The insulation resistance of a capacitor is a measure of its resistivity in DC.
Might be a little confusing, so let’s dispel any doubt with a simple principle.
Under a stationary DC voltage, a leakage current flows through the dielectric (between terminals, determined by the dielectric characteristics) and over the capacitor surfaces (between terminal and case, determined by the quality and characteristics of the insulating materials).
ZERO Leakage current: the ideal capacitor
The leakage current is zero only in ideal capacitors, thus in real word it’s not.
Insulation resistance (RINS)
This leakage current through the dielectric is usually converted to the expression insulation resistance by using Ohm's Law.
Do not confuse it with the equivalent series resistance (ESR) of the capacitor: the insulation resistance is also referred to as the parallel or shunt resistance of the capacitor.
What influence the Insulation resistance?
The insulation resistance depends on the dielectric and material characteristics; it varies also with temperature, applied DC voltage and nominal capacitance. In particular, it decreases with increased temperature, as well as increased voltage and capacitance. So, the related leakage current increases consequently.
Also the insulation Resistance has to be considered while selecting the capacitors for the application
Natural consequence of this fact is that this variable must be carefully taken into account by the customer during the design phase. Critically low insulation resistances, because of high operative temperature, high voltage applied to big capacitance and/or realized with particularly thin films can possibly cause important failures.
Aging due to long-term operations must also be taken into account because the insulation resistance tends to further decrease.
Enhanced risk due to multiple pieces in parallel
The already known critical condition of multiple pieces in parallel, in the event of clearing, can be considered a further risk factor if the insulation resistance is already very low.
As a conclusion, also for what concerns the insulation resistance, all the extreme and critical operating conditions need wide enough safety margins!