Megger brand is the model of insulation tester and it became so popular that insulation measurement is termed as 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 HV(High Voltage) lead of the megger is connected to current carrying part of the equipment and LV(low Voltage) lead is connected to ground.
Thus, the high voltage is applied between conductor part and ground part of the equipment. Let, V is the applied voltage and current flowing through conducting part to ground is 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 test. 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 though 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 lead to 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 insulation resistance value can not be correct if AC voltage is applied.
Now if DC volatge is applied to measure the insulation resistance value, the perfect insulator has high reactance value and it completely blocks the current. Thus whatever current flows through it is small leakage current which truly represent the actual leakage current. The magnitude of the leakage current shows imperfection of insulator.
The accurate insulation resistance can be done if the DC voltage is applied to measure the megger value. That is why a megger has DC generator.