Bubbling from the conductor paper insulation can happen when the temperature of the conductor is high and the paper wrapped around the conductor is sufficiently wet. In this situation, water vapor bubbles are created in the voids of the paper. These bubbles tend to rise and accumulate on the surfaces of the paper, thereby reducing the dielectric withstand between turns and sections of the winding. This can cause a turn-to-turn failure (arcing) which can result in a catastrophic failure of the transformer.