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Rh protecting the user and the public from accidental contact with any charged conductor, and on the other the necessity of protecting the distributing plant itself from injury by external forces, including atmospheric electricity. Thus it comes that in modern works for the generation and distribution of electricity the distributing plant is an important item financially, its cost ranging from about a quarter to one-half of the total capital outlay.

The supply of electricity in urban areas must necessarily be by means of cables laid underground, and the cost of these cables is one of the principal items in the cost of the distributing plant. The higher the pressure at which the current is conveyed the smaller may be the cross section of the cable, but where the supply is for general purposes, including domestic lighting, there is a limit to the pressure. Incandescent lamps of moderate candle-power cannot be made for a higher pressure than 250 volts, and even this is exceptional. The general voltage is 220, so that a supply to be generally used must be given at about that pressure.

The current in passing from the place of generation to the place of use has to pass