176 SECTIONAL ADDRESSES. 



The highest working pressure on 3-core cables so far applied in practice 

 has been 66,000 volts between phases or 37,500 volts phase to earth. The 

 construction of 3-core cables for large power transmission, i.e. with con- 

 ductors of 0.25 square inch, for example, offers great difficulties of both an 

 electrical and mechanical nature. Moreover, the short lengths in which 

 such heavy cables can only be constructed entail the use of numerous joint 

 boxes. 



It would seem that future practice in cable transmission at the higher 

 pressures must tend towards the adoption of single-core cables which 

 can be manufactured in greater lengths and with greater facilities in the 

 factory. These have the benefit of simpler jointing and are more easily 

 handled in the road. Single-core cables are now being constructed with 

 a pressure of 132,000 volts between phases or 76,000 volts to earth, and 

 cable manufacturers are confident that such cables can be applied with 

 safety. 



It is the cost of local distribution which causes a considerable addition 

 to be made to the cost of generation and transmission, and in which 

 economies must also be made possible in future extensions. As the 

 published official statistics show, the present average capital expended on 

 distribution systems in Great Britain is £21 per 1,000 units sold, including 

 the cost of sub-stations ; in numerous cases rotary converter sub-stations 

 are included, the cost of which is some seven to eight times that of static 

 transformer stations. The actual cost, for example, in Glasgow is £15.96 

 per kw.i. for rotary sub-stations and only £1.822' per kw.i. for static 

 sub-stations ; the ratio of costs in this case being 8.76 : 1. If one com- 

 pares the operating costs, including capital charges on the sub-station, 

 the rotary sub-stations required 0.1926c?. per unit sold, as against only 

 0.0334cZ. in the static sub-stations — in other words, the rotary sub-station 

 expenses are nearly six times those of the static sub-stations per unit sold. 

 In a large English city where A.C. distribution has been exclusively 

 adopted (excepting the supply to the local tramway system), the capital 

 outlay on distribution, including H.T. feeders and sub-stations, L.T. mains 

 services and meters, is actually reduced to £13.48 per 1,000 units sold. 

 The future cost where A.C. distribution is exclusively used is estimated at 

 £11.25 per 1,000 units sold. 



In ordinary town-distribution systems the capital cost depends upon 

 the number of consumers obtained per niile of main laid. 



The conditions of domestic electric services and loads are changing so 

 rapidly that it reqm'res considerable judgment on the part of the engineer 

 to lay out a distribution system with a due regard to a reasonable balance 

 between future requirements and the immediate outlay. 



It is not unreasonable to assume that future capital investment in 

 distribution will not exceed £11 or £12 per 1,000 units sold, since the bulk 

 of future distribution will be by means of alternating current and the 

 simpler static transformer sub-stations. 



Assuming that the average price of coal (of 10,500 B.Th.U. value) 

 delivered to the generating stations throughout the country is 20s. per 

 ton, it is safe to say that the average price at which electricity should be 

 available within a few years should be under ^%ths of one penny per unit. 



