Understanding the factors that can affect Megger insulation testing readings is very important. There are a number of things that can affect current, such as temperature and humidity, but also time. When a current flows along and through insulation, it is made up of a relatively steady current in leakage paths over the insulation surface.
To better get an idea if the factors affecting insulation testing, it is vital for you to understand total current and the components it is made up of. Below we have listed the three components and how each component is affected by time.
Total current is made up of three components. Each component is affected by time in a different form; therefore, it is important to be familiar with what each component is and how it can be detected.
A capacitance charging current consists of current that starts out high and then drops after the insulation has been charged to fill voltage. For example, this type of current can be compared to water flow in a garden hose. When you first turn a garden hose on, the spigot shoots out water at a higher interval, after time, that interval drops. A capacitance charging current disappears relatively rapidly as the equipment being tested becomes charged.
An absorption current also consists of an initially high current and then drops. However, an absorption current decreases at a much slower rate. This rate is all dependent upon the exact nature of insulation being tested.
A conduction or leakage current is made up of a small, steady current that is present in both through and over the insulation. When good insulation is present, the conduction or leakage current should build up to a steady value that displays a constant, applied voltage. If an increase of a leakage current is showing over time, chances are there trouble with the insulation.