Insulation Resistance Measurements

At 1000V you will be seeing all the effects insulation can throw at you in everything, the PCBs,  the relays, perhaps the flux used to solder the PCBs, and any finger prints on the exposed ends of wires -  everything will matter at this level of resistance - to get the best part of 10 gig-ohms, you are asking for  something like 100nA or less to flow, now that  that is a mere 10^12 electrons per second. - what area of insulation are you stressing -  each atom is ~ 250pico metres radius, or if you prefer to think of it as atoms, then each square mm of insulation to live metal interface exposes about  16 million million atoms, it is not reasonable over many hundreds of square mm of contact area between insulation and  live metal not to expect a detectable percentage of the atoms to have either a free electron (steady leakage current ) or a slightly wobbly one (slow polarisation and dielectric recovery)


If you really do need to measure giga-ohms on the test specimens, then you need to be gapping the insulation to give longest possible creepage paths and large clearances to keep the E field down, cleaning everything till it gleams, baking it to drive of moisture and then encapsulating as much as possible in HV laquer, or alternatively after cleaning and bake off, doing the whole test in vacuum. 

Can you test at lower voltage ? - if the resistance looks higher , you may also be suffering the onset of corona discharge to the air. This can be tracked down, either using a MW radio, or a UV detector. (or just hopefully polishing off all surfaces that are not smooth - cut wire ends make quite good needle electrodes.)


But before all that, as others have said, check if you really do need to measure 10 gigohms, or if a lower pass limit can be set - a few million ohms is normally OK for a  a wiring installation, and much easier to handle.

At 1000V you will be seeing all the effects insulation can throw at you in everything, the PCBs,  the relays, perhaps the flux used to solder the PCBs, and any finger prints on the exposed ends of wires -  everything will matter at this level of resistance - to get the best part of 10 gig-ohms, you are asking for  something like 100nA or less to flow, now that  that is a mere 10^12 electrons per second. - what area of insulation are you stressing -  each atom is ~ 250pico metres radius, or if you prefer to think of it as atoms, then each square mm of insulation to live metal interface exposes about  16 million million atoms, it is not reasonable over many hundreds of square mm of contact area between insulation and  live metal not to expect a detectable percentage of the atoms to have either a free electron (steady leakage current ) or a slightly wobbly one (slow polarisation and dielectric recovery)


If you really do need to measure giga-ohms on the test specimens, then you need to be gapping the insulation to give longest possible creepage paths and large clearances to keep the E field down, cleaning everything till it gleams, baking it to drive of moisture and then encapsulating as much as possible in HV laquer, or alternatively after cleaning and bake off, doing the whole test in vacuum. 

Can you test at lower voltage ? - if the resistance looks higher , you may also be suffering the onset of corona discharge to the air. This can be tracked down, either using a MW radio, or a UV detector. (or just hopefully polishing off all surfaces that are not smooth - cut wire ends make quite good needle electrodes.)


But before all that, as others have said, check if you really do need to measure 10 gigohms, or if a lower pass limit can be set - a few million ohms is normally OK for a  a wiring installation, and much easier to handle.