|2 + 10 House Exchange
This system provides ten stations with intercommunication facilities and
access to two external circuits, which may be automatic or C.B. public exchange lines or extension
lines of PBX systems. It employs press button intercommunication and exchange-line access,
and ideally suited to organisations with an anticipated high calling rate between
Direct press button intercommunication.
Up to five simultaneous conversations.
Direct press button access to exchange lines.
Lamp indication of exchange-line conditions.
Secrecy on exchange- line calls.
Enquiry calls (exchange line held).
Operator recall on lines to PBXS.
Exchange bell on/off.
Conference calls from any station to all or selected stations.
Continued exchange-line service if power fails.
Barred or restricted exchange-line access.
Station telephones are connected in order of priority by a maximum of 400yd (365m) of
cable. The first station on the multiple has priority over all other stations in making
and answering exchange-line calls; when in use, it disconnects all other stations from the
line. Similarly, the second station disconnects stations 3 to 10 and so on. Cabling
between stations is direct except where a station is inconveniently situated. A spur cable
is then used to minimise the length of the main cable; this does not affect the order of
These instruments have been approved by the Council of Industrial design and are available
in black, grey and ivory. For convenience of installation and maintenance each telephone
cord plug connects to an associated desk/junction box. Both the instrument cover and
handset are moulded from Acrylonitrile Butadiene styrene (ABS), a material noted for its
high gloss, light fastness, and impact strength. The telephone interior, which is built on
a pressed-steel base, includes a d.c. buzzer and an a.c. ringer, together with
printed-wiring panels, carrying:-
- transmission components, including plug-in line regulator.
- intercom-signalling contact springs. These engage with the printed wiring on
depression of an intercom button and require no tensioning as necessary with conventional springsets.
These total 15 on the press button panel; ten for intercom calls, two for exchange lines
(labelled 1 and 2), two for exchange line release (labelled R) and one for conference
(labelled C). Provision is made above the panel for four inline buttons for bell on/ off
and other ancillary operations such as applying restricted exchange line access to a
Slide-in reversible paper labels numbered 1 to 5 and 6 to 10 and protected by plastic
windows are provided for the intercom keys. Space is left for writing the names of persons
or departments on the labels.
Two pairs of lamps ('exchange' and 'seize') serve for exchange line visual signalling and
supervision. The seize lamp gives a steady signal at the originating station on seizure of
the exchange line, whereas the exchange lamp gives the following three signal
indications at all stations:-
- A pulsating signal at ringing periodicity until an exchange call is answered.
- A steady glow during an established exchange call.
- A 'wink' when a station holds an exchange line while making a call to another line.
This signal becomes steady when the user returns to the held line.
This houses screw terminals for the main cable and a 56-way connector for acceptance of
the plug ended telephone cord. Where a station is connected via a spur, the spur cable is
connected to the main cable at an auxiliary junction box similar to the desk/junction box.
Exchange-line Relay Unit
This unit accommodates relays for exchange-line switching and lamp signalling, together
with a transistor device for regulating the current supply for the signalling lamps.
Components mount on a raised platform, hinged to the base to facilitate access to
the relay wiring. Cable entry is via rubber grommets in the base.
Installation and Cabling
Systems to be connected to exchanges under the jurisdiction of the British Post Office (or
similar administrations overseas) are supplied and installed by the administration.
Systems not in this category can be customer installed.
A mains-operated power unit is normally provided, giving a smoothed regulated 50V supply
of up to 2.7A.
Note: the call lamps are fed via a transistor current-regulating device
which ensures long lamp life.
Equipment Code Numbers
Plessey Publication No. 7132 (1971)