At work we heve 3 new meggers and they have a capatence test feature. This is not a capacitor test function but a insulation capatence. How can I relate insulation capatence to insulation quality. I know Doble testing measures capatence but it is done with AC. THANKS
Hmm, Now there is one for the books. What brand/model meggers are they? Are you sure you aren't referring to Insulation Resistance?. Reason I say that, is because capacitance is the function of Insulation Resistance.
[This message has been edited by Trumpy (edited 07-25-2006).]
All along the "cable under test" there will be small capacitances, caused by the cable insulation, between cores, the longer the cable, the higher the capacitance. This is why it is necessary to discharge the charge built up by meggering a certain cable. One question, what type and voltage cables are you testing?.
Capacitive charging current is one of the 3 leakage currents you are measuring with IR testing, it tends to decay pretty quickly as the cable is charged, we typically look at the capacitive element by calculating a DAR (Dielectric Absorbsion Ratio) which is determined by dividing the 60 second reading by the 30 second reading. Different ratios apply to different types of equipment but you should always have a DAR of at least 1.0 or more (>1.6 is excellent). >8.0 DAR however would usually means the insulation has become embrittled.
While I agree that DC hipot testing should be considered destructive, newer hipot methods are not and are acceptable for routine testing, VLF seems to be the leading canadite, but there is also PD ans Tan delta tests that are as effective and non destructive.