CHAPTER XIII.

CLASS C, LOW POTENTIAL SYSTEMS. PART VII.

TEXT OF THE CODE COVERED BY THIS CHAPTER. 24. SAFETY FUSES-a. Must all be stamped or otherwise marked with the number of amperes 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 connections 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.

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

5

16...

6

3

18.... 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).

Before taking up the subject of fuses, let us consider more fully the requirements of Rule 23 (the text of which was printed in our last chapter) relating to cutouts. Section "b" may be stated as follows: "Every wire shall be protected by a fuse of such a size that it will melt before the wire becomes unduly heated." Cutout bases or "blocks" are made of marble, slate, glass or porcelain. For ordinary sizes porcelain is now universally used. It is the best material, as it is strong, is easily moulded into convenient forms, is cheap, and, when thoroughly vitrified, is a first-class insulator. When a fuse melts and a circuit carrying a current is opened, we of course have a flash, and the greater the current the greater the flash. Section "d" requires such a design of cut-out as shall prevent the blowing of a fuse from igniting any adjacent inflammable material. Safety is usually secured, first, by enclosing the cutouts in a box or cabinet with a fire-proof lining; sec

ond, by having the cut-out so designed that each fuse is covered. This last is accomplished by providing the fuse block with a mica or porcelain cover, or by using a plug, as mentioned in section "d." The "plug" is a device which connects the fuse with the circuit in the same manner that an Edison lamp is attached to a circuit. The fuse plug is just like the base of an Edison incandescent lamp, except that a piece of fuse wire replaces the wires leading to the loop or filament, and that the base has a metal cover to prevent the melted metal from blowing out. These plug cut-outs are used for circuits carrying currents up to about 15 amperes; and for small currents they are both convenient and safe.

A combination fixture," mentioned in section "c," is a chandelier or bracket, designed to carry both gas and electric lights. A circuit to a fixture ought to be protected by a small fuse if possible. From the nature of its construction, most combination fixtures have to carry small wires. Again, it would be disastrous to have an arc of many amperes formed in a combination fixture, as it might burn a hole in a gas pipe and ignite the gas.

We have seen that the branch blocks must be so proportioned that the conducting metal will carry the cur rent without overheating, and that in order that we may be sure of having connections of proper size, the blocks must be marked with the maximum number of amperes that the cut-out is designed to carry. We now come to the subject of "fuses." A fuse has been described as a piece of wire of such a size and material that it will

melt before the current becomes sufficiently great to unduly overheat the conductors of our circuit. The fuse is usually of a material which is a poor conductor, and which will melt at a low temperature. It is not essential, however, that such a material be used. Any metal may be used if the wire is made of the proper size, but by using a poor conductor and a metal which fuses at a low temperature, a wire can be made which, though of a respectable size, will be fused by a small current. The question of the best material for fuses is still a mooted one, but experience has thus far led to the use of some such metal as above described. The most common material used is an alloy of lead, such as lead, antimony and tin.

We have already seen that the fuse must be of such a size as to melt before the current can overheat the smallest wire depending upon it for protection. As fuses differ in material, the appearance of a fuse is no indication of the number of amperes which it will carry without melting. All fuses ought, therefore, to be marked so that anyone can tell how many amperes it will take to melt it. Unless this rule is observed, circuits will seldom be properly fused, and, what is more, no inspector can tell whether or not they are properly fused.

Section "b," though often violated, is one which should always be observed. This rule prohibits the fastening of fuse wires into a cut-out block by fastening the ends of a soft wire under the head of a screw. The reason for this is that a soft metal like lead cannot be held firmly in place by clamping it. The metal will

gradually but surely spread out under pressure until the wire becomes loose in its fastenings. Wherever we have a joint or contact there is resistance. The poorer the contact the greater the resistance, and consequently the more the joint will be heated by a given current.. When, therefore, our joint becomes loose it begins to heat. Again, as soon as the joint becomes loose, the surface of the fuse is exposed to the air and it quickly oxidizes. This oxidation still further increases the resistance of the joint, and soon we find that the fuse will blow with a current much smaller than that for which it was designed. The blowing of the fuse in itself would do no special harm, but the chances are that it will be replaced by a larger fuse, so that wire will not be properly protected. Aside from the question of safety, it is poor engineering to install a safety device that will not, as nearly as possible, furnish the same protection at all times. Again, in the case of small fuse wires it is necessary to have some sort of tip to the wire in order to have a place on which to mark the capacity of the fuse. Section "c" is a rule which we have already considered under the subject of cut-outs. We have spoken of a fuse as a wire in the same way that we have spoken of our conductors as wires. As a matter of fact, our conductor, if very large, may consist of a bar of copper instead of a wire, and in the same way our fuse, if large, may be (and usually is) a flat strip of "fuse metal." Such fuses are called fuse strips. Whatever the form of our conductor or our fuse, the fuse must be proportioned to the size of the smallest conductor which it has to protect. A conductor