Expert: Dave Nyce - 12/10/2006

Question
Dear all
1.What is the concept behind meggering a cable and the voltage we inject?Why we inject?
2.Let say the reading is low,a few Ohm,we conclude the cable is not good.Insulation is low.Where is the current flow(since we injected V so there must be I).Is it shorted to somewhere,thats why the resistance is low.

Answer
I would not call it "injecting" a voltage.  I would call it "applying" a voltage.  With two conductors that are supposed to be insulated, the amount of insulation can vary depending on the thickness of the insulating material, the amount of moisture, or from being contaminated with an electrically conductive substance.  The insulation could be thin in one or more spots due to improper manufacture, or due to overheating, or due to applying excessive mechanical force, etc.  So, megging the insulation is intended to check for any of these imperfections.

When the voltage is applied across two conductors that are supposed to be insulated, the megohmmeter measures the current that flows.  If the current is lesss than the specified amount, then the insulation is meeting the specified minimum.  If it fails the test, then there is a problem such as I mentioned above.

Hope this helps!

Dave