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erly protected by fuses, it can never overheat sufficiently to melt; but the safety of the construction obviously should not depend upon the carefulness of the men who place the fuses.
As the conduit is not depended upon for insulation, it is doubly important that there be nothing in its composition that will injure the insulation on the wire.
The virtues of the conduit consist in its being a complete protection to the insulation on the wire, entirely enclosing it at all points, and in its affording a smooth and ample raceway for the insertion and extraction of the wires. If conduits were allowed to be installed with the wires inside, there would not be the same necessity to the workman of joining the tubes accurately that there is when the wires must be afterwards inserted, and many installations would, through carelessness, be little better than plain installations of fished wire, with lengths of tube strung along the wire as suited the convenience of the workman. Even if it were allowable to lay strings in the tubes at the time they are placed, so that the wires might be pulled in afterward by means of the strings, there would be danger of abrading the insulation on the wire, because it would then be possible, on account of the force that could be applied, to pull wires through tubes that are too small, or in which there are obstructions that would tear the insulation. There would be lost, too, all advantage that comes from being able to remove and replace wires with facility. By requiring the conduit to be first installed as a complete system and the wire to be then pushed in, there is assurance that the joints in the tube are well made — for otherwise the wire would catch — and that there are no nails driven through the tube, or obstructions in it, for in these cases the wire could not be forced through.
If conductors are inserted before the mechanical work on the building is completed, any damage to the installation through crushing, cuts, or punctures, not only injures the tubing as a mechanical protection, but it is apt to injure the insulation on the wire at the same place. If the wire is inserted after the work on the building is completed, these damages are likely to be discovered when an attempt is made to insert the wire, and even though the wire slip through easily and the defect in the tubing is not discovered, there is at least the surety that the electrical insulation on the wire is intact.
If a tube containing a twin wire (Fig. 55) or two separate wires, were pierced by a nail and a short circuit thus caused, or if from any other cause an arc were to form in the conduit, there would be danger of the conduit's igniting before the protecting fuse could melt. With the iron-armoured conduit there is very little liability of such short circuits' forming, because the iron armour is a thick iron pipe like a gas-pipe, and is an effectual protection against nails. Even though an arc form within the conduit, harm could hardly be done, since the fuse protecting the circuit would melt before the iron would be burned through.
Rule 22 (f) is made because it is wished to have the conduit as free from moisture as possible, and by having the ends closed and the joints air-tight there is less liability of the gathering of condensed moisture.
23. Double-pole Safety Cut-outs :
a. Must be in plain sight or enclosed in an approved box, readily accessible.
[Section a. To be approved, boxes must be constructed, and cut-outs arranged, whether in a box or not, so as to obviate any danger of the melted fuse metal's coming in contact with any substance which might be ignited thereby.]
6. Must be placed at every point where a change is made in the size of the wire (unless the cut-out in the larger wire will protect the smaller).
c. Must be supported on bases of non-combustible, insulating, moisture-proof material.
d. Must be supplied with a plug (or other device for 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.
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 convenient 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