Effect of harmonic content on alternating breakdown voltages of spark-gaps and insulators

The peak values of alternating breakdown voltages were measured for rod-gaps and 12.5 sphere-gaps; the peak dry-flashover voltages of insulators also were measured. Tests were carried out for sinusoidal voltages and with waves containing third harmonics at different percentages and phase angles. The peak breakdown voltages were found to be sensitive to the instant of occurrence of the voltage peak over the half-cycle. They would increase by about 10%; if the peak occurred at the first quarter rather than the middle of the half-cycle. They would, on the other hand, decrease by about 7% if the voltage peak were retarded to the third quarter of the half-cycle. The harmonic content was found to have rather a small effect on the magnitude of the breakdown voltage if the voltage peak remained at the middle of the half-cycle.