Question

How is a megger test performed on an XLPE cable? Like, to what parts are the megger terminals connected to? And what does each result indicate? Like what does 0 mean, and what does infinity mean? Please provide me with a simplified circuit of the connection between the megger and the cable.

Answer 1

Hi champ,I will help you in this .

As you know meggar is a tool that really helps in measuring insulation resistance. It usually contains a hand driven generator which is usually powered by DC supply. Some meggar employ batteries as a source of DC power. However, majority of Meggars are usually made up of hand driven DC generator along with an ohmmeter.

Now coming to the function, whenever the hand driven DC generator is rotated with hand at a at a particular speed, EMF generated. The generated DC voltage is passed through the wires that it usually comes out and these wires are placed across the two terminals of the the cable, thereby closing the circuit. Depending upon the resistance of the cable, the DC current starts flowing. Depending on the magnitude of the current, the deflection torque is produced in the ohmmeter thereby giving the corresponding resistance of the cable.

Coming to your question, the emitter usually calibrated from zero to infinity in in which if the metre reads 0, it implies the insulation resistance value is zero ohms which inturn means the cable is shorted at some point.

If the metre reads infinity it implies that that the insulation resistance value is infinity.

Normally meggar measures the insulation resistance value in Mega ohms.

Coming to the xlpe cable which means cross linked polymer polyethylene cable, it is mainly used as underground cables and these cables have mainly two parameters

1. Insulation resistance 2. Capacitance

It is important to note that the insulation resistance of the cable is inversely proportional to the length of the cable which made the insulation resistance decreases if the length of the cable increases Whereas the original resistance value increases with increase in length provided the cross-sectional area is uniform.

But for xlpe cables, along with insulation resistance the capacitance value also plays a vital role. Capacitance mainly mainly depends on the diameter and sometimes on the length of the cable. As voltage gradient increases near the conductor compared to the sheet capacitance , IT results in severe harmonics in cable.

I have recently conducted megger test on xlpe cable and I can share the result so that it might help you

Recently in our institution there occurred a major fault in which the underground cable from the Transformer to the LT panel is burnt since the cable has gone through the roots of a tree whose roots are penetrated through the cable thereby damaging it completely. At first we are unable to find find the fault, later we have separated the cable from the Transformer and the LT panel and started testing the cable. Usually employees 350 square mm cable in which each phase R,Y,B is of 100sq.mm and a neutral of 50sq.mm.

So we started the continuity test. You should get following results for an non faulty cable

1. Short two terminals of meggar ------> zero

2. Between R,Y; Y,B ; R,B .-----------+> infinity

3. Between R,N ; Y,N ; B,N --------> 200MEGA OHMS

in our case I got reading of R,B as Zero. So the cable is shorted some where so we digged the entire trench and found fault under the trees... It's so difficult to find failf locslocaof UG.

But champ, be cautious with meggar . Don't ever touch the meggar wires when it is hand driven because edison(DC) is powerful than Tesla(AC) in taking lives. If you touch,you can meet Edison in heaven.

Stay safe... Thanks for reading... All the best.. any queries are always welcomed...