Spark ignition probability estimates for resistor and inductor circuits in methane-air atmospheres

The safety factor presently used by the US Mine Safety and Health Administration to assess the safety of electronic circuits for use in methane-air atmospheres is a factor of 1.5 on energy at the point of test plus two worst-case faults. The Bureau of Mines is engaged in intrinsic safety research that will ultimately relate the conditions of test and actual use by estimating the simple ignition probability, versus current for circuits in an 8.3% methane-air atmosphere. The experimental design minimizes the probability of not observing an ignition that should occur during a series of tests. For resistive circuits, no ignitions were observed below 2000 mA at 20 V DC, 350 mA at 30 V DC, 150 mA at 40 V DC, and 175 mA at 50 V DC. Results for inductive circuits show that ignition was not obtained below 35 mA for 600 mH, 90 mA for 100 mH, 230 mA for 10 mH, and 700 mA for 1 mH. These results imply that a threshold value exists below which spark ignition does not occur