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.
If a short circuit did happen, then would be the main and sub-main cut-outs to rely upon, but nevertheless electrical engineers consider the point of such importance that they often fix the cut-out actually on the top of the cable or wire from which the branches are being taken, so that it is literally impossible to leave any length of wire "behind" it unprotected.
Fig. 631.-A Celling-rose.
Fig 632.-Wall- socket. and Plug.
There is another appliance frequently used in the lighting of rooms, namely, a "wall-socket" or "connection-socket". This consists of a wall-piece and a plug, as shown in Fig. 632. The wall-piece contains two small terminals inclosed in a small china case, and connected through a fusible tin wire to the mains; the plug also has two terminals, which fit into two holes in the wall-piece, and so connect with the two terminals therein; the two terminals in the plug are connected to a double flexible wire, which may be attached to a lamp or other fitting. In this way, the flexible wire can be put into contact with the mains simply by inserting the plug into the china cover; a portable reading-lamp, hand-lantern (see Fig. 633), or any such fitting, having a piece of flexible wire attached, can thus be readily connected to the mains, and so provided with current.
The atmosphere is fortunately an insulator of electricity, Although varying in value considerably acoording to the amount of moisture suspended therein. If we take an incandescent lamp and attach one terminal of it to one main cable of a circuit, supplied at the time with a potential of (say) 100 volts, and then attach to the other terminal of this lamp a short piece of wire, and with the other end of this short piece of wire approach the other main cable within (say) a one-hundredth part of an inch, the intervening space of air (only one-hundredth part of an inch) would be sufficient to prevent the electricity from jumping across and lighting the lamp; it would, indeed, need a very much higher potential to do so than any used in domestic lighting. If the wire were brought closer and closer until it actually touched the main, contact would be made without a spark occuring. But if* the wire were then removed, the potential already existing in it (which did not exist before the contact was actually made) would be sufficient to bridge across a short space of air. It will be seen, then, that it is not the making of a contact, but the breaking of it. which produces a spark. In consequence of this, switches made with a trigger action are now used, by which the lever, which makes and breaks the contact, is in the latter case very suddenly withdrawn without the control of the oprator. In other words, if one were to turn a lamp out with such a switch, it would be found that, as the contact was about to be broken by one's hand turning the button, the lever inside would suddenly and quickly make the disconnection, even if one tried to make it do so slowly. These switches are termed "quick break". There are also "quick make" switches, but these an not so necessary. The advantage of the quick break is, that the spark is not allowed to exist for any greater length of time than is absolutely necessary; thus the metal parte, which would otherwise suffer somewhat, are practically undamaged.
We have hitherto spoken of insulated copper wire in a general way only: before proceeding further, it would be well to understand that the insulation of wires not only consists of cottons, silks, and other materials used in the manufacture of dynamos, Ac, but also of a complete coating of vulcanized rubber. giving a result somewhat similar to the placing of a cable inside a garden hose.
Fig. 633 -portable Hand-lantern.
The advantage of this is that the wire can be subjected to any amount of moisture, without this being able to pass through the insulation. This is of great importance, because moisture is a conductor of electricity. If a pair of wires get into a pool of water, or be placed on a damp wall, and be imperfectly insulated, a current will pass from one to the other, and the short circuit already described will in time occur.
The quality of the insulation is selected to meet the requirements of each case, but it may safely be assumed that, for domestic lighting, the insulation known as 600-megohm quality is sufficient. The expression "600 megohm" merely means that, with a high E.M.F. of (say) 400 volts (far in excess of what would be used in buildings), the insulation in question would offer a resistance of 600 million ohms, - a megohm being equal to one million ohms.
The "tree" system of wiring so far described, with its cut-outs at the branches, is only used in mills and such places, where the following system would be too costly and impracticable. It will be noticed that, in the tree system, for each lamp there must be at least two joints and one cut-out, or, if double-pole cut-outs are used, two cut-outs. The joints must, of course, be perfectly insulated.
Insulated joints are made by baring the ends of the copper wire, rolling these ends together, soldering the joints, and afterwards insulating them with a strip or ribbon of rubber, and then with waterproof tape. For a Mux with the solder on all joints, resin only should be used. These insulated joints are by no means equal to the 600-megohm wire, as regards their ability to resist moisture; excellent and costly wire is therefore much impaired by having its insulation cut in many places, and replaced with something of a comparatively worthless nature.
To avoid this, the "distributing" system of wiring has been largely adopted. It consists in taking the mains from the dynamo to two terminals on a slate slab, and on that slate slab branching these two points into (say) 12 pairs of points; then from these 12 pairs of points, taking sub-mains to twelve more slate slabs in different parts of the building, from each of which a number of circuits could radiate according to the number of lamps to be lighted. No joints are required at all with this system, and, as all the cut-outs are upon the slate slabs only, any particular cut-out is readily found and replenished, whereas with the "tree" system in an intricate building, it is often a game of "hide-and-seek" to find a cut-out when a lamp has gone out. If the slate slabs, or distributing-boards, as they are termed, are only in dry places, the wiring can be subjected to a very considerable amount of moisture before being damaged thereby.