Sometimes it is very easy to find the cause of winding failure. Here this picture shows an alternator stator winding which is burnt due to the insulation failure of separation. Two nearby phase coils are separated using an insulation paper depending the class of insulation. If the IR value of that paper goes below a minimum value there will be a winding failure. It can happen in slot also. Here the insulation failure is at coupling side winding overhang.
Next picture shows a close-up image of the winding were it failed.
Motors and Alternators 
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