Insulation resistance (IR) is a useful diagnostic for gross moisture, contamination or unintended connections. It is not the standardized proof of power-frequency or impulse withstand.
Learning objectives
Choose a safe test voltage, prepare circuits, interpret environmental effects and state the limited acceptance role of IR.
Core engineering principles
IR measures leakage under DC stress
The result depends on insulation material, surface contamination, humidity, temperature and charging time.
Sensitive devices may require disconnection
Surge arresters, sensors and electronics can be damaged or distort results if included.
Time and temperature affect comparison
One instantaneous value without conditions is weak evidence. Consistent duration and environment improve trending.
Low values require diagnosis
Segmenting circuits can locate moisture, wiring mistakes or connected loads.
IR does not replace dielectric withstand
The wave shape, voltage level and acceptance purpose are different.
Engineering application method
- Step 1: Define circuit, test voltage and exclusions.
- Step 2: Isolate, discharge and earth safely.
- Step 3: Apply DC voltage for the specified time.
- Step 4: Record value, temperature and humidity.
- Step 5: Discharge fully and investigate abnormal phases.
Practical example
A low reading on one phase may be caused by a connected arrester rather than damaged bus insulation. The test configuration must be documented before judgement.
Common mistakes
- Applying an arbitrary megger voltage.
- Testing through electronics.
- Comparing results without environment.
- Failing to discharge capacitive circuits.
- Calling IR a withstand test.
Design and review checklist
- Is the test voltage approved?
- Are exclusions recorded?
- Are environmental conditions known?
- Is the circuit discharged afterward?
- Is dielectric testing still completed where required?
Standards basis and official sources
- IEC 62271-200:2021 + AMD1:2024 — AC metal-enclosed switchgear assemblies above 1 kV and up to 52 kV.
- IEC 62271 series — High-voltage switchgear and controlgear framework.
Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.