This article describes why DC voltage is applied in an insulation resistance test. Insulation acts as a capacitor and If we apply DC voltage then the capacitor blocks the flow of DC current, and only leakage current flow due to imperfection of the insulator. Therefore, on the application of DC voltage for insulation resistance measurement gives the correct value of insulation resistance or megger value.
Megger brand is the model of an insulation tester and it became so popular that insulation measurement is termed insulation resistance measurement or megger value. In general, we say what is the megger value of the cable. Here megger value means insulation resistance value.
What does megger or insulation resistance do for measuring the insulation resistance? The megger generates high voltage and the HV(High Voltage) lead of the megger is connected to the current carrying part of the equipment and LV(low Voltage) lead is connected to the ground.
Thus, a high voltage is applied between the conductor part and the ground part of the equipment. Let, V is the applied voltage and the current flowing through conducting part to the ground be I, then the insulation resistance value is;
R = V/I
The voltage applied for insulation resistance testing may be either AC or DC. However, DC voltage is always preferred over AC for insulation resistance tests. Why is it so? First, let us understand what is insulator.
The perfect insulator acts as a capacitor with very low capacitance and it offers large reactance ( Xc=1/2πfc ) and very low leakage current flow through it. An ideal insulator must have very low capacitance.
If AC voltage is applied, an insulator offers lower reactance, and current pass through the HV and leads to the ground which does not represent the actual leakage current value. The capacitor passes the AC easily and offers very low reactance. This is the reason the insulation resistance value can not be correct if AC voltage is applied.
If DC voltage is applied to measure the insulation resistance value, the perfect insulator has a high reactance value and completely blocks the current. Thus whatever current flows through it is a small leakage current that truly represents the actual leakage current. The magnitude of the leakage current shows the imperfection of the insulator.
The accurate insulation resistance can be done if the DC voltage is applied to measure the insulation resistance value. That is why DC voltage is applied in an insulation resistance test.