more accurate in this respect than a fuse, and can be adjusted to act at any given current within a considerable range. Their accuracy and adjustability are important, but their great advantage over the fuse is the facility of resetting them compared with the trouble and delay of renewing a fuse. The fact that a circuit-breaker flies open the instant that the current rises above the value for which it is set, may be a doubtful advantage. This might happen very often when the excess of current does not last long enough to cause any harm, as explained in connection with fuses on the preceding page. The main circuit-breaker may open at the same time as one in a branch circuit, thus shutting off other parts of the system needlessly. In some forms the so-called time factor is purposely introduced, a device being added to prevent action unless the overload is of a certain duration. An example of circuit-breaker illustrated in Fig. 341, differs from most types in the fact that the arms for the two sides of the circuit are independent, so that one may open automatically if it is attempted to close the other while a short circuit still remains. Fig. 341. Automatic Circuit-Breaker. Switches. The general facts concerning these important devices are set forth in Vol. I. Chapter XXII., but there are certain additional points to be noted in connection with interior wiring. The rules for switches as laid down by the National Electrical Code will be found on pages 9, 10 and 23 of the Appendix to the present volume. Ordinarily the type used on switchboards is the simple lever knife-switch. They are sometimes made in the quickbreak form in order to reduce the duration and therefore the burning effect of the arc produced on opening the circuit. It is a fact, however, that switches are used to close the main circuits or those carrying heavy currents, but should not be used to open them except in emergency. In starting up an electrical plant, or in closing the service switch of an installation, the switches controlling the various circuits may be closed before or after the current is turned on, and a switch may be left open if no energy is required in the corresponding circuit. In case of overload, a circuit-breaker or fuse should be provided in each main or branch circuit to open it when the current exceeds a safe value, but the switch itself when carrying heavy current is to be opened only as a last resort. Under ordinary circumstances the lamps are disconnected in comparatively small groups by means of snap switches, one of which is shown in Fig. 342. In fact, the current allowed in each branch circuit is limited by the insurance rules, being a maximum of 660 watts (equivalent to 12 lamps of 16 c.p. each at 110 volts). Even in a theater or other place where many lamps are turned on or off at about the same time, it is customary to control them in groups, each having a separate switch. If the current does not exceed 3 amperes at 110 volts a single-pole Fig. 342.- switch is allowed, but for currents greater than this it must be double-pole. Break-down Switch. A building having its own generating plant may be provided with a connection to the street circuit of some electric lighting company. In case of accident to, or repair of, the isolated plant, current may be obtained from the central station. The switch which enables this to be done is called a "break-down " switch, since it is used in case the local machinery is broken down. It is usually of the double-throw type (Fig. 343); and since street circuits are often three-wire systems, and isolated generating plants are operated in most cases on the two-wire plan, the switch is generally arranged to convert from the former to the latter, as described on page 82. Fig. 343. Break-down Switch. Multi-Control Switches. It often happens that it is desired to light or extinguish a given lamp or group of lamps from two or more different points. A common case is that of a lamp which may be turned on at the foot of a staircase and turned off at the top, or vice versa. This may be accomplished in various ways: one plan indicated in Fig. 344 requires a three-way key socket at the lamp, a three-way switch at the other point of operation, and an extra wire between them in addition to the two mains that supply the current. With this arrangement the lamp may be lighted or extinguished by either switch. Main Line Three-Way Switch Three-way Socket Fig. 344. Panel Boards and Cut-out Cabinets are miniature switch-boards or sub-centers of distribution which afford means of splitting up the mains into branches and of grouping the cut-outs. The various forms used differ mainly in the styles of switches or cut-outs and in their arrangement, as illustrated in Figs. 340, 345 and 346. The two or the three main conductors are represented by parallel wires or bars of metal, from which the branch circuits are led out through switches and cut-outs. Fixtures and Sockets. The endless variety of fixtures used for supporting arc and incandescent lamps may be classed as furniture or ornament, as they are not of a character to be included in a technical treatise; the only technical features they contain being the fixture wiring and the insulated joint interposed between the fixture and the gas-pipe to which it may be attached. The sockets are described in the next chapter, on Incandescent Lamps. In a general treatise it is impossible to go into the details of interior wiring, as they depend largely upon the conditions in each particu lar case. The principles involved and the chief elements of construction have been set forth, and for further information reference may be made to the National Electrical Code printed in full in the Appendix and to the following publications : Cushing, H. C., Jr., Standard Wiring for Incandescent Light and Power, pp. 116, N. Y., 1900. Emmet, W. L. R., Alternating Current Wiring and Distribution, pp. 76, N. Y., 1898. Leaf, H. M., Interior Wiring of Buildings, pp. 195. London, 1899 (gives English Practice). |