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RICHARDSON'S BLUEBOOK OF PROJECTION
position covered with weatherproof or fireproof insulation.
Weatherproof Insulation
There are several types of weatherproof insulation, but they are used chiefly for outdoor work. (31) A form of insulation known as "fireproof" is used for interior work where the air is warm and dry.
(32) Specifications for conduit insulation are as follows, taken bodily from the National Code :
"Conduit wire shall be of approved rubber-covered type (Types R, RD, etc.), or, if in a permanently dry location, may be of the varnishedcambric insulated type (Type VC). A double braid shall be provided for conductors larger than No. 8 and for all twin, twisted or multipleconductor cables. Slow-burning insulation (Type SB wire) or asbestos covered wire (Type A) shall be used in permanently dry locations where the ambient temperature of the wire as installed, will exceed 120 deg. F. (49 deg. C). With flexible metallic conduit in wet or damp places wires shall have lead coverings (Type RL). All wires of Xo. 6 or larger shall be stranded. There shall be no splice or tap within the conduit proper."
The Two-Wire System
(33) Fundamentally all circuits are of the type. A 2-wire system is illustrated in Fig. 16.
2-wire It con
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Figure 16
sists of two wires which form the mains from which current is distributed on auxiliary 2-wire circuits. These, in their turn, serve circuits within buildings as shown at EEEE, Fig. 16, which also shows a projection lamp