Electrical insulating oil is the “life blood” of power transformers.  The condition of the oil greatly affects the performance and service life of transformers and other oil-filled electrical apparatus.  A combination of electrical, physical and chemical tests are performed to measure changes in the electrical properties, extent of contamination, and the degree of deterioration in the insulating oil. The results are used to establish preventative maintenance procedures to avoid costly shutdowns, avoid premature equipment failure and to extend the service life of the equipment.

Dielectric Strength Breakdown Voltage (kV)(ASTM D1816, ASTM 877)

This measures the electrical stress the oil can withstand.  The presence of contaminants (including moisture) will lower the dielectric breakdown voltage.

Acid (Neutralization) Number (mg KOH/gm) (ASTM D644)

The acidity of the oil increases as a result of oxidation.  The higher the acid number, the greater the extent of oxidation and the sooner that sludge will begin to form.

Interfacial Tension (Dynes/cm) (ASTM D971)

This measures the presence of polar components in the oil.  Some of these polar components will eventually form acids.

Colour (0-8 Color units) (ASTM D1500)

As transformer oil ages, it slowly changes from a transparent pale-yellow colour to an opaque brown colour. This test will help determine when the oil should be filtered or replaced.

Water Content (ppm) (ASTM D1533)

The presence of moisture represents one of the most common causes of unfavorable dielectric properties of insulating oil.  High moisture in the oil will result in low dielectric breakdown voltage.

Oxidation Inhibitor (%) (ASTM D4768)

Oxidation Inhibitor (DBP/DBPC) is added to the oil by the manufacturer to prevent oxidation of the oil.  These inhibitors react with oxygen and eventually are depleted in the oil.  The absence of this inhibitor will promote oxidation of the oil.

Power Factor (%) (ASTM D924 25ºC and 100ºC)

Power factor is a measure of dielectric losses in insulating fluids due to heat dissipation in an electric field.  High values indicate the presence of contaminants, oxidation products and metals.

Metals in Oil (ppm) (ASTM D7151)

Dissolved metals in oil can provide vital information about transformer operation and pinpoint fault locations.


Proper sampling of insulating oil is extremely important.  Flush the outlet adequately before taking the sample.  The sample should be collected in a clean and dry plastic quart bottle (1000 mL).  Follow the recommendations in ASTM Standard D923 (Standard Practices for Sampling Electrical Insulating Liquids).