A static voltage-current characteristic for the low current DC arc

Abstract

systems has failed to detect arcing faults numerous times with disastrous consequences. The fault leaves a charred furrow, known as the ''arc track'', through Kapton cable insulation. Tests by NASA and others have confirmed that the average circuit current during an arc tracking fault remains below overcurrent protection trip levels. Some in the field believe the reduction of fault current is due to the carbonized insulation, presumably, acting as a resistive element in the fault branch. It is thought that insulations which do not carbonize when exposed to an arc will eliminate the arc tracking problem. This thesis supports an alternative theory that the voltage drop of the arc itself may be solely responsible for reducing the fault current and the ''track'' is collateral damage from the arc. The voltage-current (V-l) characteristic of arcs below 50 amps was left unresolved, by research concluded in the 192O's. Recently, interest has returned to low current AC arcing, but the focus has been on the harmonics generated by such arcs as a means of detection. This thesis proposes a static V-I characteristic for a low current DC arcs supported by theory and empirical data. Two different experiments and a total of twenty four trials were made of DC arcs on bare I8AWG solid copper electrodes in geometries simulating Sow current arcs, between wires of a typical cable bundle (1-6 amps and 12-125mils long). Using multiple linear regression the data was fitted to 18 different models, new or proposed in the literature. Evaluated using the F-statistic, the best model, with a standard error of less than 3V , was a voltage source, opposed to the system voltage, dependent only on arc length. Current only influenced arc lifetime. Varc=l3v+l68v/inch*arc length This model agreed with low current (<1A) zero length sparks and a 55miI, 60 amp peak AC arc. It does not agree with the high current model developed from long AC arcs: Varc=37v/inch*arc length. The arc voltage between thin wall (7 mil) wire is predicted to be 15.5V, which, in a 28V power system, would reduce the bolted fault current by 55%, regardless of the insulation type.