Wikimiras

This is the often mentioned Megger test. It is meant to give some indication of the condition of the insulation, and is often used in maintenance procedures. The connections are the same as for the high-voltage test. The insulation resistance of a transformer depends largely on the temperature and cleanliness and dryness of the windings. Insulation resistance should be at least 1 megohm (I million ohms) for each 1000 volts of test voltage. If it falls below this figure, presence of dirt or moisture may be indicated.

Ref:
Electrical Transformers and Power Equipment by Anthony J. Pansini