How to Calculate Minimum Insulation Resistance Requirement?
How to calculate minimum insulation requirement is very important for ensuring insulation resistance healthiness. The insulation resistance test is a common routine test. We perform this test on all electrical equipment before powering up. The insulation resistance test does not indicate the localized fault in the equipment. But, it gives information about the quality of insulating material.
Every electrical equipment manufacturer does the IR test during the production process for ensuring the quality of the insulation.
As electrical engineers, we all know how to perform the insulation resistance test. But, most of the technical fellow does not know what should be the minimum insulation resistance value of the equipment.
After carrying out the insulation resistance test & getting the IR value, the next confusing point is that whether the IR value is OK for charging the equipment. Whether or not the IR value is satisfactory.
One should ensure the IR testing passes criteria before charging of the equipment. This may be possible the user either declares the good equipment as defective or defective equipment as good.
What is insulation?
We use electrical conductors like copper and aluminum in the cables, switch-gears, and almost in every piece of electrical equipment. Copper and aluminum have the best conductive properties which make them ideal conductors. The insulating material like PVC, XLPE, and rubber. etc, are most suitable for providing insulation over the conducting material. The insulating material that opposes the flow of electric current is known as insulators.
Minimum insulation resistance requirement for an equipment
R=kV +1 ,
R=resistance in megaohm(MΩ)
kV= equipment rated voltage in kV.
So if we like to check a machine-rated voltage is 33 kV, the minimum tolerated insulation resistance will be R=1+33=34 MΩ, and for a motor, with 440 voltage rating the IR value will be R=0.440+1=1.44 MΩ.
The insulation resistance should be approximately one megohm for every 1000 volts of operating voltage with a one megohm minimum. This is an empirical formula.
According to this formula, a motor rated at 6600 Volts should have a minimum resistance of 6.6 Megaohm.