with Automatic or C.B. exchange lines
This private manual branch exchange switchboard, with capacity for four exchange
lines, 18 extensions, and seven connecting circuits, has been designed in accordance with
modern needs. Compared with earlier types of switchboard, it offers the attractive
combination of improved facilities and compact design, both features having been obtained
largely through the use of miniature keys and 4 wire extension lines. The switchboard
operates from a 50v supply and its physical design features are similar to the 2+6 and
3+12 types in the cordless PMBX range.
Up to half the extension line circuits can be converted to 2-wire working by the use of
wall mounted auxiliary relay units of which there are five types for:
- Long extension lines.
- Short- and medium-distance private wires, tie lines and PBX junctions.
- Long-distance private wires, tie lines and PBX junctions.
- Medium-distance lines to PABX.
- Long-distance lines to PABX.
Construction and Finish
The switchboard is in two-tone grey. The lift-off cover is a one piece moulding made of
impact resisting light-grey plastic, whilst the plastic faced metal equipment panel and
the metal base are elephant grey. The front and rear equipment mountings hinged on
the base can be opened book fashion for maintenance, as shown in the illustration
overleaf. The switchboard is fully tropicalised and uses p.v.c. insulated wire throughout.
Keys are miniature type N9300, with ivory-coloured wedge handles tipped with inserts of
contrasting colours to assist ac- curate operating.
The lamps are behind a lens strip held by clips at the ends. When this strip is removed,
the lamp-jack assembly can be pulled forward after releasing its fixing screws.
Relays are Types N45000 (BPO 600) and N30000 (BPO 3000), protected by strip covers.
An internal terminal field permits direct termination of incoming lines. Alternatively,
for plug-in switchboard connection, a 6ft (1.83m) flexible cord with matching plug can be
supplied, together with an associated line jack.
At installations where no source of power ringing exists, a wall mounted ringing
converter, operating from 200-250V, 50Hz a.c. mains, can be supplied. Alternatively, an
inbuilt or wall-mounted transistor ringing unit can be supplied.
The separate two-tone grey table type Etelphone provided for the switchboard operator, can
be arranged for use in conjunction with an operator's lightweight headset, if required.
- Lamp signalling on all lines.
- Individual clearing signals on extension-to-extension calls.
- Automatic holding of exchange calls.
- Follow-on call trap on incoming exchange calls
- Press button operator recall on extension and exchange calls.
- Free connector signalling.
- 'Overcall' to enable calls to be answered when all connectors are engaged.
- Night service.
- Power unit operation with mains failure safeguards.
- Extension-to-exchange line limit of 935 ohms, or 850 ohms if 4-wire/2-wire auxiliary
unit is used.
- Connection of private circuits and inter-switchboard circuits without modification to
the permanent wiring of the switchboard.
The required 50V d.c. supply is usually derived from the a.c. mains via a battery
Full load current drain: 2A without auxiliary unit, 4A with auxiliary unit.
These are Plesseyphones (catalogue sheet 7159) or Etelphones (catalogue sheet 7145)
arranged for use with the switchboard 4 wire lines. For auto-working, dials a readjusted
to transmit 10 p/s with 2:1 break/make pulse ratio, and have standard numbering (i.e. 1 to
Height: 9 in (229mm)
Width: 23 in (587mm)
Depth: 15 in (387mm)
651b (29.5kg) net
Plessey Publication No. 7130 (1970)
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Click here for the GPO Switchboard No. 2/4