This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
It has already been stated that where a branch wire joins a larger wire, fuses must be used to ensure safety from fire. Fig. 170 shows one of the simplest types of fuse box, in which the fuse wire is mounted in a porcelain case. The base of the case has a wall of porcelain formed between the terminals to prevent "Arcing." The covers for these fuses are usually made of porcelain, with holes for ventilation purposes, so that the terminals may be kept as cool as possible, and that the air or the gases may escape when the fuse blows. This circular type of fuse is about 2 to 2 1/2 inches in diameter, and is suitable for circuits carrying from 5 to 10 amperes.
Fig. 171 shows a rather more elaborate type of fuse box, which is used for high voltage circuits. It possesses a very long break, varying from 2 to 3 1/2
inches, so as to avoid all possibility of "arcing," and is further protected by means of porcelain walls. These are fixed in the same manner as the smaller circular fuse boxes.
The cover, which should be well provided with ventilation holes, is held in position by the thumb screw at its centre. These boxes are made from 3 to 7 inches long, and from 1 1/2 to 2 inches wide, with a projection of from 1 1/4 to 3 inches; and they will carry a current of from 10 to 100 amperes.
The method .of connecting a branch to the mains, through fuses on each pole, is shown diagrammatically in Fig. 172. Fuse boxes of this description are useful for connecting the branches of a tree system to the mains, since by looping out, as shown in Fig. 149, joints may be avoided, and the branches may with perfect safety be proportioned according to the current they have to carry.
The most useful form of fuse for connecting the house mains to a company's supply main is shown in Fig. 173. In this the fuse wire is fixed between two terminals contained within a porcelain trough, which is protected by an iron case. The cables are carried through the insulators at the top and bottom of the iron case, and are connected to the fuse terminals by means of clamping screws. Two such fuse boxes are usually mounted side by side on a board with insulators at the corners, to form a double pole fuse, as shown in Fig. 173. This type of fuse box is designed for currents from 25 to 100 amperes. When large currents are to be carried, several strands of wire or strips of fuse metal are used
These main fuses are usually fixed by the supply company, and their property. It is nevertheless advisable to have a pair of consumer's main fuses, which should be fused to blow at slightly lower excess current than the company's fuses.
The type of switch in common use for switching on incandescent lamps is shown in Fig. 174, and is known as the tumbler switch. By moving the knob K in the direction of the arrow the contact bridge P is brought downwards and makes contact between the springs S. The mechanism and terminals are mounted upon a block of china, and protected by either a brass or china cover screwed on to the thread beneath the knob K. When it is desirable that the switches should not project as much as is shown in Fig. 174, a type of switch known as the "Flush switch" (Fig. 175) is used. In these the mechanism is mounted upon a block of china as shown at A, and attached to the bottom of a brass cup, as shown at B. These are let into the wall and attached to wood plugs or fixing blocks, and a cover, such as is shown at C, is fixed over them. Flush switches may be made into ornamental features by the use of covers such as that shown in Fig. 176. For schools and asylums where it is desirable to prevent the children or patients from tampering with the switches a form is used which can only be turned on with a special key.
Two-way Switches are used when it is desired to switch on a light from one point and to switch it off at another; thus it is usually desirable to switch on the lights on a staircase from the hall and to switch off the lights from one of the upper landings.
Fig. 177 shows an example of a 2-way switch which is of the tumbler type. It contains three terminals, A, B, and C, and two pairs of contact springs, D, E, and F, G. The points C, D, and F are connected together, G and B are connected together, and so are A and E. In the present position of the switch contact exists between C and B; while if the knob is pressed over the other way the contact is broken between C and B, and made between C and A. The method of connecting up two-way switches with a lamp circuit is shown in Fig. 178, the terminals of the switches being represented by small circles which are lettered A, B, and C to correspond with the terminals in Fig. 177.
Double-pole Switches, for currents up to 20 amperes, may consist of two single-pole switches mounted on board, with the handles coupled together by means of a wooden bar, as shown in Fig. 179.
Turn Switches. Fig. 180 shows a type of switch known as a turn switch. By turning the handle H the laminated contact C is brought into contact with the under side of the brass strip S, and a similar contact is made on the other side of the handle. On account of the fact that contact is made or broken simultaneously on both sides of the switch the name "Double-break switch " has been applied to it.
Main Switches are of three general types, and are classified according to the nature of the contact they make. These are the Brush-Contact switch, the Spring-Contact switch and the Knife-Contact switch.
One pattern of a single-pole brush-contact switch with a fuse attached is shown in Fig. 181. Two of these are often mounted together to form a double-pole switch.
A switch of the spring-contact type is shown in Fig. 182.
The knife-contact type of switch is shown in the Change-over Switch illustrated in Fig. 183.
Change-over Switches are used when electricity is obtained from two sources of supply, such as from a private plant and from a supply company's mains. They are sometimes used as a safeguard against a breakdown of the private plant, or when electricity is taken for some part of the day from a private plant and for the rest of the day from the local supply company.
The cables are connected to main switches by baring the copper conductors at their ends and sweating them into sockets connected with the contacts.
Distribution Boards may consist of a number of fuses mounted on a slate base and connected to the omnibus bars, in which case they are called Distribution Fuse Boards; or they may consist of a number of fuses and switches, when they are called Distribution Switch and Fuse Boards.
The simplest form of distribution fuse board is shown in Fig. 185, and consists of two bus bars and a number of clamping terminals, attached to an enamelled slate base and enclosed in a teak case. To prevent the chance of an arc forming between the bus bars a projecting fillet of slate is fixed to the base between them. The mains are connected at the back of the board to the terminals M, M by clamping, or by sweating the ends into sockets. The fuse wires are clamped between corresponding terminals such as T, T, and the cables for the minor circuits are clamped to corresponding terminals, such as L, L, at the top and bottom of the fuse board. A very good type of distribution fuse board is shown in Fig. 186, where the fuse wires are carried on porcelain bridges of any of the patterns A, B, or C. These bridges are supported by means of spring clips attached to porcelain blocks on the base of the board. The main cables are connected to this type of board by clamping or sweating, while the minor circuits are connected to terminals at one end of each bridge, similar to those used in switches and ceiling roses.
An example of this type of distributing board, designed for regulating three points of light, is shown in Fig. 187. The switches are not always enclosed within cases, but the fuses must always be enclosed to prevent the molten fuse metal setting fire to anything when it "blows."
It will be noticed that on all the distribution boards illustrated fuses are provided to each pole of the minor circuits; but they may be obtained with single-pole fuses only, and also with double-pole switches.
The cases used for enclosing distribution boards are usually made of teak, as this wood is very fire-resisting; iron cases can also be obtained.