The surface leakage current is dependent upon external contamination on the surface, dust, moisture, etc.

Insulation Resistance (IR) test, often called Megger test, is more than 100 years old and assumed to be a very straightforward test. During my inspection and testing work in the past 15 years in Canada, US, and internationally, I have seen different practices of performing and interpretation of IR test.
The application of IR test includes:
At the point of installation, pre-commissioning to ensure minimum specifications are met

  • Verify proper installation after a repair
  • Troubleshooting
  • Periodic preventive maintenance

The value of insulation resistance depends upon the equipment type, voltage rating and is very sensitive to the temperature. The result of IR test is typically in the range of a 10 MΩ to 100 GΩ or higher. The acceptable level of IR depends on the equipment type, voltage class, and age of equipment.

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