Megger is a brand name of Insulation Tester. Due to the popularity of this brand, we often use this term to mean Insulation Tester and Megger value to mean Insulation Resistance value. Today, there are various other make of Insulation Testers available and therefore it is better to call it Insulation Tester instead of Megger. Let us now focus our attention to why do Megger or Insulation tester has a DC Generator?
Insulation Test is done to find the insulation resistance (IR). During this test, HV lead of the test kit is connected to the conductor and LV lead is connected to the ground. For example, in case of cable, the HV lead will be connected to conductor and LV lead to the ground. High voltage (250 V, 500 V, 1000 V or 5 kV as per the cable voltage rating) is injected by the kit. This high voltage causes leakage current through the insulation. This leakage current is measured by the kit to evaluate IR value.
As we know that a perfect insulator is nothing but a capacitor having small capacitance. How? Due to small capacitance, the reactance offered will be high leading to small value of leakage current. Also, the loss in the insulator will also be zero. This is the reason, a perfect insulator should be a perfect capacitor with small capacitance.
Therefore, if we apply AC voltage during IR test then there will always be a flow of current through the capacitor and thus a leakage current to the ground. Therefore, the resistance measured by Megger will not actually represent the Insulation Resistance.
If we apply DC voltage then as we know Capacitor blocks the flow of DC, only leakage current due to imperfection of insulator will flow and hence the Resistance measured by Megger will actually represent Insulation Resistance. This is the reason, an Insulation Tester or Megger have DC voltage generator.