Why does Megger have DC generator? To know the fact behind it, we will thoroughly discuss the use of DC voltage for the megger test.
Megger is the trade mark of the company of an insulation tester manufacturer, 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.
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Megger Working principle
The megger has a DC generator inside it. The DC generator may be of manual type of or electronic. It generates a high DC voltage. The positive lead of the megger has its connection with the current carrying part. The negative lead of the megger is connected to body of the equipment.
Let, the DC generator voltage is V, and the insulation resistance is R. Then the megger or insulation resistance value of the equipment under test is,
R = V/I
We can measure the insulation resistance by applying AC voltage or DC voltage. Why we prefer DC voltage for IR measurement. We will discuss the reason of this in the next paragraph.
Why DC Voltage used in Megger Test ?
Before understanding the concept, first we must clear the current flow mechanism in the insulators.
The current can not flow through the insulator when applied DC voltage across it is less than the break-over voltage of insulator. Thus, we can say that capacitor functions like a low value capacitor. The reactance offered by the insulator is very high and, thus the current passing through it is negligible.
The insulator offers lower reactance when AC is passed through it( Xc=1/2πfc) because of higher frequency. Thus, the current flow through it is sum of the leakage current & the current due to lower reactance of insulator. These two types of current flowing through the insulator does not represent the actual value of insulation resistance. This is the reason of correct measurement of insulation resistance with use of DC voltage.
The perfect insulator has a very high reactance when AC voltage is applied across the insulator. Thus, the insulator entirely blocks the current. In the case of DC voltage, the current passing through the insulator gives the correct reading of the current. & hence the measured insulation resistance is perfectly OK.
From above discussions, it is very clear that we get the correct insulation resistance value on application of DC voltage across insulator. This the reason why the megger has a DC generator.
