Hi, In some 11kV cables while doing VLF Hi-Pot testing, the IR values seems to be decreased after Hi-Pot. What is the reason?

Infect i do agree with Mr..Manzoor Umar, however while testing a minor leakage current or charge conduct from phase to ground and hence minor decrease in Insulation Resistance (IR) can be noted. But it falls in between the acceptable value. If there is faulty leakage current, device will indicate it as fault.

If there is a very high difference in insulation resistance after applying high voltage, then there must be defect in the insulation.