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Wiring Riddle No.13 - Hipot on a 600 volt THHN wire
Had a vendor come out to test a 200 Hp motor and someone allowed him to Hipot our motor feeder that was fed from a size 5 Magnetic motor starter about 200 ft away. During testing he stated that the cable failed. Is it a practice to hipot 600 volt cables ? From my expirence I believe he put a pinhole in the cable which caused leak over . Is this possible ?
Author : Stephen Alewine
Wed, March 13th, 2013 - 08:21
In dc insulation test, sudden application or removal of the voltage can be creates a huge dv/dt and abnormal stress. The test voltage should be applied and removed gradually. If a large overvoltage is applied to insulation, the smaller air voids in insulation body will become charged. If the insulation is then suddenly reconnected to the power system, it may fail due to the addition of the system voltage to the still charged voids. Also during test, related leads should not be allowed to touch each other because this will induce leakage paths. Sharp points at the test cable connections should be avoided to prevent corona discharge. On the other hand the test circuit should be in controlled situation. In fact that mentioned way for insulation test will face some uncertainly situation and may not recommended. 
Author : Hamid - From: Iran
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