PABX No. 1+4/1

TYPE 1+4/1

The Private Automatic Branch Exchange type 1+4/1 has been designed for small factories, offices, clubs, large residences, etc., where the small number of telephones in use does not justify the installation of a private branch exchange and the expense of a switchboard operator.

This PABX provides fully automatic telephone service for all internal calls and also calls to and from the public exchange which may be CB, manual or automatic, Incoming calls from the public exchange operate a common calling bell. A telephone selected for the purpose of answering is called an attendant extension. Calls are answered from the attendant extension by lifting the handset and pressing the button. An exchange call (incoming or outgoing) is transferred from an attendant extension to another internal extension by pressing the button and dialling the required number.
Instruments used with this PABX may be wall or desk automatic telephones with push button.


  1. Automatic, secret, intercommunications.
  2. Dialling of a digit in internal calls.
  3. A maximum of one local call and one exchange call in progress simultaneously.
  4. Extensions may make calls via the public exchange network. This facility can be barred to any extension except number 4 which always has full facilities.
  5. Incoming public exchange calls are signalled on a common bell, and any extension may answer. The answering facility can be barred to any extension except number 4.
  6. Extensions may make an enquiry call to another extension whilst holding an exchange line call, if equipped with a push button telephone.
  7. If the local link is busy when a transfer or enquiry call is attempted, the users of the local link hear a warning tone.
  8. Loop resistance (local) 300 ohms Loop resistance (exchange line) 400 ohms.
  9. Night service:
    At night or any other time when it is desirable to leave the exchange line switched through to one extension, the test key is operated, i.e. pushed inwards. Extension No. 4 is then directly connected to the public exchange for incoming and outgoing calls.
  10. Mains fail:
    In the event of a failure of the current supply to the PABX, the exchange line is automatically connected to extension No. 4 Immediately a failure becomes apparent the Test Key should be operated. This is to ensure that a call which is in progress between extension No. 4 and the main exchange is not interrupted when the current supply becomes normal.
    When the current is restored the key may be restored, having first ascertained that no exchange call is in progress.

The PABX is housed in a metal box with the following dimensions:-
Height - 38 cm (15 in.)
Width - 24 cm (9,5 in.)
Depth - 23 cm (9 in.)
Weight (approx.) - 16.5 Kg (36.5lb)
The rear framework is fastened to the wall with Rawlbolts or coach screws and battens, at a suitable height for working on the equipment.

The normal working voltage or the PABX is 24 volts, d.c. No batteries are required as the standard equipment is arranged to take current from a battery eliminator operated directly from alternating current supply. The typical characteristics of the eliminator are as follows:-
Input: 110 or 220V a.c. 50 Hz single phase.
Output: 24V d.c. 1 Amp.

To install a PABX type 1 + 4/1 the following, equipment and material are required:-

  1. 1 PABX type 1+4/1
  2. 1 rectifier (see paragraph 4).
  3. 4 desk or wall automatic telephones fitted with a push button.
  4. 1 extension bell with a capacitor (Publication 304/1)
  5. cable network, including:
    a) 2 wires for the public exchange line
    b) 2 wires and 1 common wire for each extension.

Taken from sales literature 300/1 by Plessey Automatica Electrica Portuguesa (SARL)

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Last revised: October 09, 2019