Arcing Fault Currents in Low-Voltage A-C Circuits

Abstract

1. On 250-volt and 125-volt a-c circuits, arcing faults are unstable and will extinguish themselves within two cycles or less even when initiated by four number 8 copper wires on a bus with 4-inch spacing. On buses with less than 4-inch spacing, it would be expected that the arc would have more of a tendency to stabilize itself. 2. About half of the line-to-line arcing faults on the 500-volt circuit were unstable and extinguished themselves. 3. The magnitudes of the first-half-cycle values of current are dependent upon the means of initiating the fault. In using a relatively small wire for the initiating means, the current as compared to the bolted-fault current decreases as the circuit current level is increased. On the 500-volt circuits, the results of the copper bar and steel bar tests indicate that a factor of 0.9 might be applied to the bolted symmetrical current for systems whose bolted-fault currents are in excess of 40,000 amperes and a factor of 1.0 for systems whose bolted-fault currents are less than 40,000 amperes. On 250-volt circuits, the factor appears to be smaller. 4. On 500-volt circuits, the sustained currents may be less than 0.8 of the bolted symmetrical currents.