|PRIVATE MANUAL BRANCH EXCHANGES
with auto exchange lines
This switchboard is arranged for positive supervision on all calls, and has capacity
for 20 exchange and 100 extension lines with 17 cord circuits. Stock units are available
wired and equipped for 10 exchange and 30 or 60 extensions and 12 or 15 cord circuits. The
10+30 unit has wiring for up to 70 extensions, the 10+60 unit has wiring for 100
extensions and for optional multiple facilities.
Both may be extended to full capacity as required. For installations of from 1 00 up to
200 extensions, two units may be bolted together to form a two position, non-multiple
switchboard. When up to the maximum of 400 extensions are required, four units, with
provision for multiple equipment, can be similarly installed. When two or more positions
are installed en suite, each is equipped with longer connecting cords and double pulley
- Through dialling and clearing.
- Separate positive supervision on local calls: double positive supervision on exchange
- Visual ringing indication.
- Ring back.
- Visual and audible fuse alarm.
- Audible alarm.
- Exchange-line prohibition at selected extensions.
- Cord test and hold jack with called party answered supervision.
- Continued exchange line service in the event of local power failure.
- Multiple working with engaged test tone.
- Night-service working on cord circuits 1 to 1 2. Note. Extra plug-ended through-cords to
connect additional circuits for night-service working can be supplied.
A welded, pressed-steel frame, with integral plinth, forms the main structure and
component mounting, and is enclosed by laminated wood panels. These are surfaced with
hard-wearing, buff-coloured plastic, edged and secured by metal trims finished in glossy
elephant grey stove enamel. In contrast, the face panel is coloured black and the key
shelf buff linette. When two or more units are installed ensuite, adjacent intermediate
side panels are removed and the units bolted together. Similarly, both the
front panel below the key shelf, and the rear door, can be removed for convenience of
The face panel has indicators arranged in two groups on either side of the jack strips,
and includes provision for night-extension keys, fuse and night-alarm keys and cord-test
jack. In addition, the jack panel has capacity for up to 100 multiple jacks for use when
switchboards are multipled together.
The key shelf is arranged with supervisory indicators and switching keys for cord circuits
and miscellaneous facilities.
Internal equipment is positioned and mounted for maximum accessibility. Incoming cables
enter via covered apertures in the open constructed plinth, and terminate on soldered
terminal fields positioned at the bottom of the unit.
For convenience of handling (e.g. during installation) the hinged key shelf can be raised
and its apron lowered to reduce the depth by 8.5 in (21 6mm). The weight of the stock unit
varies, depending upon the extension-line capacity.
Switching keys and signalling indicators are of miniature type. The wedge-shaped
ivory-coloured key handles are tipped in contrasting colour, rendering operated keys
instantly discernible. The indicators have twin shutters that restore automatically, and,
when operated, display a black numeral on a back ground of the same colour as the
associated jack label. Operating equipment, supplied as standard, includes a lightweight
plug-in handset with rocking- armature receiver, a manually operated ringing-current
generator and a dial adjusted to transmit 10 pps with 2:1 break/make pulse ratio and with
standard numbering, i.e. 1 to 0.
Alternative equipment, available in lieu of, or in addition to, includes a lightweight
plug-in headset, a transistor ringing-unit powered from a 24V source, and a dial with any
desired numbering and adjustment.
Internal components, including relays and relay retards, are to British Post Office
specification and are afforded tropical finish, i.e. with coils suitably impregnated and
metal parts specially finished. Conductors are insulated with p.v.c., giving protection
against moisture, fire, attack by insects and fungoid growth. Connecting cords are
additionally protected by p.v.c. sleeving and nylon braiding. Additional relay equipment,
for extensions requiring the exchange line prohibition facility, is mounted externally.
The switchboard operates on 24V d.c., the busy-hour current drain being approximately 2A.
Power is normally derived from the local mains supply via a battery eliminator unit.
Alternatively, batteries with charging equipment can be supplied if preferred.
Exchange-line working is not affected by any local power failure.
Using Plesseyphones type N2020 (auto) or N1520 (c.b.), or Etelphones type N1900 (auto) or
N1340 (c.b.) as extension instruments, the maximum permissible loop resistance between
public exchange and extension is 1000 ohms.
Height: 48in (1220mm)
Width: 30in (722mm)
Depth: 32in (813mm)
Between 266lb (121 kg) net and 300lb (136.2kg) net
Plessey Publication No. 7136/1 (1971)
Also in Catalogue 60, Section 2