What is the minimum dielectric strength required for transformer oil?

Minimum dielectric strength depends on whether the oil is new, processed, or in service, plus the test method used. A common reference test is IEC 60156, which defines breakdown voltage measurement for insulating liquids at power frequency. Utilities set acceptance thresholds in specifications and maintenance guides, often higher for new oil and slightly lower for in service oil after considering moisture, particles, and equipment criticality. The key point is that dielectric strength must be high enough to prevent breakdown under expected electrical stress with margin.