I was under the impression that this test would tell you the quality to the insulation on the wiring, no? I thought it would give you some indication of the quality of the insulation covering the conductor. Is this not the case?
Step away from the my question on "stress" for a moment, you have to bear with me here as I am trying to get a clearer understanding of this.
Can someone explain to me why you perform that test? Is it to test the quality of the insulating material around the conductor?
How do you get the same results with or without insulation? Now I'm just getting confused...thanks for the reply though!
So this will tell me about bad and faulty splices or terminations then, correct?
But it won't tell me if insulation has been compromised on wiring in, lets say, a part of an attic that you can't access?
First you need to understand the megger a little better. A megger is just an ohmmeter that uses a higher voltage source for measuring resistance.
If you want to test insulation integrity of conductors in a piece of cord, you can connect the megger to two different leads and press the test button. If there is a really high reading this means the insulation is in good condition for the particular voltage you used to make the test. If there is damage to the individual conductor insulation on just one conductor you may still get a similar reading, damage of insulation of both conductors still will not likely yield a bad reading, but put some conductive contaminants between them and now you may have a high resistance path, but low enough the megger can push some current through it at whatever your test voltage is.
With metal sheathed cables and metallic raceways, you have the conductors in contact with the sheath or raceway that is grounded. If you apply a test voltage of at least the same level as operating voltage you will have some idea of condition of conductor insulation in relation to the possibility of developing a fault path to the raceway or sheath. Testing at a voltage above normal operating voltage is often done to get some predictive maintenance data, or even a simple pass/fail result.
With the knob and tube, you can still meg between conductors, but lets assume for a minute that the circuit you are testing is installed properly, has not been buried in thermal insulation or other debris, and is still attached to all the knobs, there is not much chance of anything to create a conductive path between the two conductors. So if you meg it and get a poor result, you know for certain there is something to be concerned about. But if you meg it and get a good result, you really have no idea what condition the actual insulation is in, you just know you don't have a path between the two conductors.
Go hang a couple bare wires on porcelain insulators and keep them maintained at least 4 inches apart, then connect your megger leads one to each of the bare wires. As long as there is no conductive dusts or other coatings on the porcelain insulators to create a conductive path they will test good even though they are bare. Now spray the test installation down with water and meg it. You will likely get a lower resistance but will increase as things dry out.
