No. 42 PAGE No. 32

H. G. LAMBERT and M. V. DUNN, Bach. Eng. (India)
Circuit Development Engineering Department
January 1961

A small telephone exchange has been developed for operation in rural areas where battery charging facilities are not available locally. It has capacity for twenty subscribers’ lines, and three bothway junctions, which may be associated with physical or a combination of both physical and carrier circuits. The exchange is self-contained and the authors describe its facilities and construction, together with the arrangement for charging the local exchange battery over a physical junction.

THE Company’s ‘Rurax’ (Rural Automatic Exchange) equipment meets a variety of particular requirements and adequately fulfils the demand for an efficient 24-hour telephone service in many of the remote areas of the world. A principal factor restricting the introduction of a similar service to the more sparsely populated and isolated rural fringe areas has been the problem of providing a suitable power source for exchange battery charging. In these areas a mains supply is often not available and to provide a locally-generated source would be uneconomic, bearing in mind that these communities are very small and widely dispersed, for example, on plantations, stations or farms.

A satisfactory solution to this problem has been obtained by the development of the Minirax, a small capacity exchange, complete in every detail and requiring no particular site accommodation or local mains supply for its successful operation. By its use, the needs of small and isolated groups are served with the same efficiency as are the needs of Rurax subscribers.

As its name suggests, the Minirax is a miniature rural automatic exchange. It is designed for unattended operation and primarily for use as a small dependent exchange serving twenty subscribers. The exchange provides fully automatic local service and direct bothway working over three junctions to a parent exchange-manual, auto-manual or automatic. Junctions may be physical or a combination of both physical and carrier.

Power is derived from a 24-volt 8 a.h. local battery, trickle charged over a physical junction terminated at the parent exchange. Full use is made of the junction for traffic purposes as charging is only operative when the junction is free from calls.
Special attention has been directed to the provision of equipment with low power consumption and minimum maintenance requirements, thus ensuring trouble-free service in areas where regular visits by skilled personnel are often impracticable.

Fig 1 - Basic Trunking diagram

A 2-digit numbering scheme (20 to 39) is used for local subscribers’ lines. Access to the parent exchange is gained by dialling a single digit (9).

In the trunking arrangement shown in Fig. 1, twenty subscribers are connected to an equal number of line circuits. Each of the four connecting links provided, consists essentially of two single-motion selectors of similar type, one being a 25-point linefinder and the other a 25-point connector switch, arranged to function as a combined group and final selector. Subscribers’ line circuits and junctions are terminated on the linefinder bank contacts and multipled to corresponding bank contacts of the connector switch.

Although a connecting link is normally associated with both local and junction traffic, a particular link can be restricted, by simple adjustment of strapping, to incoming junction calls only. This allows access to the Minirax should all other connecting links be engaged by local subscribers.

Fig 2 - Simplified diagram of Connecting Link


Local Calls
When a caller lifts the handset to initiate a call, the LR relay operates and an earth is applied to all connecting links. A free link is seized and the ‘start’ earth extended to operate the associated SC relay

and cause the seized link to be marked ‘busy.’ The linefinder (LF) then hunts and seizes the calling subscriber’s circuit and dial tone is returned to the caller. If the first digit dialled is ‘2’, the wipers of the connector switch (CS) step to contact ‘2’, and, subsequently, self-drive to contact ‘4’. Similarly, upon the dialling of digit ‘3’, the wipers take a corresponding number of steps and then self-drive to contact ‘14’.

At this stage the connector switch is again under control of the subscriber’s dial. Following the dialling of the second digit, therefore, the connector switch wipers are stepped to the wanted line and the conditions of the line tested.

Two relays test the P-wire of the called subscriber’s line in turn, relay G testing in the operated condition followed by the high-speed relay H, released. If the called line is engaged, an earth is present on the P-wire and relay G remains operated, causing busy tone to be returned to the caller. Should the called subscriber number be spare, N.U. tone is returned, since the absence of earth potential on the P-wire causes the release of relay G, and relay H to remain un-operated.

When the called line is free, battery potential on the P-wire causes the release of relay G, and relays H and HR to operate. Interrupted ringing is then applied to the called line and ring tone received by the caller. Connection is established upon the called subscriber lifting the handset.

Incoming Junction Call
Operation is similar for an incoming junction call. A free link is seized via the junction relay set and the linefinder hunts for the calling junction line. When it is located, dial tone is returned and the connection completed as above.

Outgoing Junction Call
On the dialling of the appropriate single digit, the junction selector relay JS is operated and the connector hunts for a free junction. Immediately this is seized, relay H operates to battery potential on the junction P-wire and disconnects the drive circuit. Relay HA operates at this stage to extend the calling line to the parent exchange and also reduce current drain by releasing all other relays in the seized link.

In the event of all junctions being engaged when a junction call is originated, busy tone is returned to the caller.


Metering, when required, takes place immediately a called subscriber answers. Local call metering is untimed. Provision is also made for unit-fee or fixed multi-fee metering of junction calls. When fixed multi-fee metering is used, the calling subscriber’s meter is operated a determined number of times according to the fee charged for a call to a particular area. To meet an Administration’s requirements, a simple relay device can be included to enable the fixed number of meter operations to be repeated at specified periods during a call.

A traffic overflow meter can also be provided to record occasions on which all junctions are found to be busy.

Coin-Box (Pay station) Lines
Any subscriber’s line circuit can be arranged for coin-box working by appropriate terminal strapping. When a call is made to the parent manual exchange, a short burst of tone is received by the operator to enable her to discriminate between the coin-box call and a call originated by an ordinary subscriber.
If the parent exchange is auto-manual, a coin-box subscriber can be barred from dialling all junction calls other than ‘90’ for the operator or ‘999’ emergency.

Forced Release
An essential feature of the unattended exchange is the forced release facility. It ensures that should a subscriber fail to dial within a specified period or fail to replace the handset on completion of a call, the common switching equipment is released for use by other subscribers. Forced release is also applied to prevent a connecting link being held for an unnecessarily long period by an earth or permanent loop fault on the line.

Local Line Limits and Short Line Facility
Satisfactory operation is possible over a subscriber’s loop of 500 ohms, inclusive of telephone resistance. To avoid unnecessary drain of the exchange battery by subscriber’s short lines, facilities are provided to enable suitable resistors to be inserted in each leg of the line.

Fig. 3 - General View of Ring and Tone Relay Set, showing top to bottom, transistor generators, control relays and miniature uniselector Fig. 4 - General view of Minirax Unit

Ringing and tones are derived from two transistor generators (Fig. 3) brought into use when a connecting link or junction relay set is seized. Both ringing and tones are interrupted by relays controlled by a miniature uniselector and pulsing circuit. The periodicity of the ringing generator is 25 c/s, and sufficient power is produced by the generator to satisfactorily operate up to six telephones arranged in parallel. The frequency of the tone generator is 400 c/s and both busy tone and N.U. tone are derived from this source; ringing tone is obtained by a combination of both generated frequencies.

Because of the proved reliability of transistors and the absence of any moving parts in either of the generator units, risk of failure is small and, hence, no standby or changeover arrangements are provided.

The exchange will normally be unattended but it may be considered advisable to provide fault indication in an adjacent room or building. An alarm cabinet, containing three lamps, two keys and a buzzer, has been designed to meet such a requirement and can be provided as an optional extra.

The exchange can be tested for satisfactory working by the staff at the parent exchange on the dialling of a chosen number (‘7’), known as the test number. Distinctive tones are returned to the caller to indicate either normal working or the nature of an existing urgent fault (i.e., ring fail, fuse alarm, P.G. or low volts).

The automatic apparatus is housed in a floor standing, pressed steel cabinet designed for attachment to a wall for stability (Fig. 4). Access is gained from the front by means of two dustproof doors fixed with quick-release fasteners.
The Minirax is wired for its total capacity to enable certain facilities not required initially to be easily added as required.

Fig. 5 - Front view of unit with both doors removed Fig. 6 - Front view of Unit with gates open to expose wiring and rear equipment. At the top of the unit the battery box lid is shown raised

Apparatus of orthodox type and proved reliability is used throughout and includes heavy-duty B.P.O. type uniselectors, relay sets and two types of relays: 3000 type and high-speed.

As may be seen from Figs. 5 and 6 the arrangement of the apparatus is compact and accessible. The upper compartment contains the alarm display panel (including low-voltage alarm regulator), line fuses, heat coils and arrestors, all mounted on a gate-type framework to provide convenient access to the carrier equipment, subscribers’ cross-connecting field, and wiring points in the rear. Because all circuits requiring external connection are terminated in this compartment, the connection of incoming lines can be completed without interfering with or exposing the apparatus below.

In the lower compartment, mounted on a similar hinged-gate structure, are uniselectors and strip mounted line circuits. Arranged on the left-hand side of the uniselectors are termination points to allow the insertion of ‘short-line’ resistors should subscribers’ line conditions demand.

With the gate open, as shown, all associated wiring is revealed together with line-circuit resistors and the miscellaneous connection block.

All relay sets jack-in to the lower shelves, the first shelf accommodating equipment for connecting links and the second, apparatus for ringing and tone supplies, miscellaneous alarms and three junctions (one physical and two carrier).

The arrangement of the junction relay sets shown is typical, one physical junction being essentially provided to permit battery charging. The junction relay set shelf wiring, however, is arranged to permit the use of either physical or carrier circuits in any of the last two positions.

A novel feature of the Minirax is the provision made for battery accommodation (Fig. 7). The battery is enclosed in a sheet metal box, the dimensions of which are 4.5 in. (11.4 cm) high, 2 ft. 0 in. (61 cm) wide, and 4.5 in. (11.4 cm) deep from front to rear. The box has a special acid-resistant finish and is freely positioned in metal channelling supported by the exchange wall-fixing bracket. The hinged lid opens to the rear and, when closed, lies in the same plane as the top of the exchange cabinet. This method of battery mounting makes the Minirax completely self-contained, eliminates the need for cable leads to remote battery positions and allows the entire installation to be inspected from one point.

The unit occupies small space, its overall dimensions being 5 ft. 1 in. (155 cm) high, 2 ft. 9 in. (84 cm) wide, and 1 ft. 4.5 in. (42 cm) deep including depth of battery box.
The whole equipment has a high quality finish and is designed to withstand tropical conditions. The cabinet is enamelled externally opaline green, and inner surfaces are finished cream to provide maximum reflection of light.

Fig. 7 - Plan view of Unit showing the 24-volt 8 a.h. Battery in acid-resistant metal box freely positioned on mounting Fig. 8 - Charging arrangements on physical junction

When three junctions consisting of one physical and two carrier circuits are used in the system, the physical junction is tested last in sequence by the connector switch to allow the maximum time to be given to battery charging. Under average traffic conditions, this period approximates 22.5 hours per day.

The basic charging arrangement is shown in Fig. 8. Charging is effected over both wires of a 2-wire junction arranged in parallel with the positive terminal of each battery connected to a satisfactory exchange earth to avoid unwanted resistance. Direction of charging current flow is indicated by arrows. Rectifier MRA safeguards against battery drainage occurring owing to the presence of an earth fault on the junction,

Relays LB and CH function as signalling relays and remain operated while the junction is traffic free and charging is in progress. A junction call made from either exchange causes both relays to release and initiate circuit conditions to seize the junction for conversation.

The resistance of a physical junction for use in battery charging is primarily governed by the need to maintain a charging rate of approximately 148 mA. Assuming the battery is charged over a 2-wire line from a 50-volt d.c. supply at the parent exchange, the total permissible resistance of the line is 100 ohms. This is equivalent, for example, to a distance of approximately 46.5 miles should 200 lb/mile copper wire be used.
If more than one physical junction is used with the system these may be paralleled to decrease the overall charging path resistance, allowing greater line lengths to be obtained.

When a charging potential exceeding 50 volts is permitted by an Administration, this may be used to advantage to provide the required charging current over even greater distances. When this procedure is

adopted a relatively inexpensive rectifier is provided at the charging source.

Fig. 9 - Voltage Regulation Circuit and the heat coil serves to disconnect the circuit in the event of a low resistance earth fault.

To prevent excessive charging of the Minirax battery during long periods when junction traffic is absent or slight, equipment is included in the first junction relay set to limit the terminal voltage to a safe level of 27 volts.

In the circuit arrangement shown in Fig. 9, the transistor is used as a sensitive switching device. Its base voltage varies with the potential of the local battery and is derived from the potential drop across resistor R4 and part of variable resistor RV1. The emitter voltage, however, does not change, but remains constant under all conditions of the battery potential owing to the nature of diode Z1.

Within normal battery voltage limits (22V to 27V) the base voltage is positive to the emitter and is therefore cut off, allowing no current flow in relay VR. When the battery voltage exceeds the upper limit of the battery, the transistor base potential becomes negative with respect to the emitter and the transistor switches to ‘on’. Collector current then flows to operate the relay. Contact VR1 in operating, short circuits resistor R2 to prolong the ‘on’ condition and prevent ‘hunting’ of the relay; additionally, resistor R5 is introduced into the charging path, thus materially reducing the charging current.

The Minirax is a significant contribution to rural telephone communications because it has the desirable qualities of small size, economy of working, adequate facilities, ease of installation, and reliable performance over a wide range of temperatures and humidities. By its use, the particular telephone requirements of small groups of rural subscribers with common interests and activities are efficiently served. In addition to this special application, the characteristics of this small exchange make it suitable for use in other situations. For example, it may be employed to meet unexpected development in defence services or to provide temporary service during delays in the scheduled erection of an exchange building or in the acquisition of a suitable exchange site.


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Last revised: March 03, 2004