Insulation resistance

The Insulation Resistance (IR) test
The Insulation Resistance (IR) test or ‘Megger’ test as it is sometimes known, is used to quantify the insulation properties of the product when subjected to a 500V d.c. test between live parts and the Earth (protective conductor) of Class 1 products (live parts and outer surfaces for Class 2).

As one of a series of "core" electrical safety tests, this test is a requirement within National, European and International legislation. The IR test should be applied following a Flash test, as a means of checking that insulation is still sound after being temporarily stressed by the higher voltage test.

In application, ‘live parts’ refers to both the Line (Phase) AND Neutral paths, and these are consequently joined together to form one test point, with the other test point being the Earth (protective conductor) for Class 1 products, or outer surfaces of the product for Class 2. Product switches should also be ON to include all parts of the supply path.

In the majority of standards, acceptable values of Insulation Resistance are a minimum of 2 Megaohms for Class 1 products and 7 Megohms for Class 2 products. It should be noted however, that with modern materials and manufacturing methods much higher levels of insulation are achievable.

On the other hand, some electronic equipment may use mains filtering devices that contain capacitive elements connected between live parts and Earth. These will have the effect of reducing the measured insulation values but not necessarily compromising product safety. Where resistance values fall below acceptable limits, it may be necessary to investigate further before determining conformance or otherwise.

Products incorporating mineral filled heating elements may also display signs of poor insulation, due to moisture ingress when the element is in a cold state. Where this is suspected, warming the element may drive out absorbed moisture enabling more realistic insulation values to be obtained.