No. 51 PAGE No. 68

A New Type of Portable Switchboard
K. J. CLARKE - P.M.B.X. Equipment Section, Engineering Department
July 1965

This article describes a portable switchboard of novel design. Composed of separate units it can be quickly assembled and connected on site for use in a variety of situations where telephone service is required at short notice.

Figure 1
fully assembled switchboards

In recent years the holding of conferences in many different parts of the world has been a feature of international and commonwealth affairs. At these times telephone communications, and particularly the provision of additional special lines for delegates and representatives, become an important aspect. To fulfil the conditions demanded in these circumstances, the communications engineers of the Foreign Office asked the Company to consider the development of a switchboard which could be easily dismantled into units of convenient size for despatch by air to any part of the world, to give telecommunication facilities at conferences at short notice.

Figure 2
The two separate portions of a single position, together with a battery eliminator
Figure 3
Rear view of switchboard with panels removed

The design of the switchboard had to meet the following conditions:-

  1. No unit should be wider than 1 ft. 9 in. (53 cm) so that when packed in a case, it could be loaded through narrow aircraft doors.
  2. Although no limit was set on weight, each unit should be as light as possible.
  3. The component units should be easy to assemble with the minimum of interconnection between units.
  4. Circuit facilities should be the same as those incorporated in the switchboards normally supplied to the Foreign Office for use in embassies.
  5. The switchboard should have capacity for 20 lines to the main exchange and 100 extension lines.
  6. The whole switchboard should be robust enough to withstand rough treatment both in use and transit.

It was obvious that the stated number of lines and the circuit requirements could not be accommodated in a one-position switchboard without exceeding the prescribed dimensions. A two-position board was therefore designed, each position normally containing equipment for 10 exchange lines, 50 extension lines and 10 cord circuits, the line equipment being in the upper part, and the cord circuit equipment in the lower part of the board, as shown in Figures 1, 2 and 3. Each position is self-contained and could be used independently where the requirements are within its capacity; see under ‘Flexibility’.

In order to provide rigid and robust units a welded pressed-steel skeleton is utilized, on to which removable plastic-faced panels are fitted. This arrangement not only makes for easy replacement of panels, if damage occurs, but also affords improved access to equipment for maintenance and adjustment (see Figure 3).

Figure 4
Hinged relay mounting frame swung out to give access to internal wiring

With one relay per extension line and two relays per exchange line, it is necessary to mount the line equipment in the top unit in such a way that access can be gained to both the jack and lamp-strip wiring and the relay equipment. This is achieved by mounting the relay bars on a hinged frame secured by two lock-screws. In this way, the relay frame can be swung out to permit access to the interior of the top unit (see Figure 4). On the lower unit a hinged frame is not necessary because access can be obtained to the wiring by removing the bottom panel and moving aside the cords and weights.

Although the external finish is not of major importance, a durable surface is necessary to minimize damage during operation and handling. For this reason, the ironwork is stove-enamelled elephant grey and the front panel, the key and plug shelf, and the side and rear panels are faced in a buff-coloured hard-wearing plastic. The lower front lift-out panel below the keyshelf is also faced at the bottom with a plastic ‘kicking‘ strip.

The facilities incorporated in switchboards normally supplied to the Foreign Office for use in embassies are provided in the Portable Switchboard. They include the connecting and supervising of calls between two extensions or between an extension and the main exchange.

If the operator remains in circuit after setting up a call, or enters the circuit during an established call, her presence is indicated by the application of ’ticker tone’ to the line.

On extension-to-exchange calls the equipment is held under the control of the extension loop to provide through clearing.
Calls to the main exchange are made by loop disconnect pulsing, using a standard dial.

Ringing current can be provided by a ringing machine, hand generator, or transistorised ringing unit.

Audible and visible alarms are provided to indicate when a fuse blows, and coupling facilities are included to enable one operator to deal with calls on two positions.

Each cord circuit includes divided battery feed and is arranged to prevent dialling-out on extension lines or ringing-out on exchange lines.

To facilitate replacement and interchangeability, standard telephone apparatus is used throughout. All relays except those for extension lines are B.P.O. 3000 type. Extension line relays are B.P.O. 600 type. A transistorised ringing generator is fitted in addition to the hand generator and a transistorised ‘ticker tone’ unit provided for the intrusion tone facility. Each section of each position has its own separate fuse panel, thus reducing to three the number of connections between the upper and lower section, i.e. battery, earth and alarm leads.

Figure 5
Plan view of upper section of switchboard showing easy access to terminal block

Connections required between positions are: 7 wires for coupling, 3 for ringing (when an external supply is used), and 2 wires for power. Connection of incoming lines needs to be simple because soldering irons or special tools may not be available. To simplify connection, all the terminals are mounted on a single plastic-faced panel at the top of the upper sections, using screw connections only (see Figure 5). There is ample room for the cable entering at the base of the switchboard to pass vertically through both sections to the terminal panel.

As stated previously, the circuits of the switchboard supplied to the Foreign Office had to conform to those standardized for switchboards installed in embassies. In these circumstances each position has capacity for 10 exchange lines, 50 extension lines and 10 cord circuits, as well as the usual common circuits and 10 auxiliary jacks and lamps which may be used for any special requirements. However, the flexibility of the equipment mounting arrangements makes possible the accommodation of almost any type of circuit, and where this is of simple design, the capacity for extension lines may be increased to 100 per position, although the maximum number of exchange lines and cord circuits remains at 10.

The circuit flexibility, convenience for transport and ease of installation of this switchboard renders it suitable for a variety of applications such as building sites exhibitions, agricultural or similar outdoor shows of a temporary nature. Its use to meet emergency conditions of various kinds can also be envisaged. In such conditions the convenience for transportation by air in containers of the type designed by the Foreign office and shown in Fig. 6 is particularly advantageous.

Figure 6
Packing cases for the two sections of the switchboard and the battery eliminator

The switchboard is designed in its present form to operate from 24 volts d.c. although, of course, circuits operating from 50 volts d.c. could be provided. The most convenient power source in both cases is a mains-operated battery eliminator, but any method of obtaining a 24 or 50 volt d.c. supply could be utilized. The busy-hour load per position will depend upon the circuit employed and in the case of the type supplied to the Foreign Office is 2 amps.

The author is indebted to the communications engineers of the Foreign Office for help in the preparation of the article and for permission to reproduce the photographs.


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