This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
We have now in our mind a number of lamps connected at various points throughout the length of the main cables and their branches, and we know that each lamp is allowing only a little current to go through it, in consequence of its high resistance; but suppose that, by some mishap, the two small copper wires supplying a lamp were to be come connected together, then this path would offer a resistance to the dynamo very much less than all the lamps put together, because, as so frequently mentioned, the copper mains and wires have been arranged of such a low resistance as to absorb practically no energy.
This fault is termed a "short circuit", as the current has found a shorter or easier circuit than its proper course.1 The result would be that all the current would go through these two wires at the point at which they came in contact, and none through the lamp, which would therefore be extinguished. Further, the two small wires, which come in contact and make the short circuit, will receive all the current the dynamo is producing, whereas they were only arranged to supply one lamp taking perhaps '6 of an ampere, and these small wires, becoming exceedingly hot, will burn the rubber, etc, with which they are covered, and this may lead to a serious fire in the building.
This danger, however, can certainly and easily be avoided. If it were not so, electric lighting would not be, as is universally admitted, the safest known artificial illuminant. The cure is based on the fact that lead and tin wires have low melting points, and sufficiently low electrical resistances to enable the following arrangement to be adopted. At the point where one of the two
Fig. 627. - Section of Lampholder.
Fig. 628. - View of Lampholder with Outer case.
Fig. 629 - View of Switch Lampholder.
1 The expression "easier circuit" is less misleading, - although never used, - as it is not a question of distance bat small wires, leading to or from a lamp, leaves the main cable, or sub-main cable (as a large branch is termed), this small wire is cut, and the two ends so produced are connected by (say) an inch of lead or tin wire of perhaps 20 B.W. gauge, so that the lamp will light just as it did previously, but the current has to pass through this inch of tin wire on its way to or from the lamp. This small piece of tin wire does not offer appreciably any increased resistance to the current, but unlike the copper wire to which it is attached, it cannot carry much more current than the '6 of an ampere without melting.
Where this method is adopted, and a short circuit happens, the current increases instantly along this small circuit to a very large quantity, and in less than a second tin' tin or lead wire melts, and automatically switches oft* the faulty circuit. Nothing could be more simple or effective. By adding one of these pieces of tin wire at each branch, and also in the main cables close to the dynamo, we make it absolutcly impossible for any circuit to take such an excess of current as will raise the temperature of the copper cables and wires more than a few degrees since any faulty circuit will be immediately and automatically cut out.
The article used for preventing this overheating of the wires is termed a fusible "cut-out", and consists of any shape of box (preferably of china) with small terminals within it. by which to attach the copper wires to the small piece of tin wire. The gauge of the tin wire would have, of course, to be varied to suit its position - that is to say, if the cut-out were fixed in the main cable near the dynamo, the tin wire within it would require to be much larger than the small-gauge piece for conveying current to one lamp.
Incandescent lamps are often suspended by two flexible wires, twisted together in the form of a cord, which enters a small fitting on the ceiling. This fitting is called a ceiling-rose, and is usually made of china. Its object is to provide terminals for the two small copper wires from the main cables, which have to be brought to that point, and also terminals for the double flexible wire referred to. In this fitting there is also a fusible wire, so that, on any fault happening (in the flexible cord, for instance), the entire length of this cord would be automatically cut out of circuit.
Mention was made just now of cut-outs being inserted near the dynamo, and to explain the importance of the position of cut-outs, we will suppose that two leaving six feet of cable between the dynamo-terminals and the cut-outs. If a fault arose in this distance, there would, of course, be no fusible wire to rely upon, but as such cables are always most securely fixed and strongly insulated, they are practically free from the danger in question. Such is not the case, however, with smaller wires; so, if the cut-outs are not fixed very closely indeed to the cable, from which the wire leading to them is connected, the danger exists of a short circuit happening behind them.
Fig. 630 - A Fusiblc Cut- out cut-outs are fixed, one on each cable, leaving six feet of cable between the dynamo-terminals and the cut-outs. If a fault arose in this distance, there would, of course, be no fusible wire to rely upon, but as such cables are always most securely fixed and strongly insulated, they are practically free from the danger in question. Such is not the case, however, with smaller wires; so, if the cut-outs are not fixed very closely indeed to the cable, from which the wire leading to them is connected, the danger exists of a short circuit happening behind them.