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

A comparison of the two system voltages, nominally 120 V
_rms
and 240 V
_rms
at 60 Hz, typical of North American residential applications, was made to determine why a series arcing fault in SPT-2 wire at a 240 V
_rms
had a significantly higher probability of fire ignition than at 120Vrms 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. 5 A
_rms
, 10 A
_rms
, 15 A
_rms
, 20 A
_rms
and 30 A
_rms
)- It was shown that the arc power, dissipated in the series arc, was greater at 240 V
_rms
than 120 V
_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 120 V
_rms
, and also to an increase in the number of arcing half-cycles at 240 V
_rms
because the higher system voltage was more likely to breakdown the gap than at 120 V
_rms
. Both these factors combined created a higher average arcing current at 240 V
_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 240 V
_rms
applications.