PAX 109


Working on Strowger principles, the type 109 Private Automatic Exchange (PAX) is designed for large organizations whose requirements range from 100 extension lines upwards.

One of the characteristics of this PAX is its capability of being extended to an unlimited number of lines by the addition of racks equipped with 50 or 100 extension lines. The number of simultaneous calls depends on the amount of switching equipment installed.

The components i.e. selectors, uniselectors, relays, etc. are identical to those used in Strowger type public exchanges manufactured by P.A.E.P.

This PAX works with desk or wall type automatic telephones, a description of which is given in the corresponding catalogue.

The extension line equipment, linefinders and selector hunters are mounted on the combined racks in two distinct groups. Each group consists of 50 line circuits, 10 linefinders, 10 selector hunters and associated control circuits and 9 or 10 final selectors.
The ringing current and tones relay set is also located on this rack.
The group selectors are mounted on racks having a capacity for 60 selectors on 6 shelves of 10 each.
A ringing current and tones relay set is supplied for each 200 extension lines.


  1. Automatic and secret communications.
  2. Calls between extensions effected by dialling, two or more digits depending on the capacity of the PAX.
  3. Dial, ring, busy, warning, and N. U. tones.
  4. Number of simultaneous conversations to meet traffic requirements.
  5. Priority facility for any extension line by dialling the digit '0'.
  6. If, during a priority call, the called extension is busy on another call, warning tone is injected into the conversation.
  7. Forced release of the equipment when an extension fails to replace the handset at the end of a call.
  8. Direct access via junction relay sets to other PAX or PBX.
  9. Two wire extension lines.
  10. All the equipment is protected with suitable devices and visual and audible alarms.
  11. The equipment is designed to work within the following limits:-
    Voltage variations:
    46 to 52 V.D.C.
    Dial Speed: 9 to 11 Hz.
    Maximum extension line loop resistance: 750 ohms
    Minimum extension line insulation resistance: 50, 000 ohms.


  1. Staff locating systems (combination of lamps-visual or by Morse code-acoustic).
  2. Watchman's clock service.
  3. Fire alarm service.
  4. Direct key calling, with or without conference (maximum of 20 and 10 lines respectively).
  5. Loudspeaking telephone.
  6. Dictation recording via telephone.
  7. Secretarial service.
  8. Party lines.
  9. Final selectors with group (PBX) hunting facility up to 10 lines.

The equipment is mounted on grey finish racks 2.286m (7'6') high, 1.371m (4'6') wide and 0.393m (1'3') deep.
The majority of the equipment is of the 'jack-in' type. This method of mounting permits the easy removal of the equipment for maintenance purposes and the extension of equipment without additional wiring.
Two-motion 100 outlet group and final selectors of the BPO type 2000 are used.
The uniselectors are of the heavy duty BPO No. 2 type: BPO type 600 relays are used in the extension line circuits, BPO type 3.000, thermal and high speed relays are used in the other circuits.
All selectors and relay sets are fitted with covers as a protection against dust.

The power equipment normally comprises a 50V battery paralleled across the output of an appropriate automatic float battery charger.

The battery should have sufficient capacity to operate the PAX for 8 to 10 consecutive hours in the event of a mains supply or charger failure occurring.

A high/low volt alarm circuit can be supplied which operates when the maximum or minimum voltages are exceeded.

A main distribution frame of adequate capacity is supplied fitted with appropriate term mating equipment, depending on the customer's requirements.

This PAX, even after it has been in service, can easily be converted into a PABX (type 211) by simply installing the additional equipment required (exchange line relay sets, attendants position etc.).


Taken from sales literature 211/1 by Plessey Automatica Electrica Portuguesa (SARL)

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Last revised: October 10, 2019