How to Check for Electrical Cable Insulation Faults

Good cable insulation has high resistance; poor insulation, relatively low resistance and cable faults can show very low resistance or a zero insulation indicating a short circuit and/or fault.. The actual resistance values can be higher or lower, depending upon on the temperature or moisture content of the insulation  

With a little record-keeping and common sense, however, you can get a good picture of the insulation condition from values that are only relative and an indication if the cable has a fault.

To check the cable insulation values we commonly use The Megger insulation tester which is a small, portable instrument that gives you a direct reading of insulation resistance in megohms. For good insulation, the resistance usually reads in the high megohm range.

The Megger insulation tester is a high-range resistance meter (ohmmeter) with a built-in direct-current generator. This type of meter is of special construction with both current and voltage coils, enabling true ohms to be read directly, independent of the actual voltage applied. The Megger tester is non- destructive; it does not cause deterioration of the cable insulation.

The generator is often line or battery-operated to develop a high  DC voltage which causes a small current through and over the surfaces of the insulation being tested. This current (usually at an applied voltage of 500 to 1000 volts or more) is measured by the meter, which has an indicating scale. which reads increasing resistance values from  0 up to infinity,