--MINIMUM CLEARANCES
BETWEEN UNINSULATED SURFACES
+------------------------------+-----------------------------+
| | Clearances (inches) |
| |---------------|---------------|
| Phase-to-phase voltage | | Phase-to- |
| (ms) | Phase-to- | ground or |
| | phase | control |
| | | circuit |
| |---------------|---------------|
| 0 to 250 ...................| 0.25 | 0.25 |
| 251 to 600..................| 0.28 | 0.25 |
| 601 to 1000.................| 0.61 | 0.25 |
| 1001 to 2400 ...............| 1.4 | 0.6 |
| 2401 to 4160 ...............| 3.0 | 1.4 |
+------------------------------+--------------+--------------+
(d) Parts whose dimensions can change with the motor operation, such
as ball and roller bearings and oil seals, shall not be used as
flame-arresting paths.
(e) The widths of any grooves, such as grooves for holding oil seals
or o-rings, shall be deducted in measuring the widths of flame-arresting
paths.
(f) An outer bearing cap shall not be considered as forming any part
of a flame-arresting path unless the cap is used as a bearing cartridge.
(g) Requirements for explosion-proof enclosures of motor assemblies.
(1) Enclosures shall be--
(g)(1)(i) Constructed of metal;
(g)(1)(ii) Designed to withstand a minimum internal pressure
of 150 pounds per square inch (gauge);
(g)(1)(iii) Free from blowholes when cast; and
(g)(1)(iv) Explosion proof as determined by the tests set out
in §7.306 of this part.
(2) Welded joints forming an enclosure shall be--
(g)(2)(i) Continuous and gas-tight; and
(g)(2)(ii) Made in accordance with or exceed the American
Welding society Standard AWS D14.-77 "Classification and Application of
Welded Joints for Machinery and Equipment," or meet the test requirements set
out in §7.307 of this part. AWS D14.-77 is incorporated by reference and has
been approved by the Director of the Federal Register in accordance with 5
U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from the American
Welding Society, Inc., 2501 NW 7th Street, Miami, FL 33125. Copies may be
inspected at the U.S. Department of Labor, Mine Safety and Health Administration, Approval and Certification Center, 765 Technology Drive, Triadelphia, WV 26059, or at the National Archives and Records Administration (NARA).
(3) External rotating parts shall not be constructed of
aluminum alloys containing more than 0.6 percent magnesium. Non-metallic
rotating parts shall be provided with a means to prevent an accumulation of
static electricity.
(4) Threaded covers and mating parts shall be designed with Class 1A and 1B
(coarse, loose fitting) threads. The covers shall be secured against
loosening.
(5) Flat surfaces between fastening holes that form any
part of a flame-arresting path shall be plane to within 0 maximum deviation
of one-half the maximum clearance specified in paragraph (g)(19) of this
section. All surfaces forming a flame-arresting path shall be finished during
the manufacturing process to not more than 250 microinches. A thin film of
nonhardening preparation to inhibit rusting may be applied to these finished
metal surfaces as long as the final surface can be readily wiped free of any
foreign materials.
(6) For a laminated stator frame, it shall be impossible
to insert a 0.0015 inch thickness gauge to a depth exceeding 1/8 inch between
adjacent laminations or between end rings and laminations.
(7) Lockwashers, or equivalent, shall be provided for all
fastenings. Devices other than lockwashers shall meet the requirements of
§7.308 of this part. Equivalent devices shall only be used in the
configuration in which they were tested.
(8) Fastenings shall be as uniform in size as practicable
to preclude improper installation.
(9) Holes for fastenings in an explosion-proof enclosure
shall be threaded to ensure that all specified bolts or screws will not
bottom even if the washers are omitted.
(10) Holes for fastenings shall not penetrate to the
interior of an explosion-proof enclosure, except holes made through motor
casings for bolts, studs, or screws to hold essential parts, such as pole
pieces, brush rigging, and bearing cartridges. The attachments of such parts
shall be secured against loosening. The the threaded holes in these parts shall
be blind unless the fastenings are inserted from the inside, in which case
the fastenings shall not be accessible with the rotor in place.
(11) For direct current motor assemblies with narrow
interpoles, the distance from the edge of the pole piece to any bolt hole in
the frame shall be at least 1/8 inch. If the distance is 1/8 to 1/4 inch, the
diametrical clearance for the pole bolt shall not exceed 1/64 inch for not
less than 1/2 inch through the frame. Furthermore, the pole piece shall have
the same radius as the inner surface of the frame. Pole pieces may be shimmed
as necessary. If used, the total resulting thickness of the shims shall be
specified. The shim assembly shall meet the same requirements as the pole
piece.
(12) Coil-thread inserts, if used in holes for fastenings,
shall meet the following:
(g)(12)(i) The inserts shall have internal screw threads.
(g)(12)(ii) The holes for the inserts shall be drilled and
tapped consistent with the insert manufacturer's specifications.
(g)(12)(iii) The inserts shall be installed consistent with
the insert manufacturer's specifications.
(g)(12)(iv) The insert shall be of sufficient length to
ensure the minimum thread engagement of fastening specified in paragraph
(g)(19) of this section.
(13) A minimum of 1/8 inch of stock shall be left at
the center of the bottom of each blind hole that could penetrate into the
interior of an explosion-proof enclosure.
(14) Fastenings shall be used only for attaching parts
that are essential in maintaining the explosion-proof integrity of the
enclosure, or necessary for the operation of the motor. They shall not be
used for making electrical connections.
(15) Through holes not in use shall be closed with a
metal plug. Plugs, including eyebolts, in through holes where future access is
desired shall meet the flame-arresting paths, lengths, and clearances of
paragraph (g)(19) of this section and be secured by spot welding or brazing.
The spot weld or braze may be on a plug, clamp, or fastening (for example see
figure J-1). Plugs for holes where future access is not desired shall be
secured all around by a continuous gas-tight weld.
(16) O-rings, if used in a flame-arresting path,
shall meet the following:
(g)(16)(i) When the flame-arresting path is in one plane, the
o-ring shall be located at least one-half the acceptable flame-arresting
path length specified in paragraph (g)(19) of this section from within the
outside edge of the path (see figure J-2).
(g)(16)(ii) When the flame-arresting path is one of the
plane-cylindrical type (step joint), the o-ring shall be located at least 1/2
inch from within the outer edge of the plane portion (see figure J-3), or at
the junction of the plane and cylindrical portion of the joint (see figure
J-4), or in the cylindrical portion (see figure J- 5).
(17) Mating parts comprising a pressed fit shall result in
a minimum interference of 0.001 inch between the parts. The minimum length of
the pressed fit shall be equal to the minimum thickness requirement of
paragraph (g)(19) of this section for the material in which the fit is made.
(18) The flame-arresting path of threaded joints shall
conform to the equirements of paragraph (g)(19) of this section.
(19) Explosion-proof enclosures shall meet the
requirements set out in table J-2 of this section, based on the internal free
volume of the empty enclosure.
