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Field testing for insulation breakdown of transformers

Per ANSI C57.94 “Recommended practice for installation, application, operation, and maintenance of dry type general purpose distribution and power transformers” Insulation resistance test values should not be used to determine if a dry type transformer is performing satisfactorily. 

Instead of Megger test, I would recommend to run a field dielectric test per "ANSI C57.12.91 10.2 dielectric tests in the field".

Per this standard, If no insulation breakdown is detected, the transformer should be considered acceptable.