PABX 5+20 (No. N22407)


5 + 20 PABX
with or without attendant

pabx5.jpg (18545 bytes)This is a single-unit equipment for use where the ultimate requirement will not exceed 5 exchange lines and 20 extension lines. The stock equipment's are for 5 + 20 lines with 4 connecting circuits, and 3 + 10 lines with 2 connecting circuits. Partially equipped units are wired for 20 extension-line circuits.

Tie lines to other private installations can be provided. The total of exchange and tie lines must not exceed 5. Both are normally arranged for automatic or C.B. working. A manual switchboard is not necessary (see Facility 3) but a small switchboard equipped for   intercepting incoming exchange calls while others are being extended can be provided for a designated extension. A typical switchboard is illustrated below.

Maximum extension-line loop resistance, including the telephone, is 800 ohms Exchange and tie-line loop resistance may be up to 1000ohms, depending upon the limits imposed by the remote exchange equipment.
Numbering and Facilities

Extensions are numbered 20 to 29 and 30 to 39. Single-digit dialling codes 8, 9 and 0 are used for:-

  1. Exchange lines
  2. Tie-lines or a second group of exchange lines

n24104a.jpg (16708 bytes)Facilities

  1. Extension - to - Extension Calls are dialled direct.
  2. Outgoing Exchange Calls are made by dialling an access digit, then the exchange subscriber's number (or instructing the distant operator if the exchange is manual).
  3. Incoming Exchange Calls are signalled to designated extensions by a bell or bells. A call on any exchange line can be answered by any designated extension and transferred.
  4. Barred Access. Extensions can be partially or completely barred exchange facilities.
  5. Enquiry Calls. An exchange call can be held while making an inquiry call to another extension. The internal conversation is secret from the exchange subscriber.
  6. Exchange Calls can be repeatedly transferred by extensions to others who are permitted to receive such calls.
  7. Trunk Offering. A designated extension can enter established   extension-to-extension connections.
  8. Right-of-way. Preference on local calls can be given to executives.
  9. Exchange-Line Switching. Exchange lines are automatically switched to selected extensions in the event of power supply failure. Exchange-line conversation is not interrupted when power is restored.

Optional Service Features

Message registration (metering), conference, extension busy lamps, direct-access keycalling, staff location and route barring.


This is a grey-enamelled pressed steel dustproof unit of small size which stands on the floor, and is supported 3in (76mm) from a wall by two brackets at the top. The rear cover is sealed and the front one clip-retained. All the apparatus is accessible from the front.

The equipment has tropical finish. Connecting wires are p.v.c. insulated. Components are as used in public exchanges in Britain and of proven reliability. They include heavy-duty uniselectors used as line-finders and connectors and Types N30000 (BPO 3000) and N45000 (BPO 600) relays.

The switches are on two hinged shelves, which can be swung down to expose the fuse panel and the connection strips on which the incoming cables terminate. The upper shelf is not fitted on the 3+10 stock unit but can easily be installed later. Strip-mounted circuits are used throughout. All are arranged on jack-in plates, excepting line circuits, but the plates for these can also be withdrawn without disturbing the connections. Transistor oscillators are used to generate ringing current and tones. These static devices do not cause radio interference and require little maintenance.

Power Equipment
The working voltage is 45-55V (50V nominal). Power is usually supplied from the mains via a 4.2A eliminator N22404C, the input to which may be 100-125 or 200-250V (40-100HZ). The eliminator mounts at the bottom of the PABX unit. Alternatively, a 1A charger and 10Ah battery may be used.

Extension Telephones
Plesseyphones (catalogue sheet  7159) or Etelphones (catalogue sheet 7145) are recommended for  extension telephones. Any other  type of telephone must have an   earth-connected, non-locking  press button and a dial pulsing at 10 pps with a 2:1 break/make ratio.


Equipment Item   Height in (mm)   Width in (mm)   Depth in (mm)
PABX unit   69 (1753) 30 (762) 14 (356)
Attendant's cordless switchboard   7.5 (198) 19 (505) 13 (346)

PABX stock unit (3+10) N22407D - 370lb (168.2kgs)
including battery eliminator - 86lb (39kgs)
PABX fully equipped unit (5+20) N22407E - 476lb (216.4kgs)
including battery eliminator - 86lb (39kgs)
Battery eliminator N22404C - 58lb (26.3kgs)
Attendant's cabinet N24104A - 43.5lb (19.8kgs)

Plessey Publication No. 7121 (1970)

Click for an article from the Ericsson Bulletin

N22002 was another version which was for unattended sites.

Type Trunks Connectors
N22407AT 10 3
N22407BT 20 5
N22407CT 15 10
N22407DT 10 3
N22407ET 20 5
N22407FT 15 10
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Last revised: October 07, 2019