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[Section c. From the nature of the question, the decision as to what is an approved case must be left to the Inspector to determine in each instance.]
a. Must be equipped with metal or other non-combustible frames.
6. Must be placed on the switch-board, or at a distance of a foot from combustible material, or separated therefrom by a non-inflammable, non-absorptive, insulating material.
[Section a. The word "frame" in this section relates to the entire case and surrounding of the rheostat, and not alone to the upholding supports.]
"Cut-out" here means fusible cut-out. Generally speaking, a cut-out is any device that makes a break in the circuit and "cuts out" any lamp, motor, or other piece of apparatus. The term is, however, becoming commonly applied to fusible cut-outs only. The fusible cut-out ordinarily consists of a block or box of porcelain arranged so that a "fuse," or piece of easily fusible metal, will form a part of the circuit. When the current becomes too great, the fuse melts, because of the heat that the current gener
Fio. 22. — Double-pole
ates in it, and the circuit is thus broken. This fuse is supposed to be so proportioned that it will melt before the copper conductors can become unduly heated by an excessive current. The fusible cut-out is frequently called a "fuse-block."
A "double-pole" device is one that acts in each of the two poles, two wires, or two sides of a circuit. It is required that cut-outs and switches be double-pole so that the cutting-off will be complete and certain.
Suppose that, as in Figure 23, the fuse is singlepole, that is, acting on one wire of the circuit only. A heavy current flowing through the circuit will, of course, melt the fuse and the flow be stopped. But suppose that the fuse is just inside the wall of the building, and that the wire A outside the wall comes into connection with a metal column or with some other conductor connected with the earth. Suppose, moreover, that there is a connection with the ground on account of poor insulation at the motor, or that the wire comes in contact with a gas-pipe somewhere along BO. (Page 59.) Then the earth will be a by-path or "shunt" round the fuse and will make a closed circuit even though the single-pole switch be open. In this case a large part of the wire will not be protected by the fuse at all. A double-pole cut-out or doublepole switch will, however, cut off all the circuit beyond it, as may be seen by comparing Figures 23 and 24.
The "switch" is a device that operates mechanically, and is simply an arrangement by means of which one can pull out of circuit, metal that bridges over a gap. An air-space or some other insulation is left, over which the current cannot pass.
When the circuit is broken, there is an arc at the point of separation. If the circuit be broken at only
one point, the arc will be greater than if there are several points of separation. With a double-pole switch having two breaks in each pole, there is thus much less arcing upon opening, than if the switch has a single-pole with one or two breaks. With a small motor running on a low-potential circuit, the current is small and the pressure low, so that the arc formed when the circuit is broken is inconsiderable. It is also of little importance here, that the switch cut off completely all the wiring beyond it. For these reasons the rules allow single-pole switches under the conditions stated.
The motor must be insulated from the ground for the same reasons that apply to the generator. (Page 58.) A motor is, however, used in all sorts of places and usually has not a special space set apart for it as a generator has, so there is need of greater precaution. Mounting the motor on filled dry wood and raising it eight inches from the surrounding floor keeps it well insulated, away from dirt that might gather on the floor, and less liable to water damage in case the floor becomes flooded during a fire. Drip-pans are required so that all surplus oil may be caught, and everything about the machine thus kept clean and less combustible.
The waterproof cover serves the same purpose that it serves in the case of the generator. (Page 60.)
Rule 1 (c) requires that a generator shall not be placed in a room where there are flyings of combustible material. It is obviously not always practicable to confine motors to rooms altogether free from this sort of thing, and where there is dust or dirt, or flyings of any kind, the motor is enclosed in a case that will isolate it from bad conditions. Usually the case advised by the underwriters is a small room or closet built about the motor and lined with sheet metal.
Rules 9 (a) and 9 (6) are made for the reasons that 5 (a) and 5 (6) are made. In the case of a motor, however, a wooden resistance-box is still more unfit, for it is used only in starting the motor, and resistance is not supposed to be left in circuit, but to be all cut out as the motor reaches full speed. When through carelessness or accident some of the resistance is left in circuit, the coils are very likely to overheat, for they are frequently of too small carrying capacity to carry for any length of time the current that will flow through them.
HIGH-POTENTIAL SYSTEMS. — OVEB 300 VOLTS.
Any circuit attached to any machine, or combination of machines, which develops over 300 volts difference of potential between any two wires, shall be considered as a high-potential circuit and coming under that class, unless an approved transforming device is used, which cuts the difference of potential down to less than 300 volts.
10. Outside Conductors — All outside, overhead conductors (including services):
a. Must be covered with some approved insulating material, not easily abraded, firmly secured to properly insulated and substantially built supports, all tie-wires having an insulation equal to that of the conductors they confine.