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This is a common situation in industry where the 3-phase motor has a resistive reactance fault (as small as 1% unbalance) PRIOR to start. Insulation has been subject to excessive heating but meets IEEE std. for safe operation. The trip occurs as reverse torque is generated due to current unbalance. Overload relays protect & function as designed.

Upon resetting the "ground fault" - insulation appears in safe range. At start, current is in excess of the "norm" in the effected phase (with the reactance fault). Current exceeds the 7-25X fla (dependent on motor design) and insulation fails! I have utilized this analysis on 3-17,500hp motors with a 95-100% success rate at predicting a pending motor insulation failure!

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closed as unclear what you're asking by winny, Charles Cowie, Warren Hill, Elliot Alderson, Edgar Brown Feb 1 at 16:21

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    \$\begingroup\$ Hi and welcome. Do you have an actual question? \$\endgroup\$ – mike65535 Jan 31 at 13:54
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    \$\begingroup\$ Comment - in the above situation Surge testing proved in-effective. This was due to faults located at the end of stator coil grouping, where applied voltage was at a lower potential! \$\endgroup\$ – Robert Stokes Jan 31 at 13:58
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    \$\begingroup\$ You still haven't asked a question. Please add it to the original post (edit link below it) rather than in the comments. \$\endgroup\$ – Transistor Jan 31 at 14:04
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    \$\begingroup\$ Robert - please edit the original post to include a clear question. \$\endgroup\$ – mike65535 Jan 31 at 14:28
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    \$\begingroup\$ You have asked a question in the comments. At the bottom of your question is an edit button. Edit your question and add your comment question to it. \$\endgroup\$ – StainlessSteelRat Jan 31 at 16:05
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Welcome to Stack Exchange! The resistance reactive fault is likely due to a short in the windings - but the short is contained within the individual phase between two adjacent turns. This leads to a shorted turn in the winding path, with the associated high current and overheating of the winding during startup. However, because the wire is shorted to the adjacent turn, there is no current path to the body or other windings, so the insulation from phase to phase or from phase to ground will still provide isolation and pass the hi-pot test (until the catastrophic failure). If the short is a single turn, the change in resistance or inductance of the winding (measured at low current) will be small.

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  • \$\begingroup\$ This situation can normally be detected by metering inductance and resistance of the winding in question. If turns are shorted, both L and R will be lower than original values and lower than a good winding. \$\endgroup\$ – K H Feb 1 at 0:19

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