Comparing 240 Vrms to 120 Vrms Series Arcing Faults in Residential Wire

A comparison of the two system voltages, nominally 120V(rms) and 240V(rms) at 60Hz, typical of North American residential applications, was made to determine why a series arcing fault in SPT-2 wire at a 240V(rms) had a significantly higher probability of fire ignition than at 120V(rms) even though the arc voltages were similar for both system voltages. Experiments using SPT-2 wire were performed per UL1699 standards at each voltage level with equal nominal steady-state non-arcing load currents (e.g. 5A(rms), 10A(rms), 15A(rms), 20A(rms), and 30A(rms)). It was shown that the arc power, dissipated in the series arc, was greater at 240V(rms) than 120V(rms) due to increased arcing current. This was attributed to the arcing path resistance being a larger percentage of the total circuit resistance, reducing arcing current at 120V(rms), and also to an increase in the number of arcing half-cycles at 240V(rms) because the higher system voltage was more likely to breakdown the gap than at 120V(rms). Both these factors combined created a higher average arcing current at 240V(rms). Furthermore, the increased arc power would result in a hotter carbonized contact gap - further reducing the breakdown strength of the gap. The average arc voltage however remained about equal for both voltage levels. These results may be useful for exploring additions to the UL1699 "fire curve" for 240V(rms) applications.