Tan Delta testing - Principle and Method

Tan Delta is a a diagnostic test conducted on the insulation of cables and windings. It is used to measure the deterioration in the cable. It also gives an idea of the aging process in the cable and enables us to predict the remaining life of the cable. It is alternatively known as the loss angle test or the dissipation factor test.


The Tan Delta test works on the principle that any insulation in its pure state acts as a capacitor. The test involves applying a very low frequency AC voltage. The voltage is generally double the rated voltage of the cable or winding.

A low frequency causes a higher value of capacitive reactance which leads to lesser power requirement during the test. Besides, the currents will be limited enabling easier measurement.

In a pure capacitor, the current is ahead of the voltage by 90 degrees. The insulation, in a pure condition, will behave similarly. However, if the insulation has deteriorated due to the entry of dirt and moisture. The current which flows through the insulation will also have a resistive component.

This will cause the angle of the current to be less than 90 degrees. This difference in the angle is known as the loss angle. The tangent of the angle which is Ir/Ic (opposite/adjacent) gives us an indication of the condition of the insulation. A higher value for the loss angle indicates a high degree of contamination of the insulation.

Method of Testing

The cable or winding whose insulation is to be tested is first disconnected and isolated. The test voltage is applied from the Very Low Frequency power source and the Tan delta controller takes the measurements. The test voltage is increased in steps upto the rated voltage of the cable. The readings are plotted in a graph against the applied voltage and the trend is studied. A healthy insulation would produce a straight line.

The test should be continued only if the graph is a straight line. A rising trend would indicate weak insulation which may fail if the test voltage is increased beyond the rated voltage of the cable.

Interpretation of the test data

There are not standard formulae or benchmarks to ascertain the success of a tan delta test. The health of the insulation which is measured is obtained by observing the nature of the trend which is plotted. A steady, straight trend would indicate a healthy insulation, while a rising trend would indicate an insulation that has been contaminated with water and other impurities.