In the late 1980s and again in the past few years, some turbine generator stator windings have apparently failed due to a mechanism variously referred to as spark erosion or vibration sparking. The mechanism can produce relatively intense sparking between the surface of the stator bar and the core. The intensity of the sparking is such that it may erode the groundwall insulation much more quickly than slot discharges. Unlike the normal loose coil/slot discharge failure process, spark erosion can happen anywhere in the winding, and not just in stator bars that are operating at high voltage. For this mechanism to occur, apparently (a) the partly conductive slot coating much be much more conductive than normal; and (b) the bar must be loose in the slot and vibrating under the magnetic forces.