Monthly Archives: April 2016

Importance of using the correct DC Test Voltages for IR testing:

A critical component of the IR test itself is the DC test voltage level used during the process. The amount of leakage current that can be measured in an insulation’s dielectric material is directly dependent on the test voltage level being applied. IEEE, NETA, and ABS standards all confirm that when performing an IR test, the higher the test voltage level used the greater the ability will be to detect any defects that may be present in the insulation materials. Those defects, such as dirt or moisture, are what breakdown the insulation materials causing the insulation resistance to drop to an unacceptable level and eventually making the equipment unsafe to operate. Typically a 500 VDC or 1000 VDC test voltage is used for low voltage equipment and either a 2500 VDC or 5000 VDC test voltage is used for medium and high voltage equipment. IEEE Std.43-2000 and NETA MTS-2011 both contain industry standard guidelines for choosing the correct minimum test voltage to be used when performing IR testing on equipment operating at various voltage levels. These minimum IR testing voltages must always be adhered to in order to accurately measure the Insulation Resistance in all electrical equipment. Any test done at a lower test voltage level is considered to be inaccurate and misleading at best.

How Important is Insulation Resistance Testing?

How significant is Insulation Resistance Testing?
Since over 80% of electrical maintenance testing involves evaluating insulation integrity, the answer is Yes, it’s a very important test. This is because electrical insulation begins to age as soon as it’s manufactured and aging causes deterioration in the performance of the insulation. Harsh operating environments will also cause further deterioration, especially where the electrical insulation is exposed to extreme operating temperatures, moisture, and chemical contamination. As a result, personnel safety and operating reliability can both be compromised. It’s extremely critical to always know the electrical condition (IR) of the insulation in your equipment at all times.

What is an Insulation Resistance Test?
The Insulation Resistance (IR) test, commonly known as a “Megger” test, is normally used as a “Spot type” test to measure the insulation’s dielectric condition at a given moment in time. The test is performed by applying a current limited DC test voltage between the conductors (Windings) and the chassis of the equipment (Ground), and then measuring any current leakage across the insulation’s dielectric materials. The current may be measured in Milli-amps or Micro-amps and then calculated into Meg-ohms of resistance. The lower the current level measured, the greater the insulation resistance.