the event of mains supply failure.

This equipment can be connected to Public Exchanges of Automatic, Central Battery and Local Battery operation. In the case of Magneto Exchanges it is necessary to provide a small group of auxiliary relays. The "C Unit" of the PABX provides all the necessary equipment for 25 Extension Lines (one for call back purposes) 4 Local Links and 4 exchange lines together with equipment for attendants cabinet, Ringing and tones, pulse supplies, etc. By the addition of a further unit ("A unit") the capacity of the PABX can be increased to a maximum of 49 extensions 8 local links and 8 or 10 exchange lines.

The equipment can be supplied with a small distribution frame or case for termination of extensions and internal wiring.
Telephones can also be supplied: these are fitted with a dial and a push button for automatic recall and transfer facilities.
An attendant's cabinet is also provided to answer incoming exchange line calls and to direct them to the required extension.

The PABX can be extended from an initial unit of 24 extension lines, up to a maximum of 49. It comprises a basic "C" unit with all the necessary equipment for complete service with 24 extension lines, 4 local links and 4 exchange lines.  A second unit "A" is available and this extends the capacity of the board to 49 extension lines, 8 local links and 8 or 10 exchange lines.

The PABX operates from 50V DC supply, usually obtained from an eliminator, which can be connected to the AC mains supply.


  1. Secrecy of communications.
  2. Extensions can dial each other, in accordance with the following 2 digit numbering scheme:-
    "C" unit - 20-43
    "A" unit - 45-69
  3. Simultaneous communications:-
    "C" unit - 4 links and 4 exchange lines.
    "C" & "A" unit - 8 links and 8 or 10 exchange lines.
  4. All extensions have access to the public exchange by dialling "0".
  5. The above facility may, however, be barred from any desired extension by the addition of a strap.
  6. Incoming exchange calls are answered and extended by the attendant. In the absence of the attendant (night service) selected extensions receive incoming exchange call answer the call, and transfer it to the required extension.
  7. An exchange call can be transferred by one extension to a second, and by the second to a third, and so on, by using a push button, which is mounted on the telephone instrument.
  8. Any extension can hold a public exchange connection whilst making an internal call to another extension and return to the original exchange connection on completion of the internal enquiry call.
  9. Priority can be given to any extension.
  10. The attendant (or a transferring extension) can offer an exchange call to an engaged extension. Warning tone is given to the engaged extension while the third party is connected to the line.
  11. The equipment works satisfactorily between the following limits:-
    Operating voltage - 46 to 52V D.C.
    Extension line loop resistance, maximum 800 ohms.
    Extension line insulation, minimum 50,000 ohms.
  12. The exchange lines are connected direct to the public exchange.  When connected to Magneto type public exchanges, it is necessary to install groups of auxiliary relays.
  13. Each extension requires 2 wires to the equipment, and 1 common earth wire.
  14. If the equipment derives its power through an eliminator provision is made to automatically switch each exchange line to a predetermined extension should the mains supply fail.


  1. This PABX uses BPO 600 type relays (line relays), BPO 3000 type relays, BPO No. 2 type uniselectors, 2000 type selectors together with other components (resistors, condensers, etc.).
  2. Attendant's cabinet - Capacity for up to 10 exchange lines.


  1. The equipment is mounted on a rack and enclosed in a Dust Proof Metal Cabinet. It is fully protected from dust and moisture by independent metal covers fitted over individual pieces of equipment.
  2. The miscellaneous relay groups comprising Ring and Tone Generator, Call Back and Transfer, Level 9 and Attendant's Cabinet relays are mounted at the rear of the "C" Unit.
  3. Rack dimensions:-
    Height - 6ft.10.25in (2.08m)
    Width - 2ft. 1in. (0.634m)
    Depth  - 1ft. 9in. (0.533m)
  4. Attendant's Cabinet dimensions:-
    9.5in. x 1 ft. 4.5in x 9 in. (0.23m x 0.34m x 0.2m)
    "C" Unit weight - 420lb (190.5Kg)"
    "A" Unit weight - 392 lb (178Kg)
    Attendant's cabinet weight - 27lb (12,25Kg)

The PABX 25/50 operates from 50 d.c. obtained from a battery eliminator or from a battery and charger. The equipment characteristics are the following:-

  1. Battery Eliminator
    Input : 110/220 V a.c., 50 Hz
    Output: 50 V d.c., 3A
  2. Battery and Charger
    a) Lead Battery 50 V. 22 AH in 10 hours
        Battery charger: Input : 110/220V a.c. 50 Hz
        Output: 50V d.c., 3 A
    b) Alkaline Battery 50 V, 22 AH in 5 hours
        Battery charger: Input : 110/220 V a.c. 50 Hz.
        Output: 50 V d.c., 5 A
    If the mains is d.c. the equipment will be fed from duplicate secondary batteries and charging panel.

The PABX cabinet is fixed to the floor and the complete installation comprises the following equipment:-

  1. One rack "C" unit with 24 extension lines, or 2 racks "C" and "A" units with 49 extension lines.
  2. One Attendant's Cabinet.
  3. 4 Exchange line relay sets if using 1 rack; 8 or 10 Exchange line relay sets if using 2 racks.
  4. 4 line finders and final selectors if using 1 rack; 8 line finders and final selectors if using 2 racks.
  5. 49 (maximum) wall or table telephones with button for priority and transfer.
  6. Distribution of Cables per installation:-
    a) 2 wires per exchange line
    b) 2 wires +1 common wire per each extension.
    c) Electric cable with 3 wires between PABX and eliminator. Electric cable with 3 wires between eliminator
        and the Mains.
    d) Group of Auxiliary relays (if the Public exchange is Magneto).
    e) A small M.D.F. or distribution case suitable for wall mounting.
    f) Cables between PABX and the M.D.F.

Taken from sales literature 304/1 by Plessey Automatica Electrica Portuguesa (SARL)

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Last revised: October 09, 2019