UL-917:2016 pdf free download – STANDARD FOR SAFETY Clock-Operated Switches.
6.6 Openings
6.6.1 An opening shall not be provided in an enclosure that houses a fuse or any portion of a circuit breaker other than the operating handle, unless the construction affords containment of electrical fault disturbances equivalent to that provided by an enclosure complying with the requirements in 6.5.4 — 6.5.6.
6.6.2 The following requirements apply to openings:
a) An opening shall not be provided in a compartment or part of an enclosure that contains field-wiring splices in a line-voltage circuit.
b) No openings shall be located in the mounting surface of an enclosure.
Exception: The following openings may be located in the mounting surface of an enclosure:
1) A mounting opening.
2) A maximum of four openings provided for the escape of air or paint during a painting process. The maximum dimension of such an opening shall not exceed 1/8 inch (3.2 mm).
3) A maximum of four unused holes provided for mounting of internal components. The maximum dimension of such an opening shall not exceed 3/16 inch (4.8 mm).
c) If the bottom surface is not the mounting surface, an opening may be provided in the bottom surface of an enclosure if the opening does not permit materials to fall out from the interior of the unit. See Figure 6.2 for an example of a construction complying with this requirement.
d) The shortest distance between an opening and the bottom of an enclosure or a wall- mounting surface shall be at least one-quarter of the enclosure height or depth, respectively, or 1 inch (25.4 mm), whichever is less.
e) Air from an opening, either forced or otherwise, shall not be directed:
1) Into a duct or into a concealed space in a building,
2) Against the mounting surface, or
3) So that a disturbance would be propagated to other equipment.
f) Not more than four holes for mounting an enclosure having a maximum dimension of 18 inches (457 mm); six holes for an enclosure with a maximum dimension of more than 18 inches, but less than 48 inches (1.2 mm); eight holes for an enclosure with a maximum dimension of 48 inches or more. Four of the holes for mounting an enclosure with a maximum dimension of 12 inches (305 mm) may be keyhole slots having the configuration illustrated in Figure 6.3. The dimensions shown in Figure 6.3 may vary if the area is equivalent. Four of the holes for mounting a larger enclosure may be keyhole slots, the dimensions of which are not specified, and which shall be judged with regard to the enclosure dimensions and configuration.
6.6.3 The smaller dimension (width) of an opening in an enclosure around a dial, adjusting knob, lever, handle, pointer, or the like shall not be more than 1/8 inch (3.2 mm) for any setting or position of the dial, knob, and the like.
6.7 Screens and expanded metal
6.7.1 The wires of a screen shall not be less than 16 AWG (1.3 mm2) for screen openings 1/2 square inch (3.23 cm2) or less in area, and shall not be less than 12 AWG (3.3 m2) for larger screen openings.
6.7.2 Perforated sheet steel and sheet steel used for expanded metal mesh shall not be less than 0.042 inch (1.07 mm) thick — 0.045 inch (1.14 mm) if zinc coated — for mesh openings or perforations 1/2 square inch (3.23 cm2) or less in area and shall not be less than 0.080 inch (2.03 mm) thick — 0.084 inch (2.13 mm) if zinc coated — for larger openings.