Effect of the Ignition Method on the Extinction Limit for a Flame Spreading over Electric Wire Insulation

Experiments for flame spreading over electric wire insulation were conducted in parabolic flights (microgravity) and on ground (normal gravity) to understand the effect of the ignition condition on the Limiting Oxygen Concentration (LOC) for electric wires under an external, opposed flow condition of 100 mm/s (typical flow velocity on ISS). Both the ignition power and the igniter heating time were varied ranging from 50 W to 110W and from 5 s to 15 s in order to investigate the ignition condition effect. Polyethylene-coated Nickel-Chrome wires with inner core diameter of 0.50 mm and insulation thickness of 0.30 mm were used as sample wires, and a 0.50 mm diameter coiled Kanthal wire was used as igniter. The experimental results show that the LOC assumes an almost constant value under normal gravity conditions once ignition occurred, whereas under microgravity conditions, the LOC gradually decreases as the ignition power or heating time increases and eventually it reaches an almost constant value. Thus, the effect of ignition condition on LOC is stronger in microgravity than in normal gravity. The difference in oxygen concentration between the maximum and the minimum LOC was about 2% within the tested range of ignition conditions. This means that additional heating is required after ignition occurs to obtain the correct LOC. Finally, the results suggest that there exists a minimum ignition power and heating time to obtain the correct LOC values for electric wire combustion, especially in microgravity. This result has the potential to improve safety aspects associated with the development of a fire safety standard for spacecrafts, and consequentially also to improve spacecraft fire safety in general.