This report presents results of recent Bureau of Mines research to establish the probability of spark ignition as a function of current (or voltage, for capacitor circuits) for resistive, inductive, and capacitive circuits in 8.3-Pct methane-air atmospheres. The data presented quantify the explosion hazard created when low-power electronic devices catastrophically fail in an explosive mine atmosphere. Previous literature dealt with levels of probability between 10-3 and 10-4. This report extends the probability data to levels as low as 10-7. This study has verified that when the probability of ignition, pi, is plotted on a log-log coordinate system versus the current (or voltage), a continuous, straight-line relationship is formed. An interesting result that can be statistically hypothesized is that at some low level of igniting current the ignition process ceases altogether.