GEC C.B. PMBX

16 - 22V DC

Strong brackets are built into this switchboard, enabling it to be fixed to a wall.  The board has a capacity of 5 lines to any automatic or C.B. main exchange and 20 extensions.  A dial and dialling keys are provided when ordered.  The wiring is in any case provided to allow them to be added easily if the main exchange is converted to automatic working.

Positive supervision is provided on all calls by the extension line indicators, which are totally enclosed and self-restoring.  The junction line indicators are of the hand-restored drop type.  Connexions are made by calling and answering cords.  The normal equipment provides eight cord circuits, but ten cord circuits are wired and can be equipped to special order any pair of lines may be connected by any pair of cords.

The operator's telephone handset is carried on a switch hook at the side of the cabinet.  Keys enable the operator to select and speak over any cord circuit, and by means of RING and RING BACK keys the ringing generator is connected to any wanted line.

The whole front of the switchboard is hinged to swing outwards, giving immediate access to all apparatus and wiring.
 

Features Common to all Central Battery Switchboards

FACILITIES
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 telephone system.

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.

CONSTRUCTION
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.

CAPACITY
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.

SIMULTANEOUS TRAFFIC
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 switchboard.

POWER SUPPLY
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 GENERATOR
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.

SUPERVISION
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.

OPERATOR'S TELEPHONE
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.

CORDS
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.

FINISH
The switchboards may be supplied with a tropical finish to combat the effects of tropical climates.

TELEPHONE INSTRUMENTS
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.

LINE WIRES
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:-

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 junction-box.

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 sources.

PROTECTION
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

Taken from the GEC Publication No. CSL. 8

Last revised: November 25, 2021