As has been stated, electrical machines and their insulation systems are subjected to mechanical, electrical, and thermal stresses which give rise to many deteriorating influences, the most significant of which are the following.
4.1 Thermal Aging
Gradual aging caused by temperatures due to normal operating loads.
Unusually high temperature from causes such as overload, high ambient temperature, restricted ventilation, and loss of cooling liquid.
Unusually high voltage such as from switching or lightning surges.
This deteriorates electrical insulation by actually conducting current over insulated surfaces, or by attacking the material reducing its electrical insulating quality or its physical strength, or by thermally insulating the material forcing it to operate at higher than normal temperatures. Included here are:
Wetness or extreme humidity
Oil or grease
Conducting dusts and particles
Nonconducting dusts and particles
Chemicals of industry
4.5 Physical Damage
This contributes to electrical insulation failure by opening leakage paths through the insulation. Included here are:
Unusual electromagnetic forces Erosion by foreign matter
Damage by foreign objects
4.6 Partial Discharge (Corona) Effects
Partial discharges which may occur at higher operating voltages may be accompanied by several undesirable effects such as chemical action, heating, and ionic bombardment.