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82 HANDBOOK OF PROJECTION FOR
in place. If the lamp sockets, receptacles, electroliers, etc., are also connected only one-half of the resistance specified in the the table will be required.
TYPES OF WEATHER-PROOF INSULATION.— There are t\vo types of weatherproof wire, viz., weatherproof and slow burning weatherproof. The insulation of the slow burning weatherproof consists of two coatings, one of which is fireproof and the other not. The fireproof coating is on the outside and comprises about 6/10ths of the total thickness of the insulation. The complete covering for size of wire from No. 14 to 0000 varies from 3/64ths to 5/64ths of an inch.
Fireproof insulation is not as susceptible to the action of heat as is ordinary weatherproof, which latter softens quickly under its influence. Fireproof insulation is .not, however, suitable for outside work, being intended for interior work in warm, dry places, such as shops and factories. When so used, underneath it, next to the wire there must be a coating of rubber.
Slow burning insulation, which is still more fireproof than the slow burning weatherproof, is intended to be used in very hot places, where ordinary insulation would soon perish. Weatherproof insulation should consist of at least three layers of braid, each thoroughly saturated with a dense, moisture-proof compound, applied in such manner as to drive out any atmospheric moisture contained in the material, thus securing a covering which will not only have high insulating power, but which also will to a great extent be waterproof. The outer covering of this insulation is pressed down to a hard, dense surface.
Wire thus insulated is intended for use out-of-doors where there will be moisture and where fireproof qualities are not necessary. In general, weatherproof wires may be used only where the supports on which the wire is mounted are of insulating material and are depended upon for insulation, the covering being regarded merely as a precaution against accidental contact with other wires, or other objects.
In addition to these there is a varnished cloth insulation which may only be used in places free from moisture.
From the foregoing we may understand that the principal difference between rubber cover and other insulation lies in the fact that rubber cover insulation may be depended upon to do the actual insulating, whereas the other must depend, at least to a considerable extent, on the wire support itself for insulation. Rubber covered wire may be used any