enclosing the fusible strip or wire) made of non-combustible and moisture-proof material, and so constructed that an arc cannot be maintained across its terminals by the fusing of the metal.

e. Must be so placed that on any combination fixture no group of lamps requiring a current of six ampères or more shall be ultimately dependent upon one cut-out. Special permission may be given in writing by the Inspector for departure from this rule in case of large chandeliers.

f. All cut-out blocks must be stamped with their maximum safe-carrying capacity in ampères.

FIG. 57.-Fuse Closet.

It is required that cut-outs or fuse-blocks be in plain sight, because when the fuse melts there is an arc for at least a short time and it is obviously dangerous to have this take place in a concealed place, where there possibly may be inflammable material. Besides this it is unlikely that the fuses will be properly inspected unless they can easily be seen. Where there are several cut-outs grouped, it is more con

venient to have them enclosed in a box built flush with the wall. The best boxes are made entirely of non-combustible material, but they are usually made of wood, lined throughout with asbestos board. (Fig. 57.) (See page 41.)

Where branches lead off from main wires, care must be taken that fuses be provided to protect the branches. Thus, in Figure 58, suppose a branch to be taken off at A. On the wires BB is a motor, and on the

wires AA are 5 lamps. Suppose the motor to require 25 ampères. Then the fuses at C must have a carrying capacity of at least 25 ampères or they will melt when the motor operates. But suppose the wires AA to be No. 14 B. & S. gauge, which is large enough to carry the current required for the 5 lamps that the wires supply, but which would overheat before the fuses at C could melt, in case there were a short circuit. It is consequently necessary to have smaller fuses at A, where the size of

K

wire changes, so that the carrying capacity of these No. 14 wires will not be exceeded. If there were no motor or lamps on the wires BB, then there would be no need of fuses at the point A, because a small fuse that would protect the wires AA could be put in the cut-out at C.

The bases of cut-outs must be non-combustible, because otherwise they are liable to ignite from the heat of the arc that forms when a fuse melts; they must be of insulating material, because otherwise there would be constant leakage; they must be waterproof, or dampness will cause leakage.

When fuses melt, the arc that is formed sometimes melts off particles of the copper or brass to which the fuse is attached, and if this molten metal flies into inflammable material it may be ignited. For this reason it is required that the fusible metal be entirely enclosed by non-combustible material, as it would be, for instance, in a cut-out like that shown in Figure 22 where there is a porcelain cover. Figures 59 and 60 give two views of a "plug cut-out." The fusible metal is soldered to brass terminals set in glass, and a brass cap screws over the top. The whole plug screws into a special cut-out base.

If the fuse be too short, the arc formed by the melting of the fuse will continue between the terminals to which the fuse is attached, and will give

out intense heat just as an arc lamp does. It is consequently of great importance that the length of fuse be enough to insure an arc of short duration. The electrical pressure, and the current at the time the fuse melts, both determine the distance the arc can hold over; but as there is always a liability, and even a probability, that the fuse will melt because of a short circuit, and consequently from a very heavy current, the correct length of fuse must not

be judged by the smallest current that will fuse it. If arcing is to be prevented, the length of fuse metal for 50 volts should be about inch, the length for 110 volts about 1 inch, for 220 volts about 1 inches, and for 500 volts not less than 4 inches.

A combination fixture" is one that is arranged so that either gas or electric lights may be used. If a large number of lamps were dependent upon one fuse, or rather, if one lamp were dependent upon a fuse large enough to carry the current for many lamps, then a short circuit in one of the lamps or its

receptacle, might cause a vicious arc that would burn through the gas-pipe and do considerable damage before the fuse could melt. Even though there be no wire in circuit small enough to require a fiveampère fuse, lamps on combination fixtures should nevertheless be arranged so that no lamp will be dependent upon a fuse larger than this.

Cut-outs should be stamped with their maximum carrying capacity so that there will be no excuse for overloading them. The parts that carry current are naturally made no larger than necessary, and an excessive current would dangerously overheat them.

24. Safety Fuses:

a. Must all be stamped or otherwise marked with the number of ampères they will carry indefinitely without melting.

b. Must have fusible wires or strips (where the plug or equivalent device is not used), with contact surfaces or tips of harder metal, soldered or otherwise, having perfect electrical connection with the fusible part of the strip.

c. Must all be so proportioned to the conductors they are intended to protect that they will melt before the maximum safe-carrying capacity of the wire is exceeded.

The largest number of ampères that a fuse will carry for an indefinite time without melting, is its