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.