heat run test on transformers

If I have to choose, I ll choose the coil that is wound on center limb, But otherwise I ll check all three,

Not sure if there is any standard on this issue.

GA. The stupid system is presently saturated, so I can't vote.

Checking resistance in all three phases is the correct procedure.

The life of the transformer depends mostly on the temperature of the insulation - the "hot spots" are the most likely place for "first failure". Consequently, although as a user you cannot identify the hot spots [the maker's problem!], it is necessary to measure all phases in a test to ensure the "temperature rise by resistance" limits are not exceeded.

thanks for the reply

during heatrun test after shutdown the resistance should be measured within how many minutes

Any standard which specifies which phase resistance to be measured?

You measure all the windings, primary and secondary e.g. 6 for Y-Y 3 phase transfo - how else can you know one winding is not exceeding specification?

You need to look at a standard e.g. IEC 76, to find the conventional limits for temperature rise and test times and corrections.

The resistance must be measured immediately the transfo is de-energised and at recorded times thereafter. Plotting the logarithm of the estimated temperature against time should give a straight line which can be extrapolated back to the actual time of disconnection, considering that de-energising and connection/reading of resistance test equipment take some time. Obviously, the time of de-energising must be the first recording written.

Without any knowledge of transformer size, the necessary time intervals cannot be given. In practice, you will have to make measurements as fast as possible until the cooling time is evident - in any case the temperature will change most quickly just after de-energise.