What is the “8 degree C (Celsius) rule”?
The “8 deg Celsius rule” (”8 ° C Rule”) is a known “rule of thumb” for estimating the thermal life of insulation of a transformer and thereby the transformer life itself. It states that “The thermal life of Class 105 insulation is halved for each increase of 8°C or conversely doubled for each decrease of 8°C. Its basis is the nine years of data concerning the relationship of tensile strength of paper, aged in oil and in air.
The rationale of this guideline is found in the so called “10°C rule”, which has been widely applied to evaluate the rate of chemical reactions;
“By this is meant that the rate of specific reaction can be expected to double for each 10° increment of temperature increase, all other factors remain constant”.
Ref: A guide to Transformer maintenance by J.J. Kelly, S.D. Myers, and R.H. Parrish
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