UL-1638A:2016 pdf free download – STANDARD FOR SAFETY Visual Signal Appliances for General Signaling Use.
3.3 Sheet metal enclosure
3.3.1 The thickness of sheet metal used for the enclosure or mechanism cover of an appliance shall not be less than that indicated in Table 3.1.
3.3.2 At any point where conduit or metal-clad cable is to be attached, sheet metal shall be of such thickness or shall be so formed or reinforced that it will have a stiffness at least equivalent to that of an uncoated (No. 16 MSG) flat steel sheet having a minimum thickness of 0.053 inch (1.35 mm).
3.3.3 A ferrous plate or plug closure for an unused conduit opening or other hole in the enclosure shall have a thickness not less than 0.027 inch [not less than 0.032 inch (0.69 or 0.81 mm) when of nonferrous metal] for a hole having a 1-3/8 inch (34.9 mm) diameter maximum dimension.
3.3.4 A closure for a hole larger than 1-3/8 inch (34.9 mm) diameter shall have a thickness equal to that required for the enclosure of the appliance, or a standard knockout seal shall be used. Such closures shall be securely mounted.
3.3.5 A knockout in a sheet metal enclosure shall be secured but shall be capable of being removed without undue deformation of the enclosure.
3.3.6 A knockout shall be provided with a surrounding surface for seating of a conduit bushing, and shall be located so that a bushing used at any knockout likely to be used during installation will not result in spacings between uninsulated live parts and the bushing of less than those indicated under Spacings, Section 10.
3.3.7 An enclosure intended for recessed mounting, and whose panel is to be flush with the surface of the wall, shall not have nonfunctional openings.
3.3.8 An appliance intended to be supported by rigid conduit shall be provided with conduit hubs, or the equivalent, having not less than five full threads.
3.4 Nonmetallic enclosures
3.4.1 An enclosure or parts of an enclosure of nonmetallic material shall have the mechanical strength and durability and be so formed that the mechanical strength of the enclosure is at least equivalent to a sheet metal enclosure of the minimum thickness specified in Table 3.1.
3.4.2 Among the factors to be considered when evaluating the acceptability of a nonmetallic enclosure of parts are:
a) Flammability;
b) Mechanical strength;
c) Resistance to impact;
d) Moisture absorptive properties;
e) Dielectric strength insulation resistance, and resistance to arc tracking; and
f) Resistance to distortion at temperatures to which the material may be subjected under
conditions of anticipated use.
All these factors are to be considered with regard to aging. See the Tests of Polymeric Materials, Section 24.
3.4.3 When a polymeric enclosure is intended for connection of conduit, the connection shall comply with the appropriate requirements for torque, bending, and pullout described in the Standard for Industrial Control Equipment, UL 508.
3.5 Ventilating and acoustical openings
3.5.1 An opening in an enclosure (including perforated holes, louvers, and openings protected by means of wire screening， expanded metal, or perforated covers) shall not permit passage of a 1 7/64-inch (6.7-mm) diameter rod.
3.5.2 The wires of a screen shall not be less than 16 AWG (1.3 mm2). Except as noted in 3.5.3,
perforated sheet steel and sheet steel used for expanded-metal mesh shall not be less than 0.042 inch (1.07 mm) in average thickness [0.046 inch (1.17 mm) thick when zinc coated].