Insulation resistance (IR) of an electrical equipment refers to the resistance between conducting part and earth.
It is expressed in M-Ohm.
It can also be measured between two circuits separated by insulation
It is measured by Megger. (500/1000/2500/5000V)
IR reveals quality of insulation of the equipment and the degree of dryness/ moisture.
The subsequent high voltage test can be avoided if IR is low. l
How Insulation resistance is measured?
Now a days, New tech company make megger available in market at voltage range of 5KV, 10KV and different voltage as per application.
Positive and Negative leads connected to equipment and inject DC voltage for st time. Megger display shown IR value.
In old technology, Megger consists of a built in mega ohm meter and a D.C. generator.
The generator is driven by hand or by a motor.
One terminal of megger is connected to conducting part and the other terminal is connected to mn 0062earth frame.
The rotor of the generator is driven by hand or motor.
The reading ( Vdc/ Idc) read by the megger gives IR value in MW.
The scale of megger is graduated from 0 to infinite in MW.
When D.C. voltage is applied to the insulation, initially the insulation draws capacitive charging current (Ic) in addition to leakage current( IL)
Initial megger reading Ro = Vdc / Idc = Vdc / ( Ic + IL)
After continuous application of D.C. voltage, the capacitive current reduces and only leakage current continue.
Megger reading after 60 seconds will be R60= Vdc/Idc
Since IL< Ic + IL, R60 > Ro
What is Polarizing Index (P.I)?
It is the ratio of IR value measured for 10 minutes to 01 minute
P.I. = IR (10 minutes) / IR (01minute)
PI gives true idea about the quality of insulation
It gives a quantitative information about the insulation with respect to moisture, dirt and contamination.
Its value depends on class of insulation of equipment
For class A , PI should be ≥ 1.5 and for class B insulation PI should be ≥ 2.0
PI less than one indicates a need for immediate reconditioning.
What are the differences between Insulation resistance & Earth resistance?
Insulation Resistance
Resistance measured between conducting part and earth or between two circuits separated by insulation.
Measured by means of megger/ megaohm.
It is of the order of tens/thousands megaohms or infinity for healthy insulation.
It reveals quality of insulation of the equipment and degree of dryness/moisture.
Earth resistance
Measured for an earthed metallic body or point.
Measured by means of earth resistance tester.
For good effective earthing, the resistance should be of the order of 0.5 Ω to 2.0 Ω.
It gives effectiveness of earthing system.