Зображення сторінки

used as would be necessary in the two-wire system running at the same voltage, with the same loss. For, as has been noticed, without the neutral wire the system is working at a pressure of 220 volts, and the copper required is only one-fourth as much as would be required with a system running at a pressure of 110 volts. If the neutral wire is the same size as one of the outside wires, the weight of copper is increased fifty per cent and becomes three-eighths as much as would be required with the 110-volt circuit.



Since electricity first came into general use there has been a constant improvement in the methods of wiring. Partly from an imperfect realization of the needs, and partly from the desire for cheap construction, all the early wiring was little more than a stringing of paths for the electricity to follow. It was soon found, however, that unless special precautions were taken to confine the current to these paths, there was danger of fire, and the service was poor and uncertain.

The first wiring was generally done with a wire inadequately covered, and the conductors were fastened directly against any convenient support by means of wooden cleats or iron staples. While everything was perfectly dry, and as long as the current was not greater than the wire could carry without excessive heating, little trouble would be found. But of course it was not possible to be sure of this dryness. Natural dampness in the atmosphere, leaky roofs, the accidental spilling of water, might at any time so affect the insulation that there would be considerable leakage. With this there would be, perhaps, a diminution in the available energy, and probably an arcing and heating that introduced a serious fire hazard.

There are two ways in which high temperatures are caused by electric currents. One is mentioned on page 13; it is the heating that takes place in any conductor when a current passes through it. A large current passing through a small conductor sometimes even causes fusion. But ordinarily much the higher temperature is produced by the formation of an electric arc, which is familiar from its practical use in the arc lamp. This arc is formed when a current passes from a conductor, through gases, to a conductor again, as explained on page 33. At ordinary potentials the current can "jump across" only a very short space, and these arcs have to be formed by the gradual separation of the conductors. Suppose that two wires when brought together complete an electric circuit; if the pressure forcing them together be now diminished, the electrical resistance at the contact is increased, and the current overcoming this resistance creates heat. If the wires are separated, there is at the instant of separation a great increase in the resistance and a proportionate increase in the heat generated; the ends of the conductors become volatilized and the result is an arc just like that formed with the carbons in the arc lamp.

A leaking current is likely to cause these arcs, because when a current passes by means of a semiconductor, it often burns away small particles and is maintained by an arc from one conducting particle to another. When a current is once established it is not easily broken, and a distance that would have at first proved a complete barrier is easily bridged by the arc with its hot, semi-conducting gases. But leakages of this kind are not the only cause of destructive arcing. One wire may come in direct contact with the other wire of the circuit, or it may come in contact with a pipe or a bell-wire in accidental connection with the other side of the circuit; and in this case there may be a "short circuit" with a very heavy flow of current, or there may be a sputter, and a burning away of the metal in contact, which results in a long arc of intense heat.

Excessive currents in multiple systems are provided against, by the use of a small wire or strip of fusible alloy, which will melt and thus break the circuit before the current has had time dangerously to heat the copper wire. These fuses are supposed to be of sufficient length to make it impossible, when they melt, for the arc to hold across the gap in the circuit, and the fuse itself is enclosed in a non-combustible case that will not be injured by the momentary heat. In series circuits, where the current is of uniform strength, the fuse cannot be used; and even in a multiple system, if the fuse is not properly proportioned, a disastrous arc may form and be maintained.

The modern methods of wiring have accordingly been designed to prevent, as much as possible, the leakage of electricity and the formation of "short circuits "; to keep local the effects of these accidents when they happen; and to make it easy to repair defects. The underwriters now require all wires not encased in approved forms of moulding or in approved makes of conduit, to be supported wholly on non-combustible insulators. It is their intention that wires shall not even come in contact with anything else. They also have made stringent regulations with regard to the insulation that is to cover the copper wire. Manufacturers submit samples of their wire, and it is subjected to severe tests before the underwriters will approve its use. No wire is allowed in concealed work, or in places exposed to dampness, except of these brands that have been approved; but where the wire is entirely exposed and is in a perfectly dry place, a somewhat inferior insulation is allowed.

It was thought in the early days that wiring would be perfectly safe and satisfactory if it were simply


« НазадПродовжити »