It should be noted that the impulse tests by the manufacturer predated IEEE Std 522 [9] of which the first edition is dated 1977. The desired rise time of the impulse was not defined until the 1992 edition which states “The rise time of applied impulses should be between 0.1 and 0.2 μs.” Since the purpose of the test in 1965 was to detect turn defects it is assumed that the rise time of the impulse was less than 1 μs. The failures were near the knuckle.