(a) The following shall be used for conducting an explosion
(1) an explosion test chamber designed and constructed to
contain an explosive gas mixture to surround and fill the motor assembly being
tested. The chamber must be sufficiently darkened and provide viewing
capabilities of the flame-arresting paths to allow observation during testing
of any discharge of flame or ignition of the explosive mixture surrounding
the motor assembly.
(2) A methane gas supply with at least 98 by volume per
centum of combustible hydrocarbons, with the remainder being inert. At least
80 percent by volume of the gas shall be methane.
(3) Coal dust having a minimum of 22 percent dry volatile
matter and a minimum heat constant of 11,000 moist BTU (coal containing
natural bed moisture but not visible surface water) ground to a fineness of
minus 200 mesh U.S. Standard sieve series.
(4) An electric spark ignition source with a minimum of
100 millijoules of energy.
(5) A pressure recording system that will indicate the
pressure peaks resulting from the ignition and combustion of explosive gas
mixtures within the enclosure being tested.
(b)General test procedures. (1) Motor assemblies being tested
(b)(1)(i) Be equipped with unshielded bearings regardless of
the type of bearings specified; and
(b)(1)(ii) Have all parts that do not contribute to the
operation or assure the explosion-proof integrity of the enclosure, such as
oil seals, grease fittings, hose conduit, cable clamps, and outer
bearing, caps (which do not house the bearings) removed from the motor
(2) Each motor assembly shall be placed in the
explosion test chamber and tested as follows:
(b)(2)(i) The motor assembly shall be filled with and
surrounded by an explosive mixture of the natural gas supply and air. The
chamber gas concentrations shall be between 6.0 by volume per centum and the
motor assembly natural gas concentration just before ignition of each test.
Each externally visible flame-arresting path fit shall be observed for
discharge of flames for at least two of the tests, including one with coal
(b)(2)(ii) A single spark source is used for all testing.
Pressure shall be measured at each end of the winding compartment
simultaneously during all tests. Quantity and location of test holes shall
permit ignition on each end of the winding compartment and recording of
pressure on the same and opposite ends as the ignition.
(b)(2)(iii) Motor assemblies incorporating a conduit box
shall have the pressure in the conduit box recorded simultaneously with the
other measured pressures during all tests. Quantity and location of test
holes in the conduit box shall permit ignition and recording of pressure as
required in paragraphs (c)(1) and (c)(4)(i) of this section.
(b)(2)(iv) The motor assembly shall be completely purged and
recharged with a fresh explosive gas mixture from the chamber or by injection
after each test. The chamber shall be completely purged and recharged with a
fresh explosive gas mixture as necessary. The oxygen level of the chamber gas
mixture shall be no less than 18 percent by volume for testing. In the absence
of oxygen monitoring equipment, the maximum number of tests conducted before
purging shall be less than or equal to the chamber volume divided by forty
times the volume occupied by the motor assembly.
(c)Test procedures. (1) Eight tests at 9.4 ±0.4 percent methane by
volume within the winding compartment shall be conducted, with the rotor
stationary during four tests and rotating at rated speed (rpm) during four
tests. The ignition shall be at one end of the winding compartment for two
stationary and two rotating tests, and then switched to the opposite end for
the remaining four tests. If a nonisolated conduit box is used, then two
additional tests, one stationary and one rotating, shall be conducted with
ignition in the conduit box at a point furthest away from the opening
between the conduit box and the winding compartment.
(2) Four tests at 7.0 ±0.3 percent methane by volume
within the winding compartment shall be conducted with the rotor stationary,
2 ignitions at each end.
(3) Four tests at 9.4 ±0.4 percent methane by volume plus
coal dust shall be conducted. A quantity of coal dust equal to 0.05 ounces
per cubic foot of internal free volume of the winding compartment plus the
nonisolated conduit box shall be introduced into each end of the winding
compartment and nonisolated conduit box to coat the interior surface before
conducting the first of the four tests. The coal dust introduced into the
conduit box shall be proportional to its volume. The remaining coal dust
shall be equally divided between the winding compartment ends. For two tests,
one stationary and one rotating, the ignition shall be either in the conduit
box or one end of the connected winding compartment, whichever produced the
highest pressure in the previous tests. The two remaining tests, one
stationary and one rotating, shall be conducted with the ignition in the
winding compartment end furthest away from the conduit box.
(4) For motor assemblies incorporating a conduit box which
is isolated from the winding compartment by an isolating barrier the
following additional tests shall be conducted--
(c)(4)(i) For conduit boxes with an internal free volume greater
than 150 cubic inches, two ignition points shall be used, one as close to the
geometric center of the conduit box as practical and the other at the
furthest point away from the isolating barrier between the conduit box and
the winding compartment. Recording of pressure shall be on the same and
opposite sides as the ignition point furthest from the isolating barrier
between the conduit box and the winding compartment. Conduit boxes with an
internal free volume of 150 cubic inches or less shall have one test hole for
ignition located as close to the geometric center of the conduit box as
practical and one for recording of pressure located on a side of the conduit
(c)(4)(ii) The conduit box shall be tested separately. Six
tests at 9.4 ±0.4 percent methane by volume within the conduit box shall be
conducted followed by two tests at 7.0 ±0.3 percent methane by volume. Then
two tests at 9.4 ±0.4 percent methane by volume with a quantity of coal dust
equal to 0.05 ounces per cubic foot of internal free volume of the conduit
box and meeting the specifications in paragraph (c)(3) of this section shall
be conducted. For conduit boxes with an internal free volume of more than 150
cubic inches, the number of tests shall be evenly divided between each
(c)(4)(iii) The motor assembly shall be tested following
removal of the isolating barrier or one sectionalizing terminal (as
applicable). Six tests at 9.4 ±0.4 percent methane by volume in the winding
compartment and conduit box shall be conducted using three ignition
locations. The ignition shall be at one end of the winding compartment for
one stationary and one rotating test; the opposite end for one stationary and
one rotating test; and at the ignition point that produced the highest
pressure on the previous test in paragraph (c)(4)(ii) of this section in the
conduit box for one stationary and one rotating test. Motor assemblies that
use multiple sectionalizing terminals shall have one test conducted as each
additional terminal is removed. Each of these tests shall use the rotor state
and ignition location that produced the highest pressure in the previous
(d) A motor assembly incorporating a conduit box that is isolated
from the winding compartment that exhibits pressures exceeding 110 psig,
while testing during removal of any or all isolating barriers as specified
in paragraph (c)(4) of this section, shall have a warning statement on the
approval plate. This statement shall warn that the isolating barrier must be
maintained to ensure the explosion-proof integrity of the motor assembly. A
statement is not required when the motor assembly has withstood a static
pressure of twice the maximum pressure recorded in the explosion tests of
paragraph (c)(4) of this section. The static pressure test shall be
conducted on the motor assembly with all isolating barriers removed, and in
accordance with §7.307 of this part.
(e)Acceptable performance. Explosion tests of a motor assembly
shall not result in--
(1) Discharge of flames.
(2) Ignition of the explosive mixture surrounding the motor
assembly in the chamber.
(3) Development of afterburning.
(4) Rupture of any part of the motor assembly or any panel
or divider within the motor assembly.
(5) Clearances, in excess of those specified in this
subpart, along accessible flame-arresting paths, following any necessary
retightening of fastenings.
(6) Pressure exceeding 110 psig, except as provided in
paragraph (d) of this section unless the motor assembly has withstood a
static pressure of twice the maximum pressure recorded in the explosion tests
of this section following the static pressure test procedures of
§7.307 of this part.
(7) Permanent deformation greater than 0.040 inches per
U.S. Department of Labor | Mine Safety and Health Administration (MSHA) | 1100 Wilson Boulevard, 21st Floor | Arlington, VA 22209-3939 www.msha.gov | Telephone: (202) 693-9400 | Fax-on-demand: (202) 693-9401