Insulation Resistance Test with Fluke 1652

[South Spark]

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?

 

[malcolmsanford]

You often find the the size of the circuit you are testing will be a factor in how long it takes to get a result.

 

[telectrix]

also, sometimes a slight bit of damp can give you a low reading, rising as the 500v dries it out.