No. 44 PAGE No. 10

K. J. CLARKE - P.M.B.X. Equipment Section, Engineering Department
January 1962

The article describes, with particular emphasis on equipment detail, a new range of indicator and lamp-signalling floor-pattern private branch exchanges of most attractive design. These p.b.x’s. are suitable for use as single position or multiple switchboards in magneto, c.b. and auto systems and each accommodates a maximum of 100 extension lines, 20 exchange lines and 17 cord circuits. Operation is from a 24-volt d.c. supply. A novel building-block arrangement for the main equipment components allows a basic switchboard to be adapted with minor modification to suit differing capacity and circuit requirements, thus permitting switchboards identical in size and general appearance to be provided with features incorporated to suit customers’ specific needs. Chief among the other benefits to be obtained from these switchboards are good telephone service, reliability and simplicity of handling.

The design of floor-type private branch exchanges has followed a conventional pattern for nearly a quarter of a century and although attempts have been made from time to time to
eliminate unnecessary detail and create more pleasing lines, their angular form has remained substantially unchanged. Similarly, the material used for the main structure of the switchboards has been timber of various types, generally stained and polished but occasionally faced with a durable plastic. Modern design in the wider spheres of industry has however given impetus to a desire for floor-pattern p.b.x’s. in pleasing colours and of more attractive shape and practical construction.

To meet this need a new range of switchboards identical in basic design has been developed. The complete series consists of one magneto switchboard, three c.b.11auto types with indicator-signalling facilities and one c.b./auto lamp-signalling version. A concise summary of the standard service features provided by each is presented in Table 1.

The low silhouette of the new switchboards represents a pleasing departure from previous wood-constructed types. The desirable characteristic of low height, fundamental to good design and of particular importance from the operator’s
point of view, has been obtained without sacrifice of extension or exchange-line capacity by adoption of standard overall height, width and depth measurements of 4’ 0” x 2’ 6“ x 2’ 8” (122 x 77 x 81 cms.). A well-balanced combination of aesthetic and practical design has been obtained by sloping the face equipment backward and the plug-shelf downward to present an angle of 100° between the two. This feature, clearly seen in the switchboard illustrated on the preceding page, not only enhances the switchboard’s modern lines but, in addition, provides improved visibility and consequently greater ease and efficiency of operation.

Summary of the Standard Facilities provided by each type of Switchboard

Colour and finish complete a distinctive appearance. The casing, composed of removable panels, is in ‘buff-linette’ (a light brown colour with a faint grain pattern), while the keyshelf, mushroom-tinted and edged in brown, contrasts with the black plug-shelf and face equipment framing. The panels, formed in laminated wood, are faced in a melamine plastic providing a smooth and easy-to-clean exterior.

Figure 1
Rear view of typical Switchboard with cover removed

Covered apertures for cable entry are located in the base and, since this is of open construction, an alternative means for cable access is afforded from below. To enable the p.b.x. to be easily manoeuvred in confined spaces prior to its installation, for example through narrow doorways and corridors, it is arranged to reduce the overall depth of the switchboard to the convenient dimension of 1’ 11.75” (59 cms.). This is achieved simply by raising the hinged key-shelf, removing the kicking panel and withdrawing four screws within the switchboard to allow the apron to be pulled forward and lowered.

A feature of considerable importance is that the p.b.x. is not restricted to a particular circuit - an inherent drawback of earlier types of switchboards. With minor modifications it can be adapted to accommodate any standard or special circuit for auto, c.b. or magneto working. The switchboard’s range of utility is further extended since it is equally suitable as a single position or multiple p.b.x.

This method of case construction provides a light yet robust covering and offers the additional advantage of allowing the standard colour scheme to be changed to suit particular tastes and decor.

The elimination of wood as the main constructional material meant that an alternative method of forming a rigid framework had to be devised. This problem was solved by making a welded pressed-steel skeleton, with an integral plinth to give added strength and complete stability. Wood screws from inside the framework secure the top and keyshelf-apron panels, while the side panels are held in place by neat metal trims in anodised bronze. The front bottom panel, or ‘kicking’ panel, simply lifts out and the back panel, giving access to relay equipment in the rear of the board, is locked by two captive screws.
The flexibility of the new board stems from the equipment arrangement. As shown in Figure 1, all the rear equipment with the exception of the fuse panel is carried on a single metal frame bolted firmly to the switchboard’s framework. Space is available within the frame for a joint total of 140 line,
cord-circuit and miscellaneous relays, all of B.P.O. 3000-type; six terminal blocks for apparatus interconnection and external line terminations, and 17 cord fasteners. The face equipment is similarly mounted and accommodates lamps or indicators and associated jacks for 100 extension lines, indicators and jacks for 20 exchange lines and, in addition, a 100-line multiple-jack field for use when the switchboard is included in a suite. This unit-frame construction enables the basic board to be stocked and units to be readily added to suit a wide range of circuit and capacity requirements.

In the magneto version, manually restored indicators of the twin-shutter type are used for extension and exchange-line signalling, but for supervision, the totally enclosed disc type is employed. On the other hand, c.b. auto switchboards utilize fully automatic indicators throughout or, alternatively, a combination of automatic and manually-restored indicators, all of twin-shutter type1.

Figure 2
General view of CB/Auto Indicator-Signalling Switchboard

When manually restored indicators are included on c.b./auto switchboards as shown in Figure 2, these are employed on exchange lines only and arranged in the lower section of the left-hand panel below one group of extension-line indicators. This extension-line group and its corresponding one in the right-hand panel are composed of strips of 10 indicators mounted to align with associated central-panel jacks below the space provided for the multiple-jack field. Each strip of extension-line indicators is protected against dust and incidental damage by a clear plastic shield while manually restored exchange-line indicators are similarly but individually guarded.

Easy identification of calling lines is ensured by clearly inscribed stile-strip legends and printed numerals on the indicator-protection shields. In addition, differentiating colours are used for extension-jack labels and each group of indicators. As an indicator operates in the left-hand group the twin shutters swing open horizontally to display the extension’s designation against a background of pink, while in the other group an operated indicator shows the calling extension’s number surrounded in green. Since the jack labels are coloured to coincide with their respective indicator labels, improved ‘speed of answer’ is assured.

All areas of the switchboard facing the operator are within easy reach. Keys for night service and miscellaneous use, such as ‘night alarm’ ‘fuse alarm’ and ‘battery cut-off’, are located at the top of the left-hand panel, while in the upper section of the corresponding outer panel, miscellaneous indicators for ‘fuse alarm’ ‘ring indication’ and ‘dial guard’ etc. are mounted.

Below the face equipment, on the plug-shelf, are the switchboard cords. Provision is made for up to 17 pairs, and these are accommodated in removable cord seats of long-wearing nylon pressed into the plug-shelf panel. Keys and supervisory indicators associated with the cords are arranged in double-row units mounted within metal frames in the adjoining key-shelf. Here, space is also available for the latest ‘trigger’ dial. This, when incorporated in the switchboard, is offset towards the operator and all fixing details are concealed by a pleasantly contoured plastic shroud.

The supervisory indicators, viewed through clear-plastic windows raised above the key-shelf surface, are of twin-shutter pattern, except in the magneto switchboard where, as previously mentioned, disc-type indicators are employed.

Figure 3
General view of CB/Auto Lamp-Signalling Switchboard

The keys are of the well-proved miniature type 2 used extensively in our B.P.O. approved cordless switchboard range. Their smooth action and specially designed ivory-coloured handles contribute to a main objective - simplicity of operation. The key handles are wedge shaped, offering a better grip than old-type lever keys, and coloured inserts in their tips serve as ‘flags’ to indicate at a glance the position of the keys.

In the front apron and conveniently to hand is the rotating-magnet generator. This is mounted directly opposite the twin jack serving the operator’s plug-in handset which rests, when not in use, on the Cradle at the side of the switchboard apron. The handset, similar to that used on the Etelphone instrument, is light in weight to prevent fatigue and equipped with a retractable cord, allowing the operator complete freedom of movement.

Although the rotating-magnet generator is provided as standard, space is available in the switchboard apron for a transistor ringing unit, supplying a 25-cycle ringing current with power sufficient for the simultaneous ringing of 10 bells. This unit may be utilized to supplement or replace the hand generator; alternatively, any existing means of power ringing can be used.

Because of the space saved by the use of lamps in the c.b./auto lamp-signalling board (Figure 3) the face equipment occupies only two panels, adequate room remaining for directory storage on the left hand side. Despite the advantage of space conservation gained by the. use of lamps, these are not employed throughout for all signalling purposes. Lamps are used for extension working and, together with their associated jacks, are arranged in both panels. But exchange lines are equipped with indicators, as may be seen in the lower section of the left-hand panel. These are the standard combined indicator-jack type which automatically restore when the exchange call is answered. To suit particular requirements, these indicators may be replaced by a similar type which is manually restored.

For exchange line signalling, mechanically-locking indicators are preferred to lamps, since they are independent of the switchboard’s power supply. Thus exchange calls can be signalled and answered even in the event of power failure. If however the switchboard power supply can be regarded as reasonably dependable, lamp signalling can be incorporated in the exchange lines in place of indicators.

The layout of the switchboard’s relay equipment, keyboard and ancillary items is, in general, as previously described for the indicator-signalling board. Similarly, all relays are of B.P.O. 3000-type and these, together with all other components, are designed to withstand wide extremes of temperatures. Plastic-covered wire is used throughout and maximum protection against dust is given to vital components.

When Etelphone-type instruments are employed for extensions, the permissible loop resistance between an extension and the exchange is 1,000 ohms.

All c.b./auto switchboards are designed for a supply voltage of 24V but will operate satisfactorily within the wide range 18 to 28 volts. The c.b./auto switchboards when equipped to full capacity each have a busy-hour current consumption of 2A.


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Last revised: August 14, 2003