Fuse Plugs.
—Every electric circuit is liable to occurrences known as short-circuiting or “shorting.” This is a technical term describing a condition where, by accident or design, the wires of a circuit are in any way connected by a low-resistance conductor or by coming directly into contact with each other. In case of shorting, the resistance is practically all removed and the amount of current which flows through the circuit is so great as to produce a dangerous amount of heat in the wires. If the covering of a lamp cord becomes worn so as to permit the bare wire of the two strands to come together, a “short” is produced. Immediately, the reduced resistance permits the electric pressure to send an amount of current through the wires, greater than they are intended to carry. When this occurs an electric arc will form at the point of contact with the accompanying flash of vaporizing metal and the wire will finally burn off. Fires started from this cause are not uncommon.
To guard against accidents from short-circuiting, every electric circuit should be provided with fuses which, in cases of emergency, are intended to melt and thus break the circuit. Fuses are made of lead-composition or aluminum and are used in the form of wire or ribbon-like strips, of sizes that will carry a definite amount of current. They are designated by their carrying capacity in amperes. As an example: a 2-ampere fuse will carry 2 amperes of current without noticeable heating, but at a dangerous overload the fuse will melt and the circuit be broken. Should a short-circuit be formed at any time, the rush of current through the fuse will cause it almost immediately to melt, and stop the flow of current. They are, therefore, the safeguard of the circuit against undue heating of the conducting wires.
When an open fuse blows (melts), the heat generated by the arc, formed at the breaking circuit, is so sudden that there is frequently an explosive effect that throws the melted metal in all directions, and in case it comes into contact with combustible material a fire may result. To do away with this danger, fire insurance companies in their specifications of electric fixtures state what forms of fuses will be acceptable in the buildings to be insured. These specifications are known as the Underwriters Rules and may be obtained from any fire insurance company. The fuses, or fuse plug, as they are commonly called, generally occupy a place in a cabinet or distributing panel, near the point where the lead wires enter the building. The cabinet contains the porcelain cutouts for sending the current through the different circuits; the fuse plugs form a part of the cutouts, one fuse to each wire. The cabinet contains besides the cutouts a double-poled switch to be used for shutting off the current from the building when desired.
Fig. 230.—Electric cabinets.
Cabinets for this purpose are made in standard form of wood or steel to suit the condition of service. These cabinets may be obtained from any dealers in electrical supplies or the cabinet may be made a part of the house since they are only small shallow closets. Fig. 230 represents such a cabinet as is used in the average dwelling. It is made of a light wooden frame set between the studding of a partition at any convenient place. The bottom of the cabinet is made sloping to prevent its being used as a place of storage for articles that might lead to trouble. The cabinet is sometimes lined with asbestos paper as a prevention from fire but this is not necessary as the fuse plugs and their receptacles, when of approved design, are sufficient to prevent accident.
The main wires which supply the house with electricity—marked lead wires—are brought into the cabinet as shown in Fig. 231 and attached to the poles of the switch S. In passing through the switch the lead wires each contain a mica-covered fuse plug F, that will be described later. The current at any time may be entirely cut off from the house by pulling the handle H, which is connected by an insulating bar and the contacts N of the switch. When the handle H is pulled to separate the contact pieces, all electric connection is severed at that point.
Fig. 231.—Electric panel containing cutout blocks, fuses and switch.
The wattmeter for measuring the current is placed at the points marked meter, as a part of the main circuit. The main wires in the cabinet terminate in the porcelain cutouts, from which are taken off the various circuits of the house. In the figure, three such cutouts are shown making three circuits marked 1, 2, and 3. In circuit No. 1, the fuses are marked F. These wires are joined to the main wires at the points marked C and C´. The number of circuits the house will contain depends on the number of lights and the manner in which they are placed. The circuits are intended to be arranged so that in case of a short, no part of the house will be left entirely in darkness.
Fuses for general use are made in two different types—the plug type and the cartridge type—each of which conforms to the rules of the Underwriters Association. Those most commonly used for house wiring are the plug type shown in Fig. 232 and indicated in the figure just described. These plugs are made of porcelain and provided with a screw base which permits their being screwed into place like an incandescent lamp. The front of the plug is arranged with a mica window which allows inspection to be made in case of a short, the blown fuse indicating the circuit in which the trouble is located. Another style of the same type of plug, known as the re-fusable fuse plug, permits the fuse to be replaced after the wire has been destroyed by a short.
Fig. 232.—Mica covered fuse plug.
Fig. 233.—Cartridge fuse.
Fig. 234.—Plug receptacle for cartridge fuse.
The second type is commonly known as the cartridge fuse plug from its general appearance. This fuse is shown in Fig. 233. The fusable wire is enclosed in a composition fiber tube, the ends of which are covered by brass caps which afford contact pieces in the fuse receptacle and to which are fastened the ends of the fuse wire. These fuses are very generally employed in power circuits and others of large current capacity. The small circle in the center of the label is the indicator. When the fuse burns out, a black spot will appear in the circle. It is sometimes desirable to use the cartridge fuse plug in receptacles intended for the mica-covered type. The use of the cartridge fuses under this condition is effected by use of a porcelain receptacle such as is shown in Fig. 234; the cartridge fuse is simply inserted into the receptacle which is then screwed into the socket in place of the mica fuse.
In order to avoid any possible chance of overloading the wires of a circuit, fuses are installed which are suited to the work to be performed. Suppose that there are ten 40-watt lamps that may be used on a circuit, each lamp of which requires 4⁄11 ampere of current.
110 × C = 40 watts
C = 40⁄110 = 4⁄11 ampere per lamp.
Ten such lamps require ten times 4⁄11 ampere or 40⁄11 = 3.7 amperes to supply the lamps.
A fuse that will carry 3.7 amperes of current will supply the circuit but a 5-ampere fuse will permit an increase in the size of the lamp and will fulfill all the necessary conditions. If, however, an electric heater requiring 7 amperes were attached to the circuit, the fuse being intended for only 5 amperes would soon burn out. When a fuse burns out it must be replaced either with an entirely new receptacle or the fuse wire must be replaced.
It sometimes happens that in case of a blown fuse there is no extra part at hand and a wire of much greater carrying capacity is used in its place. It should be remembered that in this practice of “coppering” a blown fuse, has taken away the protection against short-circuiting with its possibility of mischief.
When a short occurs, the cause should be sought for. It cannot be located and on being replaced a second fuse blows, the services of an electrician should be secured.