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The insulation failure of multi-turn coils may cause significant damage to hydro generators including both the stator winding and core. A number of stator winding faults have been occurring on hydro generators in the last ten years as result of a turn-to-turn insulation failure. The insulation failure is complex in nature and is attributed to various design and operating factors, thus making the interpretation of the turn insulation condition extremely difficult. The purpose of this paper is to provide typical cases of failure and to share the experience in investigating these turn-to-turn faults, in order to increase knowledge about turn insulation failure. The requirements of insulation design and testing for new stator winding are specified, especially turn insulation. Surge protection to reduce the turn insulation degradation is also presented as the lesson learned from these cases. Some suggestions are also made that may enable utility owners to reduce the risk of multi-turn coil winding failures.