The overriding factor contributing to transformer failure is insulation breakdown. This occurs for two reasons: age, and operation at higher than rated temperatures.
Regardless of size or rating, all transformers have essentially the same design life. All dry type transformers are designed to last for 20 years at correct operating temperatures. However, life spans of 40 or 50 years are on record. The windings in a transformer are separated by insulating materials: (e.g. insulating paper). Each material has a temperature (referred to as the temperature class) beyond which it begins to break down and fail. There are three temperature elements that add up to the temperature class:
The winding - Current passing through a wire produces heat. (Very little in a lamp cord vs. a lot in a toaster element.) The average operating temperature within a coil is referred to as the winding rise.
The hot spot - The average temperature is just that, an average. Operating tests have shown that there are temperature variations from one point to another in a winding. Therefore, hot spot standards have been set which allow for an additional temperature rise of up to 30°C somewhere inside the coil.
Ambient temperature - This is the temperature of the air in the room in which the transformer operates. This limit has been set at 40°C (104°F) .
These three figures add up to the temperature class for each of the four insulation systems. Note, however, that transformer heat limits are always referred to by the average winding rise temperatures above a 40°C ambient: 55°C, 80°C, 115°C or 150°C.
