|PRIVATE AUTOMATIC BRANCH EXCHANGES
5 + 20 PABX
with or without attendant
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:-
- Exchange lines
- Tie-lines or a second group of exchange lines
- Extension - to - Extension Calls are dialled direct.
- 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).
- 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.
- Barred Access. Extensions can be partially or completely barred exchange facilities.
- 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.
- Exchange Calls can be repeatedly transferred by extensions to others who are permitted
to receive such calls.
- Trunk Offering. A designated extension can enter established
- Right-of-way. Preference on local calls can be given to executives.
- 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.
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
eliminator mounts at the bottom of the PABX unit. Alternatively, a 1A charger and 10Ah
battery may be used.
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.
||Height in (mm)
||Width in (mm)
||Depth in (mm)
|Attendant's cordless switchboard
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.