Discharge lighting will randomly trip 6ma GFCI devices - be it Fluorescent or HID.
Must be the Lamp(s) charge state "falling" back through the Grounded Conductor, which comprises the imbalanced load, and ultimately trips the GFCI (that's the only thing I can see causing the random trip issues).
As to the continuity between Line and Equipment Ground, there should be an extremely high Resistance value - above 10 Meg Ohm.
Should find a very low Resistance between L-N, unless the Ballast is a straight Linear Reactor without an Autotransformer section, in which case there will be a very high L-N Resistance; not >10 MOhm, but above 100KOhm.
In your case, maybe a fixture has a pinched lead grounding out.
That would definitely trip the GFCI, as some level of current will be flowing on the equipment Grounding Conductor - resulting in an imbalance between L-N at the GFCI device.
The way you describe the circuit charging up (went from 500KOhm to 700KOhm), sounds like a Capacitor is being charged in at least one of the connected fixtures.
If these are HPS (High Pressure Sodium) or PSMH (Pulse Start Metal Halide) type Lamps, then the Ignitor may be what is charging between Line and Eq. Ground.
If you plan to Megger the fixtures, only megger the Ballast's Windings. Disconnect the Capacitor and the Ignitor (if used).
Also remove the Lamp from the socket, and disconnect the input branch circuit (L+N) before testing with the megger.
Connect one of the megger's leads to the Ballast Winding's input termination; and the other megger lead to the fixture's housing.
A quick and dirty method to see if your Ohm Meter was seeing a Capacitor is to place the Ohm Meter on a lower setting - something in the 100K range; then take a reading like you did before.
1: First discharge the circuitry by physically shorting the Cord Cap's blades from L-N, L-G and N-G
(use a piece of steel stud as a shorting device!!!)
2: Connect the Meter's Leads, and verify the "charging characteristics" explained below:
*a: Reading should begin at a low number, like 100 - 500 ohms,
*b: Resistance should rapidly increase,
*c: Resistnace should taper off at around 100K, or display "Infinity"
If the Meter's readings do the opposite, you are most likely reading an Ignitor.
Good luck.
Scott
