GEC C.B. TELEPHONE SWITCHBOARD
FLOOR PATTERN (WITH INDICATOR SIGNALLING)
CS3320 series (Standard) & CS3420 series (Tropical)
16 - 22V DC
Two sizes - of 50 and 100 extensions - comprise this range of switchboards that
are adaptable to provide internal intercommunication only or to work into any
automatic or C.B. public exchange as well. Wiring is complete for 10
junction lines, which are equipped when specially ordered.
Totally-enclosed self-restoring disc indicators, fitted with transparent
dustproof covers, give negative supervision of all calls. Calling and
answering cords are used to set up calls. The 50-line size is equipped
with 10 cord circuits and wired for 12 the 100-line, equipped with 12 and wired
for 14. Ringing and speaking to wanted extensions is facilitated by
combined RING AND SPEAK keys.
For the operator a telephone headset or handset is provided as required.
The hinged key shelf, and lift-out doors at the front and the back of the
cabinet, provide easy access to all apparatus and wiring.
Features Common to all Central Battery
Each of these switchboards permits its operator to set up calls between its
extensions. Provision is also made on the switchboard for one or more junction
lines to the public exchange. When these junction lines are equipped, the
operator can set up calls between the switchboard extensions and the public
To almost all the switchboards keys are fitted which, in the absence of the
operator, connect the exchange lines directly to any selected extensions. These
extensions may then be used exactly as if by an ordinary subscriber on the
public exchange, provided the instruments are fitted with dials or ringing
generators if the public exchange is automatic or magneto.
All boards are provided with a buzzer which, if switched into circuit, gives
audible indication of incoming calls.
Cat. No. CS5100/5200 series switchboards have a highly polished walnut
veneer cabinet. The other boards described are housed in matt polished hardwood.
All cabinets are sturdy in construction and allow easy access to the wiring and
apparatus they contain.
All switchboards in the range are fully wired for the maximum number of
lines quoted, but can be supplied only partially equipped so that both immediate
needs and future additions are catered for with the greatest economy.
The number of calls that may be passed through the switchboard
simultaneously is determined by the number of cord circuits or connecting
circuits with which the switch board is equipped. This number is quoted in
the specification tables given in this Catalogue for every size in each range of
Power for operating the switchboards may be obtained over power leads from
the main exchange. Alternatively, power may be drawn either from AC mains
via a mains unit or from a local battery, preferably float-charged, which need
have no more than a 10 ampere-hour capacity.
Ringing current is normally drawn from the compact and powerful hand
generator, but on cord switchboards provision is made for the connexion of an
external source of ringing current, such as ringing leads from the main
exchange, a local vibrator, or a ringing machine, to facilitate operation.
The hand generator then acts as a standby.
Lamps or indicators inform the operator when a call is pending, is in
progress, or has been completed. Positive supervision is said to be
provided when the replacing of a handset at the end of a call lights a lamp or
operates an indicator negative supervision, when the replacing of a handset
extinguishes a lamp or restores an indicator.
Switchboards are supplied with an operator's handset or headset telephone as
required. The circuit of the operator's telephone has great transmitting and
receiving efficiency with full sidetone suppression.
G.E.C. switchboard cords are of great durability. The tinsel used in them is
tested to ensure a life of 200,000 reciprocations without conductor fracture.
The switchboards may be supplied with a tropical finish to combat the
effects of tropical climates.
The instruments recommended for extensions are the Gecophone, the G.E.C.
Table Telephone, and the Muraphone, described in Catalogue Leaflets STL 9, 10
and 12 respectively. Extension instruments requiring the facility of dialling
direct into an automatic public exchange, when connected at the switchboard to
the junction lines, should be fitted with dials.
Each extension requires a two-wire connexion to the switchboard. Any
adequately insulated twisted pair, suitably protected if exposed to the risk of
damp or of mechanical damage, may be used, provided the line loop resistance
does not exceed
200 ohms for switchboards without line relays.
600 ohms for switchboards containing line relays.
The following table gives examples of maximum lengths of line wires:-
|SIZE OF COPPER CONDUCTORS
||MAXIMUM PERMISSIBLE LENGTH
The line loop resistance is the resistance of both wires of a pair in series.
DISTRIBUTION, CABLING AND PROTECTION
The simplest method of connecting extension instruments to the switchboard
is to run the line wires of each instrument directly to the terminal strip
within the switchboard. Economy in line wires may often be effected by running a
multi-core cable from the switchboard to a junction-box, so situated that only a
short length of line wires is then needed to connect the instrument to the
A distribution frame often simplifies the distribution scheme, particularly with
a switchboard of large capacity. The wires from the switchboard, which are
usually run in a multi-core cable, are terminated on the distribution frame to
which the extension instruments are then wired. The use of a distribution frame
simplifies alterations to numbering, line testing and the connexion of
additional lines, and also provides protection against any high voltage and
heavy currents that may accidentally be introduced from extraneous local
In installations without distribution frames, line wires run outside the
building should be protected against lightning by protection apparatus that is
supplied if ordered.
Catalogue Numbers and Specification Table
||No. of Lines
| Std. and Trop.
||Exchange (wiring only)
||54 x 21 x 29
||137 x 53 x 74
||54 x 27 x 29
||137 x 69 x 74
Taken from the GEC Publication No. CSL. 8