Effect of Sheathing on the Testing of Machine Coils Using Transient Submicrosecond Impulse Voltage

This paper gives a comparison of test results for impulse breakdown level of the turn insulation in electrical machine coils with and without grounded metallic sheath, using a Voltage surge with a risetime of the order of 0.2 ¿s. It was found that the breakdown of the machine coil in the sheathed condition was indicated by a large drop in the first peak voltage, along with a complete change in the voltage waveform.