I'm using a Fluke 1652c as my MFT. When I do the Insulation resistance tests at 500v - sometimes the result is displayed immediatley i.e >500 Megohms , other times the resistance seems to climb slowly, peaking around a figure. As this figure is generally massively in excess of the required 2 Megohms should I be worried about it ? Why would the level of resistance climb if the voltage remained consistent?