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THE purpose of this paper is to report on work undertaken in response to a request, made to the Institute, to determine the minimum dielectric strength of oil for high potential testing of oil circuit breakers. Many tests were made with 60-cycle and impulse voltages. The tests were made with a rod to plane gap, cone to disc gap, and on oil circuit breakers of commercial types and sizes. The results of these tests have been collected, analyzed, and are included in tabular form in the paper. The impulse strength of oil circuit breakers was found to be practically unaffected by the 60-cycle breakdown strength of the oil within the range of 16 to 30 kv, as measured in a standard test cup. The 60-cycle dielectric strength of oil circuit breakers is shown not to be changed in direct proportion to the differences in the dielectric strength of oil as measured in a test cup with 60-cycle voltage. The effect of low dielectric strength oil on voltage breakdown is discussed and conclusions are drawn which show that it is desirable to test all oil circuit breakers with oil having dielectric strength of 22 kv or higher.