IR Value Chart | IR Value Table | IR Value of Motor | IR Value Full Form | IR Value of Transformer
IR stands for Insulation Resistance — the resistance offered by insulating materials to prevent current leakage. Measured in Megaohms (MΩ), it’s a vital parameter for ensuring the safety of motors, transformers, and cables.
Why is IR Testing Important?
Insulation resistance testing helps detect moisture ingress, insulation aging, or damage. It ensures safe operation, prevents hazards, and meets international safety standards.
What is IR Value?
IR Full Form: Insulation Resistance
IR Value is the resistance to leakage current measured in MΩ using an Insulation Resistance Tester (Megger).
Factors Affecting IR
- Moisture ingress
- Heat
- Mechanical damage
- Contamination
- Aging
Low IR values can cause leakage, short circuits, fire, or equipment failure.
IR Testing Ensures:
- Safe operation
- Preventive maintenance
- Early fault detection
IR Value Chart
| Rated Equipment Voltage | Test Voltage (DC) | Min. IR Value | Remarks |
|---|---|---|---|
| Up to 250 V | 500 V | ≥ 1 MΩ | Control cables, small equipment |
| 251–500 V | 500 V | ≥ 1 MΩ | LV power circuits |
| 501–1000 V | 1000 V | ≥ 1 MΩ | LV panels, feeders |
| 1.1–11 kV | 2500 V | ≥ 5 MΩ | MV motors, transformers |
| 11–33 kV | 5000 V | ≥ 10 MΩ | HV switchgear, XLPE cables |
| Above 33 kV | 5000 V+ | OEM Standard | PI/DAR ratio may be used |
IR Value of Motors
New motors: ≥ 100 MΩ.
Used motors: IEEE 43 formula → IR (MΩ) = (Rated kV + 1) × 1 MΩ
Example: 6.6 kV motor → IR ≥ 7.6 MΩ.
IR Value of Transformers & Cables
| Equipment Type | Test Voltage | Test Between | Purpose |
|---|---|---|---|
| Power Cables | 500V–5kV | Conductor to Earth, Phase to Phase | Detect insulation failure |
| Motors/Generators | 1–5kV | Winding to Earth, Phase to Phase | Check winding insulation |
| Transformers | 1–5kV | HV to Earth, LV to Earth | Verify dielectric strength |
| Lighting Circuits | 500 V | Line to Neutral, Line to Earth | Avoid short circuits |
Instruments Used for IR Measurement
- Megger — analog/digital insulation tester
- Digital IR Testers — auto-ranging, memory, Bluetooth
- Analog Testers — rugged, field-friendly
- Multifunction Testers — IR + RCD, loop, voltage
- PI/DAR Testers — for advanced insulation analysis
- Online IR Monitoring — real-time checks without shutdown
IR Testing Procedure (Summary)
- De-energize & isolate system
- Discharge stored energy
- Connect megger correctly
- Select proper test voltage
- Apply voltage for ≥ 60 seconds
- Record readings in MΩ
Frequently Asked Questions (FAQ)
What is the full form of IR?Insulation Resistance.
What is a good IR value?Above 1 MΩ is generally considered safe.
How often should IR tests be done?During commissioning, scheduled maintenance, and after repairs.
What does low IR indicate?Possible moisture ingress, insulation aging, or faults.
What instruments are used?Megger, Digital IR Tester, PI/DAR Tester, Multifunction Tester.
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