Electronic measurement of surge-crest voltages

QUICK determinations of crest surge potential values by a simple direct method are often desirable in the laboratory to supplement the cathode-ray oscillograph which records the entire surge phenomenon. The generally used sphere-gap method of potential measurement has, however, the disadvantages that a number of tests must be made in the process of adjusting for the breakdown distance, and sometimes the effect of time lag^1,2 of breakdown must be considered. In particular, it has been found in routine laboratory surge testing in connection with insulation co-ordination that sphere-gap measurements are erroneous for short duration potentials even with irradiation³ of the gap when making tests of breakdown on the front of a sharply rising potential wave. Also on such chopped waves it is rather unlikely that the sphere-gap setting would be very close to the breakdown value and accordingly a large number of tests have to be made. It is at times essential that the actual breakdown voltage be recorded in a single test since some test objects can be broken down only once.