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A 500 MW generator failed catastrophically, it had an internal short that resulted in a phase to phase short. The generator stopped so suddenly that it sheared off 24 2 ½” bolts in the coupling between the generator and the turbine. The problem was traced to the phase leads resonance shifting to 120HZ as the generator aged. This problem resulted in the annual testing of the phase leads for the unit that had the failure and also on its sister machine. One of the key lessons learned during many hours of testing these generators was that for electrical components, the stiffness of the insulating materials is affected significantly by temperature. For instance, if a phase lead tested as acceptable at room temperature, it might experience a 10-15 HZ drop when it was at operational temperature, thereby being unacceptable in the loaded state. The extent of this effect was discovered by accident when the windows to the turbine building were left open on a cold night. It was discovered that the natural frequency results on the phase leads were significantly different when the leads were cold versus at room temperature. The lesson to learn here is to always be observant. The natural frequency of a vertical pump which is made of steel will not vary much whether the surrounding temperature is zero or 100 degrees, but electrical components that are covered with insulation can change.