Fault Voltage Drop and Impedance at Short-Circuit Currents in Low-Voltage Circuits

In the paper are presented new data on (1) arc-voltage drop and current-limiting effects for arc faults in bar-conductor structures, (2) current-limiting effect of insulated-cable arc faults, (3) cable impedance in an iron conduit, and (4) impedance of low-voltage switchgear circuit assemblies, derived from tests at short-circuit currents in circuits with open-circuit line-to-line voltages of 208-600 volts at 60 cycles. The currents ranged from less than 10,000 amperes to values as high as 100,000 amperes rms in some cases. The new data dispel several illusions with respect to accidental faults in low-voltage bus structures and other low-voltage gear. Contrary to inferences frequently drawn from previously published data, the paper shows that (a). Faults in the typical low-voltage bus structures tested are not self-clearing. (b). Fault arcs do not show the drastic current-limiting effect which they are frequently assumed to exert upon short-circuit currents. Instances are shown where actual fault currents reach 100 per cent of the rms symmetrical values calculated for bolted faults. This study shows a self-clearing tendency for representative types of accidental faults in cable, but it was found that a large current-limiting action prior to current interruption cannot be depended on. The new data, supplementing existing impedance data, definitely indicate the possibility of reasonably accurate calculation of short-circuit currents in low-voltage circuits.