Normally when Megging a piece of electrical apparatus we just hook up our leads and go… but there is an industry standard out there that tells us to hook up our Black lead to the insulated piece in question (wire/windings/ buss, etc) and our Red lead to ground( forget the guard for the question), counter intuitive as that may be…
I thought the reason why was electrokinesis, but cannot remember, nor can I remember the industry standard.
Does anybody here have the industry standard reference and supporting information? T
With older insulation, electroendosmosis causes a lower reading to be obtained with the positive terminal connected to the grounded side of the insulation under test.
Modern insulating materials give little difference in the reading, if any, regardless of which way the leads are connected. Each megohmmeter terminal outputs equal voltage at opposite polarity.
In an electric field, water molecules align themselves so that their positive ends face one way and their negative ends face the other. Once aligned, the water molecules favor negative charges. For this reason, I tend to always put the negative lead on the device being measured.
There are exceptions with transformers. A senior tech once told me the Megger guard terminal is the same potential as the negative terminal. I’m not sure if this holds true with modern test equipment, but I think this should be taken into consideration when guarding equipment that could be damaged by a high test voltage, such as a low voltage transformer winding.
Sorry I can’t provide a standard but you can find many references to this in Megger literature. Check out this discussion in the archives for more:
Ty very much for your reply! Means alot!