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carrying capacity. It is better to mark the fuse with its carrying capacity than with its fusing-point, because the amount of current that the conductors are to carry is determined by their size, and it is only necessary to choose a fuse with the same carrying capacity that the wire

has. If the fuse were

marked with its fusing

SH

point, a careless or igno- FIG. 61.-Fuse with Tips of Hard

allow

Metal. Fuse-link or Safety-catch.

rant person might altogether too much margin in choosing the size of fuse for a given case. It is important that the fuse itself be marked, not only so that one will know definitely what is used, but because when the fuse melts it is then known what size is required properly to replace it.

Figure 61 shows a fuse having tips of copper soldered to the fuse-wire. These are called "fuselinks" or "safety-catches." Fuse-metal being soft and inelastic cannot be properly fastened under the head of the binding screws in the cut-out. It is apt gradually to work loose, thus making a poor contact that is a frequent cause of overheating in the cutout. The copper tips furnish a true, hard, and elastic terminal that can be securely clamped. Moreover, these fuse-links with tips of harder metal are more accurate as regards their fusing-point than is fuse

wire alone. A fuse-wire of a given diameter will have different fusing-points for different lengths of fuse; for when the fuse is very short, the metal to which it is attached conducts the heat away from the fuse so fast that a large current is necessary to generate heat fast enough to melt it.

When the

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FIG. 62. Showing Different Fusing-points for Different Lengths of the Same Fuse-wire.

fuse is long, the heat is not taken away from the fuse-metal so fast, and a smaller current will melt it. With fuse-links, however, the length of fuse-metal is fixed because it is soldered to the copper terminals, and the manufacturer is careful to choose a size that will have the proper fusing-point. Figure 62 illustrates this change in fusing-point with the change in length of fuse. With the size fuse-wire for which this curve is true, it will be noticed that when the

fuse is 5 inches or longer, the fusing-point is very close to 24 ampères, but that when the fuse is 1 inch long, for instance, the fusing-point is 45 ampères.

If the fuse is to protect the wire, it is obvious that it must melt before the carrying capacity of the wire is exceeded. It is, of course, not possible for the fuse to heat to the melting-point instantly when an excessive current flows, but as the wire cannot instantly heat to a dangerous temperature either, the fuse properly proportioned is a good protection against this particular kind of danger.

Setting fire to combustible material by direct contact is not the only danger to be apprehended from overheated wires. The insulating properties of the covering are injured by excessive heat, and if the wires become hot and are exposed to ground connections, leakage troubles arise. The carrying capacities given in the following table are supposed, however, to be low enough to prevent this.

25. Table of Capacity of Wires :

It must be clearly understood that the size of the fuse depends upon the size of the smallest conductor it protects, and not upon the amount of current to be used on the circuit. Below is a table showing the safe-carrying capacity of conductors of different sizes in Brown & Sharpe gauge, which must be followed in the placing of interior conductors:

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NOTE. By "open work" is meant construction which admits of all parts of the surface of the insulating covering of the wire being surrounded by free air. The carrying capacity of 16 and 18 wire is given, but no wire smaller than 14 is to be used except as allowed under Rules 18 (a) and 27 (d).

The size of wire that is chosen for a circuit is
determined by the amount of current it has to carry,
and the fuse is determined by the size of wire. If
the fuse be always chosen with regard to the size of
wire only, the wire cannot be dangerously small and
still be used, because when a
when a current above its

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carrying capacity is sent through it, the fuse will melt.

26. Switches:

a. Must be mounted on moisture-proof and non-combustible bases, such as slate or porcelain.

b. Must be double-pole when the circuits which they control supply more than six 16 candle-power lamps, or their equivalent.

c. Must have a firm and secure contact; must make and break readily, and not stop when motion has once been imparted by the handle.

d. Must have carrying capacity sufficient to prevent heating.

e. Must be placed in dry, accessible places, and be grouped as far as possible, being mounted — when practicable upon slate or equally non-combustible back boards. Jack-knife switches, whether provided with friction or spring stops, must be so placed that gravity will tend to open rather than close the switch.

If switches are not mounted on waterproof and noncombustible bases, arcing and heating caused by accidental conditions, will be apt to set the base on fire.

A single-pole switch draws a longer arc than a double-pole switch draws, because the single-pole switch breaks the circuit in only half as many places; and moreover, since it is on only one side of the circuit, it does not break all connection with the wire beyond

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