The expected life span of the distribution transformers above 100 kVA capacities is about 35 years and in which of up to 100 kVA capacities is about 25 years. But experience displays that most of transformer failures start to occur even before 20 years of its life.
Until a few years ago, distribution transformer manufacturers incorporated several more safety factors in design. In present years, manufactures have adopted the cost-advantages approach in the design of transformers, that just about manages to satisfy the needs of IS specification. The result is that they compromise on both quality and reliability requirements of IS specification. Although the transformers so manufactured meet the requisite standards while tests are conducted before and immediately after installation, they fail to do so after a few years of being in operation because of ageing.
Therefore, several transformers are unable to serve the expected full life period and even if they are in service, they are quite likely to fail before the expected full life because of lower reliability. Hence, giving top priority to the replacement of those in-service transformers in which have served their full-life period will decrease transformer failure.