Power-factor or dissipation factor ( tangent of the loss angle) testing of insulating oil shall performe in accordance with American Society for Testing and Materials (ASTM) D 924, Standard Test Method for Dissipation Factor (or Power Factor) and Relative Permittivity (Dielectric Constant) of Electrical Insulating Liquids as below: 
The overall PF test results of oil-filled power and distribution transformers indicate the insulation condition of the solid windings, oil, barriers, bushings, etc. The overall PF value for individual windings-to-ground and inter windings insulation of modern oil-filled transformers should be 0.5% or less, corrected to 20