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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:
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 current above its
carrying capacity is sent through it, the fuse will melt.
a. Must be mounted on moisture-proof and non-combustible bases, such as slate or porcelain.
6. 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 the switch. (Page 76.) For these reasons the singlepolo switch is thought suitable only for small tap circuits and its use is limited to branches having on them not more than six 16 candle-power lamps. The making of a connection that completes a circuit, and the breaking of a connection that interrupts a circuit, are spoken of as the “make and break.”
Rule 26 (c) is made to provide against the arcing in the switch that would be caused by poor construction.
If the parts of a switch that conduct the current are too small, or if the surfaces that come together do not make sufficient connection,
the switch will heat just as Fig. 63. — Jack-knife Switch. a wire that is too small will heat. Resistance is introduced and wherever current is forced through resistance there is heat.
The “jack-knife switch” is shown in Figure 63. It will be seen that if the switch were mounted so that it would be closed by moving the handle down, it would be closed by anything accidentally falling on it, or possibly by a jarring of the supports. This unintentional closing might cause a short circuit somewhere, as at a motor, for instance. Stops of some kind, either springs or friction arrangements, are usually provided, but they are not always trustworthy, and it is just as easy and as convenient to mount the switch so that it tends to fall open rather than shut.
27. Fixture Work:
a. In all cases where conductors are concealed within or attached to gas-fixtures the latter must be insulated from the gas-pipe system of the building by means of approved joints. The insulating material used in such joints must be of a substance not affected by gas and that will not shrink or crack by variation in temperature. Insulating-joints with soft rubber in their construction will not be approved.
[Section a. Insulating-joints to be approved must be entirely made of material that will resist the action of illuminating gases and will not give way or soften under the heat of an ordinary gas-flame. They shall be so arranged that a deposit of moisture will not destroy the insulating effect and shall have an insulating resistance of 250,000 ohms between the gas-pipe attachments, and be sufficiently strong to resist the strain they will be liable to in attachment.]
b. Supply-conductors and especially the splices to fixture wires, must be kept clear of the grounded part of gas-pipes, and where shells are used the latter must be constructed in a manner affording sufficient area to allow this requirement.
C. When fixtures are wired outside, the conductors must