This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
Fig. 32 shows two methods of preserving equal E. M. F.'s at all points. At (v) is shown a loop circuit, in which the drop from the dynamo terminals to any lamp is the same. It will be seen that the length of circuit through c-y-x-h-a for the lamp l is equal to that through c-n-z-h-a for the lamp l1, so that all will burn with equal brightness. A closed loop circuit is shown at (w), one feeder from the dynamo d being connected to the inner ring, and one to the outer ring. It will be understood that this is diagrammatic only, for in practice these two circles would be of the same diameter insulated from each other, of course, the lamp leads being connected on all around at the required points.
45. Tap circuits are those wires making actual connection to the lamps from the submains. There are several methods of arranging these wires, the simplest being as shown at A, Fig. 33. This may, however, result in the further lamps burning quite dimly, if there are a number on the circuit. A better method is B, which bears some resemblance to the loop system, and is a very good way of arranging the leads. A third method, shown at C, is like a feeder and main on a small scale, and that shown at D is a modification which gives better distribution of pressure. The first mentioned may be used when the number of lights is small, and that marked C for double the number, since the connection is at the middle. In small tap circuits it is not usual to allow for any drop of potential, since the wires are short and the loss is insignificant.
46. There are, broadly, two systems of house wiring: the tree system and the closet, or cabinet, system.
The tree system is illustrated in Fig. 34. The general distribution of wires is somewhat similar to the trunk and ranches of a tree, hence the name. The mains a b, c f are carried through the building, being heavier at the lower end which carries the entire current. If, however, the mains are only of moderate length, they may be of equal size throughout. In the tap circuits the rosette cut-outs t, t, etc. are similar to that shown in Fig. 15, lamps with key sockets being attached. When keyless sockets are used, a switch such as m is placed in some convenient position.
47. The closet system, shown in Fig. 35, is more generally used than the tree system. Separate feeders are run to various distributing centers, and from these points the smaller, or tap, circuits start. The feeders in the diagram are a b and c f, making connection between the dynamo and the closets k and k1. In these closets the wires to each group of lamps are branched off. The two distributing boards may both be arranged in one closet, unless there is a considerable number of circuits, making a double closet desirable. They are usually lined with asbestos, and painted outside with insulating paint. Cut-outs are always connected where a conductor branches off, and frequently in the leads of each lamp. On entering the closet, the leads are generally separated into two groups, one consisting of the positive, and the other of the negative, wires, in order to reduce the possibility of short circuits. The placing of switches is subject to convenience, one or more being used to control the lamps for each room, and main switches are installed for each floor of a house. One large switch is always located at the dynamo or at the point where the wires enter the building, so that the whole system may be disconnected when required.
48. The transformer system is sometimes used in house lighting. The transformer is a stationary piece of apparatus consisting of two insulated coils of wire, surrounded by a mass of iron, the whole enclosed in an iron waterproof case, Fig. 36. The insurance regulations require that the transformer be placed outside the house, and it is usually bolted to the outer wall or carried on a near-by pole. The current is supplied to one of the coils at a pressure of perhaps 2,500 volts, and the house current is taken from the terminals of the other coil, at a pressure suitable for the lamps. The first of these coils is called the primary, and the wires are shown at p,p'; the other coil is the secondary, and its terminals are indicated by the letters s, s'. In this transformer, fusible cut-outs f, f are provided for the primary circuit, being inserted from the lower end of the cylindrical corner brackets on the case.