The dielectric test is carried out using an insulation tester (Mega-ohm-meter). The insulation test of the winding is conducted and the values noted after 1 minute. The test is continued and the reading is noted after every minute. Ten readings may be taken.
The readings thus taken are plotted on a graph with respect to time. The curve obtained will look like curve A in the diagram. This curve A indicates that the insulation resistance increases as time increases. This is because the insulation gets polarized as the test voltage is applied.
As the insulation gets polarized less and less charge carriers are available for the current. This causes the resistance to increase.
If the curve is flat (Curve B) or drooping downwards, it indicates that the insulation is in bad condition and should be repaired.