A Statistical Determination of Spark Ignition Safety Factors in Methane, Propane, and Ethylene Mixtures in Air.
The Bureau of Mines conducted an investigation to statistically determine the mean current or voltage for resistive, inductive, and capacitive circuits that would cause spark ignitions in 8.3% Methane- air, 5.3% Propane-air, and 7.8% Ethylene-air mixtures. Each mean ignition current or voltage was determined on the basis of 100 trials. Each trial consisted of 400 revolutions, 200 in each polarity, at 80 rpm of the standard breakflash apparatus. In general, spark ignition curves published in ul 913 for resistive and inductive curcuits represent approximately the mean current values determined in this study, while published capacitive circuit values in methane are considerably less than the mean voltage values from this study, especially for a capacitance of less than 100 uf. For resistive and inductive circuits, spark ignitions were obtained at currents considerably below those in the published curves, but this study was not optimized to find minimum ignition values. This report defines "safety factor" as applied to intrinsic safety testing as the energy ratio of the mean values of ignition current or voltage for two test gases. The safety factors presented in this report are not constant with voltage or current and, therefore, are not recommended for use.