What IEC standard defines BDV limits for transformer oil?

The key IEC document for BDV measurement is IEC 60156, which defines the test method for AC breakdown voltage of insulating liquids using standardized electrode shapes, gap settings, and voltage rise rates. It is a method standard rather than a single universal pass fail limit, because acceptance criteria can vary by utility practice, equipment class, and whether oil is new, treated, or in service. Many asset programs combine IEC 60156 results with moisture, tan delta, acidity, and DGA to decide treatment.