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The New Zealand Railways Magazine, Volume 1, Issue 10 (March 21, 1927)

Automatic Signalling — (General)

page 24

Automatic Signalling

The advent of electric automatic signalling into New Zealand marks the beginning of alternating current as an agent in signalling work in this country. It is true that numerous electric appliances and systems connected with signalling were in use previously, such as Morse and telephone, Winter's block, the tablet system, lock and block, the electric pneumatic interlocking at Dunedin, and numerous electric devices designed to meet necessary requirements as they arose. These, however, were all actuated or controlled by direct current electricity generated by cell power, and were the latest and most efficient for New Zealand conditions as they were successively installed. The expansion of hydroelectric power, giving a comparatively cheap supply of alternating current in practically every district served by trunk lines of railway, has enabled this country to retain its place in the forefront of British enterprise in safety appliances on railways.

Shunt Signal, Otahuhu.

Shunt Signal, Otahuhu.

The British railways, hampered by the possession of more complicated signalling systems, were not in a position to experiment on a large scale with automatic signalling, which reached its successful commercialisation first in the United States. Sporadic installations of automatic signalling were, however, in use in Britain, notably on the London Underground Railway during the process of the greater expansion of these systems.

It was left to New Zealand to pioneer the way in the British Empire with the first installation of single line automatic—that from Lower to Upper Hutt, which has completely justified the enterprise shown. This section, and the double line from Lower Hutt to Lambton, were fitted with arm signals, known as three position upper quadrant, but since these sections were opened further developments have taken place and subsequent installations of automatic have been fitted with daylight colour light signals, which are rapidly becoming standard, the essential methods of controlling the operation of the system, remaining however, practically the same.

Advantages of the System.

Though the capital cost of this up-to-date signalling system may seem to outsiders somewhat unwarranted by the density of traffic in New Zealand, this criticism is fully met by the fact that the enterprise actually shows a dividend in the saving of signalling staff, the extra traffic facilities gained and the increased capacity of the lines. In addition the power supply line accompanying such systems provides facilities for working such additional appliances as may be required along the route in the near future, some of which, of great potential utility, are now past the experimental stage in the United States and elsewhere. The maintenance staff will not exceed that required for earlier systems (allowing for due expansion) when this system is sufficiently advanced for economical organisation, and present general maintenance men are fully trained in the new standards. The operation current consumption is also very low for light signalling, and the efficiency is such that the percentage of signal failures per thousand movements is negligible.

page 25

Increased Capacity.

For unidirectional traffic the capacity of the line is increased perhaps tenfold where long or slow sections exist between crossing stations. For two way traffic on single lines the system permits with safety of unattended crossing loops and, at comparatively small annual cost, doubles or trebles the volume of traffic previously possible.

Speed Signalling.

A notable departure from previous principles of signalling is made by the three position signalling, either arm or colour light. These new signals indicate to the driver not only whether to proceed or not, but tell him the condition of the line ahead and at what speed to travel. On approaching a station the driver knows by the signals whether his route is set and clear. The signalman may make a mistake with regard to the condition of the road, but the electrified track will correct this and give the proper warning signal to the driver, should his route be occupied or fouled. On the open track the signals show the track conditions for a mile or so ahead and this certainty assists drivers in maintaining time and eases the nervous strain on the “human element.”

Home Signal, Westfield.

Home Signal, Westfield.

The Main Features.

These consist of (1) A reliable power supply of alternating current throughout the system.

(2) A continuous series of electrified bonded tracks insulated from each other.

(3) Signals electrically operated, and controlled by those tracks and situated at appropriate intervals.

(4) The necessary instruments and devices to operate and co-relate these signals to their controlling track and to one another.

(5) Shelters to house these instruments and connections.

(6) Connecting wires to join up all the various units detailed above into a continuous system.

Power Supply.

The power supply required for operating purposes is supplied from the most convenient hydro-electric scheme, but supplemented, if required, by a standby plant at each source of railway supply, in case of power failure of the hydro-electric mains or machines. This standby plant is actuated by power of different origin to the regular supply, so that the chances of a complete breakdown in power are negligible. The time required to switch in the alternative supply is less than two minutes. The standard New Zealand three-phase 50 cycle alternating current is used at 3,300 volt pressure, which pressure is suitable for overhead transmission without undue cost in line metal for the amount of current required for signalling work, and the distances from the source of supply. The overhead line conveying this power supply is graduated in size of metal used, from the source of supply to the furthest point (about 50 miles) so as to avoid voltage drop (lessening pressure) beyond say 5 per cent. Variations of voltage due to a supply defective in this particular are corrected at the supply end of the railway power line by an automatic device, which immediately adjusts any variation, thus maintaining a constant voltage. The need for a constant voltage or pressure is that the many instruments and devices used on this system are somewhat sensitive, and variations of voltage beyond a certain point affect their efficiency, which should be maintained at full.

How Power is distributed.

Wherever power is required along the route, to operate motors, signals or instruments at stations, intermediate signals or sidings, etc., an overhead line transformer is bridged across two of the power wires and the 3,300 volts pressure between these two phase wires passes through the primary side of the transformer. The secondary side of the transformer draws off (by induction) a portion of current reduced in pressure to 110 volts and transmits this through page 26 wires leading down the pole to the desired point, known as a “location.” These line transformers are so adjusted that the gradual slight drop in voltage (pressure) in the power line, as the distance from the point of intake increases is compensated for; thus the secondary (or low) side of the transformer always supplies current at 110 volts pressure wherever situated on the power line. The work of the main units such as point motors, lever locks, relays, etc., used in operating require current of 110 volts, but certain relay work, track current, signal lighting and diagram lights, etc., require voltages of various pressures (12 volts or lower), supplied by smaller transformers, which reduce the 110 volt supply to any voltage, 12 volts or lower as required.

The Insulated Track.

This consists of the permanent way bonded together at each joint and divided at suitable intervals into sections by insulated joints on each rail of the track. These sections are thus insulated from each other, and the sleepers and ballast are of sufficiently high resistance to insulate the rails from the ground; as very low voltage is used for track electrification (below 5 volts) and leakage to ground is negligible at low pressure owing to the size of metal in the rails providing the easier path for the current. The current fed into each section of track flows up one rail and down the other within its own section, and this current operates a relay or relays situated at the end or both ends of this section of track. Where a track carries a relay at each end as on single line working the current is fed to the centre of the track and flows both ways to the ends of the section returning on the other rail to the centre.

Three Position Colour Light Signals. Method of showing Clear, Caution or Danger through insulated track and three-position relay.

Three Position Colour Light Signals.
Method of showing Clear, Caution or Danger through insulated track and three-position relay.

Relays are electrical devices for opening or closing circuits. For track work they consist roughly of two kinds: those which operate two sets of contacts (switches)—known as two position—and those which operate three sets of contacts (switches)—known as three position. The two-position relays close one set of contacts (switches) when current is flowing through their operating mechanism and open that set and close another set when current is cut off.

The three position relays are differently operated as they open and close two sets of contacts (switches) according as the current flows through their operating mechanism from right to left or from left to right. They also open both these sets of contacts (switches) and close a third set if the current is cut off by a vehicle being on the track or other cause.

It will be seen from this that the insulated track can provide, through its relay, for three-different operations, that is, three circuits can be switched in by the relay, one to light a green light, or one to light a yellow light or another for a red light according to the position of the contacts (switches) provided in the relay.

(To be continued.)