What should be the dielectric strength of transformer oil in operation?

In service, acceptable dielectric strength (BDV) depends on the utility’s limits, voltage class, and moisture control strategy. New mineral oil is typically much higher than in service oil, but operators often set minimum BDV thresholds to ensure adequate insulation margin and reduce partial discharge risk. Low BDV usually points to moisture, particles, or aging byproducts, so BDV is interpreted together with moisture, acidity, and DGA results rather than alone.