Short Answer:

A megger is a special instrument used to measure high resistance, especially for checking the insulation resistance of electrical cables, motors, and equipment. It generates a high voltage (usually between 250V to 1000V or more) and measures the resistance offered by insulation to the flow of current.

The working of a megger is based on Ohm’s law. It applies high DC voltage to the insulation and checks how much current leaks through it. The less the current leakage, the better the insulation. This helps ensure electrical systems are safe and not at risk of short circuits or shocks.

Detailed Explanation:

Megger

A megger (short for megohmmeter) is an electrical testing tool used to assess the quality of insulation in wires, cables, motors, transformers, and other electrical equipment. Proper insulation is crucial in preventing electric shock, short circuits, and fire hazards. The megger helps by applying a high voltage and measuring the resistance of the insulation in megohms (millions of ohms).

Insulation resistance tests are critical in preventive maintenance, commissioning, and troubleshooting of electrical systems, especially in industrial and high-voltage installations.

Principle of operation

The megger works on the basic principle of Ohm’s Law, which states:

R=VIR = \frac{V}{I}R=IV​

Where:

• RRR is resistance (in ohms or megohms),
• VVV is the voltage applied,
• III is the resulting current.

The megger applies a high DC voltage across the insulation of the component under test and measures the small leakage current that flows through the insulation. The resistance is then calculated and shown on the display.

• High resistance (in megohms) means good insulation.
• Low resistance indicates poor insulation or damage, which may lead to electrical hazards.

Components of a megger

1. Hand-crank or battery: Older meggers have a manual crank to generate voltage, while modern ones are battery-operated.
2. DC voltage generator: Produces high voltage for testing (commonly 250V, 500V, 1000V, or more).
3. Measuring circuit: Senses the small leakage current and calculates insulation resistance.
4. Display (analog or digital): Shows resistance reading in megohms (MΩ).

How to use a megger

1. Turn off the power to the equipment being tested.
2. Disconnect the component to isolate it from the system.
3. Connect the test leads of the megger—one to the conductor and one to the insulation or ground.
4. Set the test voltage as per the equipment rating.
5. Press the test button (or crank the handle in manual models).
6. Read the insulation resistance displayed on the screen.
7. Compare the result with standard values or manufacturer’s guidelines.

Important: Never use a megger on live circuits.

Applications of a megger

• Testing insulation resistance of cables, windings, and equipment.
• Checking motor insulation during maintenance.
• Preventive testing in substations and switchgear.
• Verifying safety after electrical installations.
• Troubleshooting faults in high-voltage equipment.

Safety and care

• Always follow safety instructions while using a megger.
• Ensure equipment is completely de-energized.
• Select the correct test voltage to avoid insulation damage.
• Use protective gear when testing high-voltage equipment.

Conclusion:

A megger works by applying high DC voltage to a piece of electrical equipment and measuring the resistance of its insulation. It uses Ohm’s law to calculate resistance by measuring the leakage current. High insulation resistance indicates a safe and healthy system, while low resistance warns of faults. It is an essential tool for electrical safety, maintenance, and troubleshooting.