The main causes of failure in a transformer result from a compromise of the internal insulation. This results from the cumulative effects of aging, mechanical damage or, in some circumstances, design or manufacturing faults.
Insulation within most transformers is essentially paper impregnated with oil. This insulation can be compromised during the transformer's life by mechanical stress (vibration) or certain aging processes degrading the paper and the oil. These processes are well understood by Schneider Electric.