Test on Transformer Oil

If all else fails, read the instructions.

Depends on the end use of transformer and how heavily loaded it is. An example I use is Arc furnace xformers. These are in most cases run to the max allowed and then some if production can get one more once of steel out. I have these test done on a yearly basis. Some of the other units suppling power around the shop I only pull every couple of years. But, that depends on my main concern of what happens if I loose it. How does it affect production, cost of losing production and so on. Along with the oil testing, thermal imaging is a good idea. To look at your bushings and other connections as well.

Hope this helps.