A very significant maintenance procedure for checking out the working condition of an electrical system is called IR test or Insulation resistance. It can be said that ht system or equipment is fine when insulation resistance is high.
Megger is an insulation resistance measurement tools for finding the equipment’s insulation resistance. Let us see how is the IR calculated?
Define Insulation resistance
There is an application of direct current (DC) and not alternating current (AC) and this is the first thing while doing the Insulation resistance test.
A permissible level of DC voltage is applied on the device and thus there will be current flow. While checking the insulation test for the earth equipment, there is no means of physical connection to them. The micro amps amount of current will be present between them.
As we all know that R= (V/I), this equation is for calculating the insulation resistance R. Another important point to note is that the total value of measured insulation resistance is small internal resistance of equipment + insulation resistance. The value of R will be low if the strength of the insulation is high you will have low value of R and if the insulation is weak there will be higher current with low IR.
Insulation resistance is affected by the following factors:
Conductive leakage current (IL)
It is a small amount of current that flows through normal insulation, from a conductor to ground or between conductors. You will find an increase in current as there is a deterioration in the insulation and becomes predominant after the absorption current vanishes. It is time independent and fairly steady this is the most significant current for measuring insulation resistance.
LEAKAGE CURRENT AND CAPACITIVE CHARGING (IC)
In a raceway two or more conductors run together and they act like a capacitor. A leakage current flows through the conductor insulation because of this capacitive effect.
As the DC voltage is applied the current lasts only for a few seconds and falls out after the insulation is charged with full test voltage.
The capacitive current is higher than the conductive leakage current in a low-capacitance device but it vanishes usually by the time we start recording the data. It is significant for letting the reading settle out before recording it.
While testing higher capacitance equipment the capacitance leakage current charging can hold on for a longer duration before settling out.
Leakage current and Polarization absorption (IA)
Polarization of molecules inside the dielectric medium causes absorption current. There is a higher current for the first few seconds in a low-capacitance equipment and reduces slowly to zero. The higher capacitance device or wet and contaminated insulation dealing will have no reduction in the absorption current for a longer duration.