Insulation expected on an flyback transformer

[gkmaia]

I am testing the insulation between windings of a 220v to low voltage 100khz flyback transformer.

I am applying 1Kv with my insulation tester and I am getting really low results. For some winding pairs I get values  above 5Gohms.

But for others I get something at the 2Mohm range. I imagine that is way too low and for those windings the insulation between them has fried. Is that correct?

What is the expected minimum threshold insulation resistance between transformer windings at 1000v I should expect?
Should I expected different insulation resistance on ferrite vs iron transformers?

[Siwastaja]

It is possible those windings with smaller insulation resistance are not supposed to be insulated from each other. For example, it could be a primary winding, and a feedback winding, which are both referenced to the primary side. Or, you could have two output windings. In these cases, functional isolation is enough.