Insulation Resistance to determine faulty section of wire

[Gyo]

Imagine you have a piece of long wire that consist of the copper conductor and insulation (EPR or XLP). There was a puncture on a small section of the wire. I have seen an engineer using a Meggar with one probe on the puncture area, and the other probe on the copper conductor to determine the insulation resistance of that defective area. Is this a valid test? Why?

 

[7anoter4]

I don't think a megger could detect such a small pinhole in one point of insulation, but the "Spark Tester " would.
See:

http://www.zumbach.com/e/product/spark_testers.asp

http://www.google.co.il/url?sa=t&rc...urce=web&cd=3&sqi=2&ved=0CDcQFjAC&url=http://

http://www.prysmianusa.com/export/s...WK4YE4&usg=AFQjCNH3zUAznWPuQjDaauIGZ5aHKGe8ww

 

[waross]

That may depend on the answer to several questions that the engineer doing the test must answer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rhatcher]

There are a lot of variables in a question like this so it is not possible to accurately judge the actions of the engineer that you refer to. However, we can discuss general cases.

Case 1: Assume that the diameter of the puncture is greater than the diameter of the meggar probe. You insert the meggar probe into the puncture until you hit the bottom. The meggar will accurately measure the insulation resistance of the remaining insulation or, if the puncture is complete, the probe will contact the conductor and the meggar will show zero insulation resistance.

Case 2: Assume that the diameter of the puncture is smaller than the diameter of the meggar probe. You place the probe over the puncture site. The meggar will measure the insulation resistance of the air that occupies the puncture cavity. If the resistance of the air gap is sufficiently large, you may not detect the puncture nor be able to determine if it completely penetrates the insulation.

A better method would be to immerse the wire in an insulated (plastic) bucket of water with the ends of the wire outside of the water. Place one meggar probe on the conductor and the other probe into the water. The water will penetrate any punctures no matter how small and will allow a measurement of the remaining insulation or, if the puncture is complete, it will contact the conductor and show zero insulation resistance. This method is similar to the method used to test high voltage gloves.

 

[jraef]

Or you assumed it was a megger and it was actually a TDR, Time Domain Reflectometer.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Gyo]

Thanks for the response.
I was wrong in my first post. It was actually a splice between 2 wires. The splice was hand-taped. So the damage is more like a crack.
I was there when the Megger test was done to test the insulation resistance of the wire. It is more like rhatcher's Case 2, the probe of the megger is a crocodile clip. The probe was clipped around the suspected joint, and the other probe to the conductor. I had some electrical background but I struggle to make sense out of the test being conducted. I would thought when the wire is energised by 500V, the megger is measuring total leakage current. I assumed it is like a long capacitor with parallel current crossing the insulation. But I can't see how the megger can measure an area of the insulation of the splice. I would like an independent opinion on this.

 

[Gyo]

The suspected joint was being repaired, and the whole wire will be submerged in a bucket. But by doing so, the insulation test could not pinpoint the precise location should the wire fail the test or displayed low insulation reading.

 

[Gyo]

jaraef, we do not have a TDR.