ELECTRIC DISTRIBUTION AND WIRING. 305 



usually tested for electric strength by subjecting them to a voltage 

 about twice as great as they are to withstand in service. 



Wires on pole lines are provided with an insulating covering 

 only when it is desired to reduce the risk of accidental contacts 

 with adjacent wires. Thus the high-voltage lines which supply 

 arc lamps in city streets should have insulating coverings to re- 

 duce the danger of accidental contacts with telephone and tele- 

 graph wires. On the other hand the wires of a very high-voltage 

 transmission line are usually bare, inasmuch as any ordinary 

 insulation would be inadequate to ensure protection in case of 

 accidental contacts with other wires or with persons. 



123. Insulation of underground, house and station wires. In 

 the installation of underground, house and station wires, two 

 things should be kept in mind, namely : (a) The insulation of the 

 wires, and especially the employment of insulating material that 

 will be absolutely water-proof, and (b) the protection of the 

 wires and their insulating covering from mechanical injury. The 

 only water-proof insulation is a covering of rubber or a covering 

 of fibrous insulation encased in a seamless lead sheath, and the 

 most satisfactory mechanical protection is that which is afforded 

 by an iron pipe or by a vitrified clay conduit laid in concrete 

 underground, or built in the walls and floors of an office building. 



124. Supplementary references. A full description of the great 

 variety of structural details involved in pole lines and under- 

 ground distributing wires and cables, and a full discussion of the 

 details of station- and house-wiring including the questions of 

 location and design of junction boxes, switches and cut-outs, are 

 beyond the scope of this text. A very complete treatment of pole 

 lines and underground wires and cables is given by A. V. Ab- 

 bott in his book entitled Electrical Transmission of Energy, Van 

 Nostrand, 1905. A wealth of practical information concerning 

 outside and inside wiring is given in the National Electrical Code 

 which is described on page 220 of this text. Valuable informa- 

 tion concerning wires of all kinds, iron, steel and copper, bare 

 and insulated, may be found in the little book entitled Wire in 



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