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What is insulation resistance between HV and LV winding?
100M╬й
200M╬й
50M╬й 500M╬й
1M╬й
200M╬й
Insulation resistance (IR) between HV and LV windings of a transformer is a measure of the dielectric integrity of the solid insulation separating the primary and secondary coils. A minimum value of 200M╬й is typically recommended for health verification during routine maintenance using a Megger.
Insulation resistance (IR) between HV and LV windings of a transformer is a measure of the dielectric integrity of the solid insulation separating the primary and secondary coils. A minimum value of 200M╬й is typically recommended for health verification during routine maintenance using a Megger.
RIRтАЛ=IleakageтАЛVDCтАЛтАЛ тАФ Ohm's Law applied to DC insulation testing
The measurement is performed using a Megger (insulation tester) by applying a high DC voltage (usually 500V or 2500V depending on the rating) between the HV and LV windings. The instrument measures the leakage current and displays the resistance based on Ohm's law, R=V/I. Low resistance values indicate degraded insulation, moisture ingress, or carbon tracking.
Tests are performed using a Megger with voltage levels typically 500V, 1kV, or 2.5kV.
IR values are temperature-dependent and should be corrected to a standard temperature (e.g., 30┬░C).
A declining trend over successive maintenance cycles indicates progressive insulation degradation.
The transformer must be isolated and discharged before and after the IR test.
Non-destructive test method
Easily identifies moisture or contamination in oil/solid insulation
Cannot detect local winding faults or turns-to-turns shorts
Highly sensitive to surface moisture and humidity
Periodic condition monitoring of power transformers
Post-installation/pre-commissioning checks
Post-repair quality control
Standard: IEEE C57.152 suggests that for a new transformer, IR values should be significantly higher, often in the G╬й range.
Option 200M╬й serves as a pragmatic threshold for operational safety in many industrial field environments.
1M╬й is considered dangerously low and indicates an imminent risk of insulation failure.
B is correct тАФ 200M╬й is the generally accepted minimum benchmark for insulation resistance between HV and LV windings for standard power transformers in field conditions.
Always remember that insulation resistance is temperature-dependent: if the temperature rises by 10┬░C, the resistance value approximately halves.