The Basics of Insulation Resistance

Insulation Resistance

Taking insulation resistance readings is one of the most basic tasks in regards to electrical installation and maintenance. Insulation resistance readings are performed in order to provide verification of the integrity of any insulating materials such as cable and wire or motor and generator winding insulation. 

It is vital that any type of electrical insulation has the opposing characteristic to the conductor in order to be resistant to the current flow and therefore keep it in within the conductor. Find a great range of IR testers at RS Components.

Why insulation resistance testing matters

Regular testing can help to analyse and predict how equipment is currently performing and how it will perform in the future. Detecting problems at an early stage can help reduce the need for massive repairs and save money.

Insulation resistance testing is regularly performed as a form of electrical testing and preventive maintenance for the likes of cables, electrical machinery, rotating machines, switches and transformers. It helps to detect possible electrical issues in order to reduce the likelihood of premature equipment replacement and repairs. 

How insulation resistance testing works

The execution of insulation resistance testing is carried out with the application of a constant voltage to the particular item of equipment and measuring the current flow. The test makes use of a high DC voltage, which has the result of a small current flowing through the surface of the insulator. 

There are three components in the total current:

  • Capacitance charging current
  • Absorption current
  • Leakage current

Capacitance charging current is fairly high on start-up but experiences an exponential drop within seconds and when the reading is being taken is usually negligible. 

Absorption current decays at an ever-decreasing rate and several minutes may be necessary before it reaches zero, depending on the nature of the materials used in the insulation. 

Leakage current is actually quite constant over time.

Basic test equipment for insulation resistance testing

The basic item of equipment that serves as an IR tester is known as Megohmmeter (sometimes simply referred to as a “megger”). Contaminated insulation can be detected by the observation of the absorption current over the course of a particular time period. 

Data from insulation resistance tests can be used for the establishment of a trending pattern that has deviations from the baseline info to allow for the evaluation of the insulation. The test results, which are normally delivered in Megohms, depend on factors such as the temperature of the insulation material and the humidity of the environment during the test. All readings must therefore be corrected to the base temperature, usually around 20 degrees Centigrade. 

IR testers are available in a wide range of different output voltages and sizes dependant on the specific nature of the application. 600V class equipment can normally be tested by a handheld 1000V Megohmmeter, while bigger sets that are intended for use on higher voltage equipment can actually output as much as 15,000V and sometimes even more. 

Periodic testing is by far the best way to implement preventive maintenance for electrical equipment.