IR Value Chart | IR Value Table | IR Value of Motor | IR Value Full Form | IR Value of Transformer

IR stands for Insulation Resistance. It measures the resistance offered by insulating materials to prevent current leakage. Measured in Megaohms (MΩ), IR is a key indicator of the health and safety of electrical equipment like 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 electrical hazards, and complies with international testing standards.

What is IR Value?

IR Full Form: Insulation Resistance

IR Value is the measure of resistance offered by insulating materials to current leakage. It is expressed in Megohms (MΩ) and measured using an Insulation Resistance Tester (commonly called a Megger).

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Why is Insulation Resistance (IR) Important?

Insulation materials in motors, transformers, and cables can degrade over time due to:

• Moisture ingress

• Heat

• Mechanical damage

• Contamination

• Aging

Low IR values can lead to current leakage, short circuits, fire hazards, or equipment failure.

IR testing ensures:

• Safe operation

• Preventive maintenance

• Early fault detection

IR Value Chart | IR Value Table

Rated Equipment Voltage	Recommended Test Voltage (DC)	Minimum Acceptable IR Value	Remarks
Up to 250 V	500 V	≥ 1 MΩ	Control cables, small equipment
251 V – 500 V	500 V	≥ 1 MΩ	LV power circuits
501 V – 1000 V	1000 V	≥ 1 MΩ	LV panels, feeders
1.1 kV – 11 kV	2500 V	≥ 5 MΩ	MV motors, transformers
11 kV – 33 kV	5000 V	≥ 10 MΩ	HV switchgear, XLPE cables
Above 33 kV	5000 V or higher	OEM Standard	PI/DAR ratio may be used

IR Value of Motors (New and Used)

New motors should have IR ≥ 100 MΩ. For used motors, the IR should follow IEEE 43 standard:

IR (MΩ) = (Rated kV + 1) × 1 MΩ

Example: For 6.6 kV motor, IR ≥ 7.6 MΩ

IR Value of Transformers and Cables

Equipment Type	Test Voltage	Test Between	Purpose
Power Cables	500V–5kV	Conductor to Earth, Phase to Phase	Detect insulation failure
Motors/Generators	1000V–5000V	Winding to Earth, Phase to Phase	Check winding insulation
Transformers	1000V–5kV	HV to Earth, LV to Earth	Verify dielectric strength
Lighting Circuits	500V	Line to Neutral, Line to Earth	Avoid short circuits

Instruments Used for IR Value Measurement

Accurate IR testing depends on the use of reliable instruments. These include:

1. Megger: The traditional insulation resistance tester, either analog or digital.

2. Digital Insulation Resistance Testers: Offer auto-ranging, memory, and Bluetooth features.

3. Analog Insulation Testers: Simple and durable; preferred in field conditions.

4. Multifunction Testers: Include IR testing plus RCD, loop, and voltage testing.

5. PI & DAR Testers: For polarization index and dielectric absorption ratio tests.

6. Online IR Monitoring Systems: Provide real-time monitoring without needing shutdown.

IR Testing Procedure (Summary)

1. De-energize and isolate the system.

2. Discharge any stored energy.

3. Connect the megger properly.

4. Select appropriate test voltage.

5. Apply voltage for at least 60 seconds.

6. Record readings in MΩ.

Frequently Asked Questions (FAQ)

What is the full form of IR?

IR stands for Insulation Resistance.

What is a good IR value?

High and stable IR values—usually above 1 MΩ—indicate good insulation.

How often should IR tests be performed?

During commissioning, regular maintenance, and after repairs.

What does a low IR value indicate?

Moisture ingress, insulation aging, or internal faults.

What instruments are used for IR testing?

Megger, Digital IR Tester, PI/DAR Tester, Multifunction Tester.

What is PI (Polarization Index)?

PI = IR@10min ÷ IR@1min. PI ≥ 2 indicates good insulation.

How do I test IR safely?

De-energize, discharge, use PPE, and follow proper connection and test procedures.