Resistance of Low-Voltage AC Arcs

Arcing tests conducted in the electrical laboratory indicate that the most probable value of resistance of the low-voltage arc in ohms is equal to 40 divided by the arcing current to the 0.85 power. It is believed that this equation is approximately correct for 120 to 600 volt, 60-Hz ac stable arcing as ir a typical arcing burn-down in a panelboard or switchboard in which the arc length is approximately 21/2 inches. At any point in the low-voltage system having an impedance of R + jX, the most probable value of arcing current is equal to V/(Rarc + R + jX). The calculation is made by a process of iteration starting with the bolted fault value of current. Curves of arcing current versus bolted fault current simplify the problem of calculating the arcing-current value.