PABX 10+50 (No. N22845)

10 + 50 PABX

pabx1050.jpg (22571 bytes)This is a single-unit equipment having capacity for 10 exchange lines, 50 extension lines and 7 connecting circuits. A desk-type cordless switchboard is supplied for dealing with incoming exchange calls, and assistance calls. The stock unit, equipped for 4 exchange lines, 30 extension lines and 4 connecting circuits, is fully wired to facilitate extension. Up to 5 tie lines to other private installations can be provided. The total of exchange and tie lines must not exceed 10. Both are normally arranged for auto or C.B. working. Maximum extension-line loop resistance including the telephone is 800 ohms. Exchange and tie-line loop resistance may be up to 1,000 ohms depending upon the limits imposed by the remote exchange equipment.

Numbering and Facilities

Numbering Extensions are numbered 20 to 69. Single-digit dialling codes 7, 8, 9 and 0 are used for (a) exchange lines (b) tie lines or a second group of exchange lines (c) assistance (d) night service answering.


  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). Alternatively, calls may be set up by the PABX operator. The connecting circuits are used only for the setting up of a call.
  3. Incoming Exchange Calls are received at the switchboard. Digit keys are used for extending the calls 'Ring when free' facilities are provided.
  4. Barred Access. Extensions can be partially or completely barred exchange facilities.
  5. Enquiry Calls. An exchange call can be held by the PABX operator or an extension while making an internal enquiry call. The internal conversation is secret from the exchange subscriber.
  6. Call Transfer. Exchange calls can be repeatedly transferred, without operator assistance, by extensions to other extensions permitted to receive such calls.
  7. Trunk Offering. The PABX operator can enter established connections; warning tone is given.
  8. Right-of-way. Preference on local calls can be given to executives.
  9. Assistance. Extensions dial a single digit to call the PABX operator.
  10. Manual Extensions. Immediate calling of the PABX operator by lifting the handset can be arranged for up to three of the extensions.
  11. Night Service. Incoming exchange calls are signalled by d.c. bells. Any extension not full barred can answer by dialling 8. Alternatively, one or two extensions may be selected to answer calls.

Optional Service Features

Message waiting; message registration; conference; trunk barring; extension busy' lamps; direct access key calling and staff location.


This is a double-sided, pressed steel dustproof unit with lift-off steel-framed Perspex doors at the front and rear.  The exterior is enamelled opaline green; the interior and apparatus covers are cream. The equipment has tropical finish. Connecting wires are p.v.c. insulated.

Components are of standard BPO type and proven reliability. They include N30000 (BPO 3000) and N45000 (BPO 600) relays, heavy- duty line-finder uniselectors (BPO No. 2) and 100-outlet 2-motion selectors N46600 (BPO 4000). If specially ordered, selectors N43200 (BPO 2000) can be supplied. Excepting strip-mounted line and miscellaneous relays, all circuits are on jack-in mountings to facilitate extension.

On one side of the unit are line relays, exchange and tie-line relay sets and fuse panels; on the other, the finder uniselectors, 2-motion selectors and connection strips. Connection strips ('line' and 'exchange') are mounted under a cover at the side of the unit and serve for the distribution of incoming lines, cross-connection being made by means of screw terminated jumpers. Line protectors are provided complete with heat coils and fuses on a separate wall mounting for 10 circuits when ordered.

The 25Hz and 400Hz ringing and tone frequencies are generated by transistor-type oscillators powered from the 50v exchange supply. These static devices require little maintenance and do not cause interference with radio signals.

Cordless Switchboard
n24125a1.jpg (10913 bytes)The design of the small two-tone grey switchboard accords with contemporary taste and facilitates maintenance. The metal front panel and internal rear equipment frame are both hinged to the metal base so that they can be opened, book fashion, when the ABS copolymer plastic drop-on cover is removed. 

Keys have wedge handles with tips of contrasting colour to enable operated keys to be easily discerned. Fifty lamps, arranged in two rows, are 'extension busy' lamps, fitted only when specified.  The upper row of double-throw keys and associated lamps are for exchange-line, tie-line, manual extension, assistance and miscellaneous circuits. The lower row of keys, again of double-throw type, serve as key-sender digit keys when operated downwards, and provide for exchange and extension release, flash, test, speak, etc., facilities when operated upwards.

Lamps can be removed without using an extractor.  The dial has standard numbering, 2:1 break/make pulse ratio, and a plastic fingerplate.  A grey plug-in handset with capsule receiver and grey braided extensible cord is provided, and 6Oft(18.29m) of p.v.c. insulated cable to connect the switchboard to the PABX unit.

Power Equipment
The working voltage is 45 to 55V (50V nominal). If a reliable a.c. mains supply is available, a battery eliminator N22405D may be used. It is designed for inputs of 100 to 125V or 200 to 250V, 40 to 100 H z, has a 7.5A output and mounts on a wail. Alternatively, a 3A charger and 20Ah battery may be used.

Plesseyphones (catalogue sheet 7159) or Etelphones (catalogue sheet 7145) are recommended for extension telephones. However, any telephone suitable for 2-wire and earth or 3-wire working, may be used, provided it is equipped with a non-locking press button and incorporates a dial with a 2:1 break/make ratio and pulsing speed of 10 pps.

Weights (approx.)

Equipment Item   Net Weight lb (kg)
PABX equipped 4+30   750 (340.9)
PABX fully equipped 10+50   992 (450.9)
Battery eliminator N22405D   57 (25.9)
Attendant's cabinet with busy lamps N24125A1T   40 (18.2)
Attendant's cabinet without busy lamps N24125A2T   38 (17.2)

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Last revised: October 07, 2019