Abstract :
Herman Halperin (Commonwealth Edison Company, Chicago, Ill.): Doctor Race´s findings on the mechanism of tracking in high-voltage cable insulation agree with our findings in single-conductor solid-type 66-kv cable (“Studies of Stability of Cable Insulation,” Halperin and Betzer, AIEE Transactions, volume 55, 1936, pages 1074–82). We found, however, that tracking can start also at the sheath. While in our tests tracking was apparently started by gaseous ionization, free gas was not present in Doctor Race´s samples and breakdown occurred at high stresses in the free oil space. Observations in Chicago on joints and on cable with expanded sheath showed that oil layers of considerable thickness may break down under lower stresses than thin oil films. The thickness of the oil film is, therefore, an important factor to consider.
