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The New Zealand Railways Magazine, Volume 2, Issue 7 (November 1, 1927)

Electric Interlocking

Electric Interlocking.

As previously stated the essential difference in principle from mechanical interlocking consists of the interlocked area being track circuited, enabling automatic protection of running roads to be effective for train running movements. Shunting, however, can be given only a certain measure of automatic protection, on account of the necessity for the greatest flexibility in this work and the proved value of giving the trained signalman a certain amount of freedom.

As in Mechanical interlocking, the locking plate is the chief agent in preventing conflicting movements of signals, points, etc.

There is no mechanical connection between the interlocking machine and the points, traps, signals, etc. The operation of these units is affected through an electric circuit controlled by the lever. The points are moved by a motor, embodying a facing point lock and detecting devices, the motor being placed at the points.

Current cannot be supplied to operate the motor if a vehicle is standing on the points fouling the track and also any wrong adjustment or breakage of the permanent way will prevent the motor from operating until the defect is remedied.

Running signals, whether of “colour light” or motor worked arm pattern, are operated by electric circuits which are “switched in” by the levers in the cabin.

A signal operating circuit, in addition to passing through contacts on its own controlling lever, passes through contacts on all points levers used in setting up the correct route and also through the relay contacts of all tracks which will be fouled on that route.

All these contacts must be in the correct position or the signal will not clear.

Shunting signals are controlled in the same way, but their operating circuits do not always pass through the track relay contacts, as it is obvious that, in shunting, an engine may require to pass to an occupied road. The power required to operate the various units and instruments used is supplied from a transformer centrally situated or several transformers are provided at convenient points if the yard is over 24 chains in length. Main feed wires tapping these transformers are led through the yard, and current is drawn from these at convenient points (known as relay box locations) and at the signal cabin.

All the roads in the interlocked area are electrified and divided into separate tracks, so that each pair of points is situated in its own fouling track and, where points fouling tracks do not adjoin, the road between them is a separate track. All these tracks are shown on the illuminated diagram in the cabin, each with its own light which goes out when that track is occupied or fouled. The signalman can thus see at a glance the condition of his yard and can note whether his approach or departure tracks outside the station are occupied. The wires required for all this control are laid in wooden trunking throughout the yard and connected up to the various units at the cabin or in the relay boxes placed at intervals through the yard.