(a) A thorough inspection of the telephone or signaling device will
be made to determine its adequacy and permisibility. Tests may be made to
check the electrical characteristics and constants of the various parts, and
determine the adequacy of the insulation and other parts of features of the
device.
(b) In addition, compartments of explosion-proof design will be
tested while filled and surrounded with explosive mixtures containing varying
percentages of Pittsburgh natural gas2 and air, the mixture within
the compartment being ignited by a spark plug or other suitable means. For
some of the tests bituminous-coal dust will be introduced into the
compartment in addition to the explosive mixtures, and the effects will be
noted. A sufficient number of tests will be made under the foregoing
conditions to determine the ability of the compartment to retain flame
without bursting. Even though the surrounding mixtures are not ignited, the
compartment will not be considered as having passed the tests, if flames are
discharged from any joint or opening; if excessive pressures are developed
or if serious distortion of the compartment walls take place.
2 Investigation has shown that for test purposes Pittsburgh natural
gas (containing a high percentage of methane) is a satisfactory substitute
for pure methane.
