§ 39.234. Character of switches and disconnectors.
(a) Capacity. Switches used otherwise than as disconnectors shall have a rated capacity sufficient to insure safe interruption, at the working voltage, of the greatest current which they may be required to carry continuously, and shall be marked with the current which they can safely interrupt. Rating means that they should operate successfully at 50% overload in amperes and at the working voltage under the most severe conditions which they are liable to meet in practice.
(b) Disconnectors. Disconnectors shall be of suitable voltage and ampere rating for the circuit in which they are installed and shall be accessible only to properly qualified persons. They shall also be protected by signs warning against opening the switches while carrying current in excess of the safe opening limit. Interlocking arrangements should be used to prevent opening of such disconnectors under loads beyond their safe opening capacity.
(c) Locking or blocking. Means shall be provided so that switches controlling motors, storage batteries, transformers, electric furnaces, and similar utilization equipment may be locked or blocked in the open position and plainly tagged to prevent careless closing while work is being done on the equipment controlled by them, unless live and moving parts of controlled equipment which would cause a hazard are so guarded as to render locking or blocking unnecessary. Small capacity snap switches, if near machines and in plain sight from parts of the machines controlled, are exempted. Switches of any size are exempted if the installation comprises only one motor, and the switch is in plain sight from parts of the machines operated by the motor. Locking should be utilized instead of blocking, wherever parts of the machinery driven are remote from the point of control.
(d) Good contact. Switches, controllers, and rheostats shall be so constructed as to make and maintain good contact. Knife switches shall maintain such alignment under service conditions that they may be closed with a single unhesitating motion.
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