PAX 50/100

Automatic Switchboards
PAX 50/100 Type

This Private Automatic Exchange PAX ) is designed for organizations requiring between 20 to 100 telephones. It can be supplied initially as a 50 line unit ( which can be only partially equipped ) and later on can be extended to 100 extension lines by the addition of a second 50 line unit.

Each unit, when partially equipped can be extended line by line up to its maximum capacity.

Similarly each unit can be equipped with up to a maximum of 7 simultaneous conversation circuits, which can be installed one by one, when the unit is partially equipped. This corresponds to a maximum of 14 speech circuits for 100 lines.

The exclusive use of relays and. uniselectors in this exchange ensures that maintenance is kept to a minimum and is extremely easy.  This PAX works with desk and wall automatic telephones, descriptions of which are given in the respective catalogue.

By fitting additional equipment, various optional services can be made available, such as tie lines, code calling, watchman's service, fire alarm, etc.

Numbering Scheme
Extension Lines

1st - Unit - 10 to 59
PBX groups commence with numbers 12, 22, 32, 42 and 52

2nd - Unit - 60 to 99 and 00 to 09
PBX groups commence with numbers 62, 72, 82, 92 and 02
Numbering of lines in a PBX group need not be consecutive.

Single or two digit numbers within the numbering range of the uni~ With single digit numbering, 10 extension lines are forfeited for each tie line group and with two digit numbering, one extension line for each individual tie line.


  1. All conversations are secret.
  2. Seven simultaneous calls maximum per unit.
  3. Priority facility for any extension line.
  4. Two wire extension lines.
  5. Maximum extension line loop resistance, including that of the telephone is 1 ,000 ohms.
  6. Dial, ring, warning and busy tones.
  7. Protective fuses in the various circuits.
  8. Minimum extension line insulation resistance is 50,000 ohms.
  9. Forced release of the register, after a pre - determined time interval, if a fault occurs on a tine or when an extension fails to start or complete the dialling of a call.


  1. Key calling with or without conference facility (maximum 10 or 20 lines respectively).
  2. Secretarial service.
  3. Tie lines to other private installations.
  4. Staff-locating (Code call) system.
  5. Loudspeaking telephones.
  6. Fire alarm system.
  7. Watchman's clock service.
  8. Automatic hunting over a PBX group of extension lines or a group of tie lines by dialling one or two digits. Any number at lines (in the same unit) can be included in a group and the numbers need not be consecutive.

PAX Units
Each 50 line unit is enclosed in an enamelled pressed-steel dust proofing cabinet which stands freely on the floor, no wall supports being necessary. Hinged doors at both the front and rear give complete access to the equipment. The dimensions
of each unit are:-
Height - 1,793 m (5' - 10.5')
Width - 0.890 m (2' - 11')
Depth - 0.325 m (1' - 0.7')
The weight of each unit when fully equipped is approximately 185 Kg (408 Lbs). The equipment is tropically finished and connecting wires are PVC insulated.. The principal components include the well proved BPO No. 2 type, heavy duty uniselectors, used as linefinders and connectors, and BPO 3000 and 600 type relays.

Each 50 line unit includes two registers, each one containing two relay magnetic counters. A transistorised ringing and tones generator is also included in each unit and these static devices, requiring little maintenance do not cause radio interference.

The nominal working voltage of the PAX is 50V D.C. which can be obtained from a battery eliminator (rectifier) incorporated in the cabinet and suitably spaced from the switching equipment.  Alternatively an automatic float charger connected in parallel with a battery can be used in those instances where an uninterrupted supply is essential.

Characteristics of typical power equipments are as follows:-

Battery eliminator (rectifier)
Input: 110 or 220 - A.C. 5OHz
Output: 50V D. C. 3A (with a second unit 6A.)

Supply obtained from a lead-acid battery and charger.
Battery: 50V, 24 cells, capacity 30AH at the 10 hour rate (with a second unit 48 AH).
Charger: Input : 110 or 220V - A.C. 50 Hz.
Output: 50V. D.C. 5A. (with a second unit - 8 A).

Supply obtained from an alkaline battery and charger.
Battery : 50 volt, 38 cells, capacity 30AH at the 5 hour rate (with a second unit - 46 AH).
Charger : Input : 110 or 220V - A.C. 50 Hz.
Output : 50V D. C. - 5A.( with a second unit 8A.)
The capacities indicated are the normal ones for feeding an exchange equipped up to its maximum capacity (50/7 and 100/14).

Desk and wall type telephones of PAEP manufacture, as illustrated in the respective catalogue sheets, are usually recommended, but any automatic telephone with a dial pulsing at 10 ' 1 impulses per second with a 2:1 break to make ratio may be used.

Taken from sales literature 202/1 by Plessey Automatica Electrica Portuguesa (SARL)

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Last revised: April 18, 2022