The reason high insulation resistance is desirable in capacitors is due to what is called leakage current. Taking the “bucket of water” analogy from Part One of the Capacitor Basics Training Module, leakage current can be described as the shown image here. The water is the charge, and the material that makes up the bucket is the dielectric material. It does not matter how good the material is, there will always be losses which contribute to “leakage current”. What will vary is the level of leakage the capacitor will have according to the dielectric material properties. On the right, a simple circuit diagram is shown on how a capacitor is charged through a charge resistor and voltage source. Immediately after applying a DC voltage to a capacitor, current begins to flow, and the capacitor is gradually charged over time. The current will decrease exponentially over time until the capacitor is fully charged. However, there will still be some remaining current flowing through the capacitor due to the finite insulation resistance. This current is called the leakage current and is used to calculate the insulation resistance using Ohms law as seen on this slide. KEMET recommends a charge time of one hundred and twenty seconds to ensure the capacitor is fully charged and that the remaining current being measured is only due to the leakage current. Note that since the leakage current is very small for ceramic capacitors, datasheets only specify insulation resistance.