Relays.

The key to the success of Automatic Signalling is the relay. Essentially, the relay is a device for switching in an electric current for performing work of some kind. The work may be that of selecting which light (green, yellow or red) is to show in a signal and lighting it to the correct colour; it may be that of lifting a lock from a signal lever or again it may be the withholding of current from another relay for a set time (say 2 seconds) in order to allow proper co-ordination in a series of electrically controlled movements of other relays and machines. In fact relays seem to have no limit to their possibilities for the automatic control of operating electric currents.

Do not think, however, that relays are of one form only. Though standardised for mass-production purposes, in appearance and in most of their fittings, their functions, operating currents, and the number of contacts (switches) embodied, vary widely;—so does their price! Adequately to describe their complicated structure and functions is impossible in this short article. A slight glimpse only can be attempted. I have already roughly described two types of “track” relays as regards their contacts or switching equipment: two-position, and three-position relays.

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Two position relays may be operated (energised) by a single current of electricity passing through their coils (these are known as “single element”), or by two separate currents from different circuits acting together. In the latter case they are called “double element” relays.

Single element relays, which are always of the two-position type, are operated (energised) by low voltage (say two volts) when controlled by a track, and higher voltage (110 volts) if line controlled.

Double element relays may be of either the two or three-position types, and the two currents operating them may be both of 110 volts, or one of 110 volts and the other of low voltage (say two volts), the low voltage side, in the latter case, coming from the track. Double element relays thus have dual control, which means that two separate factors must co-ordinate in their operation.

Again relays are used which wait for a second or two before closing their contacts after their operating current is applied. Others will wait for as long as twenty-five seconds before deigning to open; of course all being timed “according to plan.”

Some relays have small motors embodied, to open and close their contacts (a very positive and reliable type). Others are of the magnet and coil variety, but the most fascinating are the Vane type relays, with their eddy currents principle and other complications.

With such an array of mechanism to draw upon it becomes a simple matter to control the operations of the nimble spark, which moves at the rate of a mere 186,000 miles per second, or thereabouts.

Other Instruments.

After dabbling in relays, other instruments seem a trifle commonplace, but perhaps the “reactance” and “resistance” coils, being appendages to relays, may be interesting. Insulated tracks, as current-carrying circuits, may vary considerably, both according to the nature of the ballast and also with weather conditions. As the tracks are the medium of signal control by passing trains, their vagaries must be suffered, but these can be softened by judicious handling. To this end the current delivered by the track to the relays is passed through a “resistance,” which lessens the current in a fixed ratio, or through a “reactance,” which limits the current in a progressive ratio.

A relay requiring two currents for its operation (double element relay) needs those currents to be out of phase with other; that is, one operating current phase angle to lead that of the other, producing an effect on the driving mechanism of the relay somewhat similar to the lead given to one of the driving rods on a pair of locomotive wheels for the purpose of avoiding the dead centres.

This is where the “reactance” gets in its work. A “reactance” in a circuit has the effect of altering the phase angle in that circuit, and, being placed in the track feed to the relay, produces a stronger torque in that relay for closing its contacts; the effect being comparable to the conditions governing a yacht beating to windward; the greater the angle her direction makes with the wind the faster she sails. These adjustments put the relays in the best condition to be least affected by variations in the track circuit due to the weather or other cause. Transformers have been dealt with, and lightning guards, fuses, terminals, etc., are commonplaces of all electrical installations.

Housing.

All relays, small track transformers, resistances, reactances, lightning guards, fuses, terminal blocks, etc., are housed in wooden boxes at the foot of each signal, the wiring from the tracks and signals is led, in wooden trunking, to these boxes, where the mechanism of the system is protected from weather and dust, and concentrated for ease of maintenance.

Wiring.

On double line no overhead wiring from location to location is required (with the exception of the main power supply line), as the rails are the medium of control from signal to signal. page 36 The wiring required for each signal at “locations” is:—

(1) The wires from the track to the track relay in its box.

(2) The signal lighting wires, which are selected and switched in by the relay.

(3) The feed wires to the next track section behind the signal. These also pass through the track relay contacts, whose position determines the polarity of the current sent back along the track behind.

On single lines, separate control wires on the pole line are required to co-ordinate the signal indications between crossing places.

This is necessary, as the number of movements for which the rail circuit can provide is not sufficient to meet all conditions that occur for two way traffic on single lines equipped with wayside sidings.

To secure directional automatic control for single line, an ingenious method of wiring certain of the relays makes them permissive in their action. They are then known as “stick” relays, because, when energised, they pick up, and when their energising current is cut off they still “stick” up because of a second current passing through their own contacts and thence through their energising coils. “Up” traffic will cause the up “stick” relays to close their contacts and will hold them closed, thus setting up the correct signalling, but they remain unaffected by “down” traffic. There is, therefore, a separate series of these relays required for each direction.

The action of the “stick” relay is shown in the diagram below. A train from the left drops A and B track relays, thus causing feed to energise D (stick relay) through C (signal relay) which is in the energised (yellow on green) position. When the train has passed C Signal, A relay picks up, and C relay drops to the de-energised position, thus cutting off “feed X” to D relay. “Feed Y” then operates through the de-energised position of C relay, and continues to energise D relay through its own contacts.

A train from the right cannot energise D relay, because relay C is in the de-energised position, therefore “feed X” cannot operate, and “feed Y” cannot energise relay D, unless “feed X” is first operated so as to pick up the contacts on relay D.

Two systems of overhead wiring are in use on single lines, the “three wire” and the “four wire.” The “four wire” is the later, and simplifies the wiring layout, but it is slightly more costly. All wires are fitted with naming fags at every terminal point, which enables the maintainer to trace every circuit with facility.

Recapitulation.

The rolling stock protects itself by putting the signal in its immediate rear to danger on double lines, and on single lines by also putting the opposing signals to danger as far as the next crossing place. This happy result is attained because the train short-circuits the track, which is a current-carrying circuit, and cuts off current (juice in the vernacular) to the relay, which is operated by the track; and the relay (in its de-energised condition) switches “in” the red light to the signal passed. Very simple! but wait! On single lines, all the opposing signals ahead to the next crossing place must also go to danger before the train receives its departure signal. Well! Before the departure signal is cleared the current to the relays controlling the opposing signals is cut off, causing them to go to “danger.” Each section of track between signals, as the train reaches it, also cuts off current to the opposing signals, which remain at “danger” until passed.

The overhead control wires on single lines provide for this and other necessary movements. On both single and double lines the second page 37 signals behind the trains successively return to clear, so that a following train can advance at the proper interval. Of course it is not quite so simple as that; but the tangle of wires and instruments seems to know that this is the result expected and, properly fed and tuned up, they certainly manage!

In actual practice the simple system detailed above is complicated by the special work required at branches, stations, wayside sidings, unattended crossing loops; and for level crossing warning devices, linking up with tablet systems, lock and block, mechanically equipped stations, etc. All these require special treatment according to the facilities desired, and come under the general term of “interlocking,” which will be the subject for the next article.