[Code of Federal Regulations]
[Title 30, Volume 1]
[Revised as of July 1, 2002]
From the U.S. Government Printing Office via GPO Access
[CITE: 30CFR18.31]
[Page 96-97]
TITLE 30--MINERAL RESOURCES
CHAPTER I--MINE SAFETY AND HEALTH ADMINISTRATION, DEPARTMENT OF LABOR
PART 18--ELECTRIC MOTOR-DRIVEN MINE EQUIPMENT AND ACCESSORIES--Table of Contents
Subpart B--Construction and Design Requirements
Sec. 18.31 Enclosures--joints and fastenings.
(a) Explosion-proof enclosures:
(1) Cast or welded enclosures shall be designed to withstand a
minimum internal pressure of 150 pounds per square inch (gage). Castings
shall be free from blowholes.
(2) Welded joints forming an enclosure shall have continuous gas-
tight welds. All welds shall be made in accordance with American Welding
Society standards.
(3) External rotating parts shall not be constructed of aluminum
alloys containing more than 0.6 percent magnesium.
(4) MSHA reserves the right to require the applicant to conduct
static-pressure tests on each enclosure when MSHA determines that the
particular design will not permit complete visual inspection or when the
joint(s) forming an enclosure is welded on one side only (see
Sec. 18.67).
(5) Threaded covers and mating parts shall be designed with Class 1A
and 1B (coarse, loose-fitting) threads. The flame-arresting path of
threaded joints shall conform to the requirements of paragraph (a)(6) of
this section.
(6) Enclosure requirements shall be based on the internal volumes of
the empty enclosure. The internal volume is the volume remaining after
deducting the volume of any part that is essential in maintaining the
explosion-proof integrity of the enclosure or necessary for the
operation. Essential parts include the parts that constitute the flame-
arresting path and those necessary to secure parts that constitute a
flame-arresting path. Enclosures shall meet the following requirements:
Explosion-Proof Requirements Based on Volume
----------------------------------------------------------------------------------------------------------------
Volume of empty enclosure
-----------------------------------------------
Less than 45 45 to 124 cu. More than 124
cu. in. in. inclusive cu. in.
----------------------------------------------------------------------------------------------------------------
Minimum thickness of material for walls \1\..................... \1/8\ eq> eq>
Minimum thickness of material for flanges and covers............ \2\ \1/4\ thn-eq> thn-eq>
Minimum width of joint; all in one plane \4\.................... \1/2\
eq> eq>
Maximum clearance; joint all in one plane....................... 0.0024,5............................ eq> eq> eq>
Maximum clearances; joint in two or more planes, cylinders or
equivalent:
(a) Portion perpendicular to plane \6\...................... 0.0087,8 spacing; joints all in one plane............... (\16\) (\16\) (\16\)
Maximum bolt spacing; joints, portions of which are in different (\9\) (\9\) (\9\)
planes.........................................................
Minimum diameter of bolt (without regard to type of joint)...... \1/4\ eq> eq>
Minimum thread engagement \10\.................................. \1/4\ eq> eq>
Maximum diametrical clearance between bolt body and unthreaded \1/64\8,11,12.......................... eq> eq> eq>
Minimum distance from interior of enclosure to the edge of a
bolt hole: 8,13
Joint--minimum width 1........................... .............. .............. \14\ \7/16\
Joint--less than 1 wide.......................... \1/8\ eq>
----------------------------------------------------------------------------------------------------------------
Cylindrical joints
----------------------------------------------------------------------------------------------------------------
Shaft centered by ball or roller bearings:
Minimum length of flame-arresting path...................... \1/2\
eq> eq>
Maximum diametrical clearance............................... 0.020
eq> eq>
Maximum diametrical clearance............................... 0.006 (\1/4\ minimum) will be acceptable provided the diametrical
clearance for fastening bolts does not exceed \1/32\.
\15\ Shafts or operating rods through journal bearings shall be at least \1/4\ in diameter. The
length of fit shall not be reduced when a push button is depressed. Operating rods shall have a shoulder or
head on the portion inside the enclosure. Essential parts riveted or bolted to the inside portion are
acceptable in lieu of a head or shoulder, but cotter pins and similar devices shall not be used.
\16\ 6 with a minimum of 4 bolts.
(7) O-rings, if used in a flame-arresting path, shall meet the
following:
(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 (a)(6) of this section within the outside edge of
the path (see figure J-2 in the appendix to subpart J of part 7 of this
chapter).
(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
within the outer edge of the plane portion (see figure J-3 in the
appendix to subpart J of part 7 of this chapter), or at the junction of
the plane and cylindrical portion of the joint (see figure J-4 in the
appendix to subpart J of part 7 of this chapter); or in the cylindrical
portion (see figure J-5 in the appendix to subpart J of part 7 of this
chapter).
(8) 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 (a)(6) of this section for the material in which the fit is
made.
(b) Enclosures for potted components: Enclosures shall be rugged and
constructed with materials having 75 percent, or greater, of the
thickness and flange width specified in paragraph (a) of this section.
These enclosures shall be provided with means for attaching hose
conduit, unless energy carried by the cable is intrinsically safe.
(c) No assembly will be approved that requires the opening of an
explosion-proof enclosure to operate a switch, rheostat, or other device
during normal operation of a machine.
[33 FR 4660, Mar. 19, 1968, as amended at 57 FR 61209, Dec. 23, 1992]