Insulation resistance or IR test is a very important maintenance procedure to check out the integrity or health of an electrical system. If the insulation resistance is found high, we can say an equipment or system is fine otherwise its not.
Now there is insulation resistance measurement tools called megger to find the insulation resistance of any equipment. We can handle the tools but what about the working principle of this tools or how the IR is calculated.
What is Insulation resistance
The first thing is that when doing Insulation resistance test we are applying direct current (DC) not alternating current(AC).
Now as R=(V/I), this simple equation led is to get the insulation resistance. R=insulation resistance.
But remember that the total value of measured insulation resistance= small internal resistance of equipment+insulation resistance.
If the insulation strength is high, value of R will be low and if the insulation is weak the IR will be low with high current.
Factors that affect the Insulation resistance
Conductive leakage current (IL)
Conductive current is a small (micro-amp) amount of current that normally flows through insulation, between conductors or from a conductor to ground.
This current increases as insulation deteriorates and becomes predominant after the absorption current vanishes. Because it is fairly steady and time independent, this is the most important current for measuring insulation resistance.
Capacitive charging leakage current (IC )
When two or more conductors are run together in a raceway, they act as a capacitor. Due to this capacitive effect, a leakage current flows through conductor insulation.
This current lasts only for a few seconds as the dc voltage is applied and drops out after the insulation has been charged to its full test voltage.
In low-capacitance equipment, the capacitive current is higher than conductive leakage current, but usually disappears by the time we start recording the data. Because of this, it is important to let the reading “settle out” before recording it.
On the other hand, when testing high capacitance equipment the capacitive charging leakage current can last for a very long time before settling out.
Polarization absorption leakage current (IA )
Absorption current is caused by the polarization of molecules within dielectric material. In low-capacitance equipment, the current is high for the first few seconds and decreases slowly to nearly zero. When dealing with high capacitance equipment or wet and contaminated insulation, there will be no decrease in the absorption current for a long time.