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ELECTRICAL CONDUCTORS 77
ferrule contact fuses. EF arc types of porcelain base DPST knife switches used in theatres for various purposes.
How To Install Switches
(53) In a projection room switches must be located to allow for quick and easy manipulation.
(54) The tendency of knife switches such as are shown in Figures 19 and 20 to fall shut through the action of gravity must be avoided. The handle of double pole switches should swing sidewise. The handle of single throw switches should point upward.
(55) The knife blade switches should be examined at least once each week if used often. (56) Contact D, Fig. 19, should be snug to insure good electrical contact and to hold the blade in any position. Contact C should always be in good condition. Often the clip is split in its center, as is shown in C. Make certain that both portions of such clips make good contact with the blade. Keep insulation base clean to prevent current leakage. All portions of the switch should be snug and tight at all times including the cross bar and handle.
Enclosed Switches
(57) An enclosed switch has an individual protective covering, usually of sheet metal. The cover protects the switch from contact with various objects which might cause a short circuit or injury to the switch parts and also protects the user.
(58) Connections of enclosed switches should be arranged so that the blade will be electrically dead when the switch is open. (59) It is a rule that should be observed with all knife switches.
The following is taken verbatim from the National Electrical Code:
"Single-throw knife switches shall be so placed that gravity will not tend to close them. Double-throw knife switches may be mounted SO that the throw will be either vertical or horizontal as preferred, but it" the throw be vertical a locking device shall be provided, so constructed as to insure the blades remaining in the open position when so ■ "When practicable, exposed knife switches shall be so wired that blades will be dead when the switch is open."