UL 551-2009 pdf download – Transformer-Type Arc-Welding Machines.
4 References 4.1 Any undated reference to a code or standard appearing in the requirements of this standard shall be interpreted as referring to the latest edition of that code or standard. 5 Terminology 5.1 In the following text, a requirement that applies only to a particular type of product is so identified by a specific reference in that requirement to the type or types of products involved. Absence of such specific reference or use of the term product indicates that the requirement applies to all types of products unless the context indicates otherwise. CONSTRUCTION 6 General 6.1 An enclosure, an opening, a frame, a guard, a knob, a handle, or the like shall not be sufficiently sharp to cause a risk of injury to persons during normal maintenance or use. Exception: A sharp edge that must be exposed to enable the product to perform its intended function. 6.2 A welding lead or electrode holder that is attached to a product shall be acceptable for the purpose. 7 Frame and Enclosure 7.1 A product shall be formed and assembled so that it will have the strength and rigidity necessary to resist the abuses to which it may be subjected, without increasing the risk of fire, electric shock, or injury to persons due to total or partial collapse with resulting reduction of spacings, loosening or displacement of parts, or other serious defects. 7.2 A product shall be provided with a case or cabinet that encloses all live parts.
7.7 Sheet metal to which a wiring system is to be connected in the field shall have a thickness not less than 0.053 inch (1 .35 mm) if uncoated steel, not less than 0.056 inch (1 .42 mm) if galvanized steel, and not less than 0.075 inch (1 .91 mm) if nonferrous. 7.8 Other than as specified in 7.7, a sheet-metal enclosure shall not be thinner than specified in Table 7.2 or 7.3, as applicable. Exception: The minimum thickness of a reinforced enclosure without supporting frame may be less than specified in Tables 7.2 and 7.3 but not less than 0.020 inch (0.51 mm) for uncoated steel, 0.023 inch (0.58 mm) for coated steel, brass, aluminum, or copper if when subjected to compression and torsion forces, its strength and rigidity are shown to be not less than the corresponding properties of an enclosure of the same maximum length and width having the required thickness of metal. See Compression and Torsion Tests, Section 38.