UL1004-1 Rotating Electrical Machines –General Requirements
First Edition – September, 2008
Second Edition -
September 19, 2012
12.1 General
12.1.1 Ventilation openings for machines intended for field installation shall comply with the requirements
in Section 12, Ventilation Openings.
12.1.2 A ventilation opening in the top of a Type 1 enclosure shall be covered by a hood or protective
shield spaced above the opening when there are energized components below the opening.
12.1.3 Any ventilating opening in a Type 1, 2, or 3R enclosure (enclosure types per the Standard for
Enclosures for Electrical Equipment, Non-Environmental Considerations, UL 50) shall comply with Section
13, Accessibility of Uninsulated Live Parts, Film-Coated Wire, and Moving Parts.
12.1.4 The area of an opening covered by a louver, a perforated or an expanded-metal mesh panel that
is thinner than the enclosure shall not exceed 1290 cm2 (200 square inches).
12.1.5 The diameter of the wires of a screen shall be at least 1.3 mm (0.05 inch) if the screen openings
are 323 mm2 (0.5 square inch) or less in area, and shall be at least 2.1 mm (0.08 inch) for larger screen
openings.
12.1.6 Perforated sheet steel and sheet steel employed for expanded-metal mesh shall be at least 1.1
mm (0.04 inch) thick uncoated steel for mesh openings or perforations 323 mm2 (0.5 square inch) or less
in area. For larger openings, at least 2 mm (0.08 inch) thick uncoated steel shall be used.
Exception: In a small device where the indentation of a guard or enclosure does not adversely affect the
performance or reduce the spacings below the minimum value specified in Section 18, Spacings,
expanded-metal mesh of uncoated steel not less than 0.51 mm (0.02 inch) thick may be employed when:
a) The exposed mesh on any one side or surface of the device so protected has an area not
more than 464 cm2 (72 square inches) and has no dimension greater than 304.8 mm (12
inches); or
b) The width of the opening so protected is not greater than 88.9 mm (3.50 inches).
12.2 Barriers used with ventilation openings
12.2.1 A barrier interposed between ventilation openings and live parts to prevent the emission of flames
or molten metal from an enclosure shall comply with 12.2.
12.2.2 The barrier shall be of such dimensions and so located that any straight line drawn from any arcing
part past the edge of the barrier intersects a point in the ventilating opening plane that is at least 6.4 mm
(1/4 inch) outside of the edge of the ventilation opening. See Figure 12.1.
12.2.3 A sheet-metal barrier shall be at least 1.35 mm (0.053 inch) thick uncoated steel, or 1.91 mm
(0.075 inch) thick aluminum.
Exception: A metal barrier may be of thinner metal provided its strength and rigidity are not less than that
of a flat sheet of steel having the same dimensions as the barrier and of the specified thickness.
12.2.4 A nonmetallic barrier shall be at least 6.4 mm (1/4 inch) thick and shall be supported to provide
mechanical strength and rigidity.
Exception: A nonmetallic material may be less than 6.4 mm (1/4 inch) thick if it is:
a) Located so that it is not subjected to mechanical damage during installation; and
b) Supported to provide mechanical strength and rigidity.