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

Power Supply

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.