A typical transformer which is operated at its rated temperature rise has a life span, where it can be operated in a dependable reliability, of around 20 to 30 years. Transformers can still be operated even if it has already exceeded its expected life span but in lesser reliability. The following curve shows the typical damage curve of a transformer.
It means that, although, the transformer can still be operated, the probability of failure is high and, continuously, becomes higher as time passed by. However, if the temperature is operated at a temperature rise 8 degrees lower than its rated value, its life span can be increase by half of its projected span.
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