Insulation Resistance

Paper and plastic film capacitors usually have insulation resistance values ranging from 6000 to 12000 M.

The insulation resistance is given in Ohm. This is not quite explicit because the insulation resistance changes for a time after voltage is applied - the self-discharge constant = Ris x C is also used to measure the quality of the insulation.

The time constant gives the time in seconds during which the voltage between the terminating wires of a charged capacitor decreases to 37% due to self-discharging.

With capacitance values in the µF range, the time constant at the time of delivery is usually between 2000 and 4000 seconds.

Humidity which penetrates into the capacitor winding, lowers the insulation resistance.
An appropriately thick casing should therefore be provided, according to how much humidity the capacitor will be subjected to.

Good insulation resistance is necessary for capacitors which are used to block off DC voltage and for storage capacitors in which a particular voltage rate has to remain unchanged for a longer period of time.