Megger | Working Principle Types History Uses of Megger

History of Megger

Sydney Evershed invented the first practical insulation tester in 1889. Megger was registered as a trademark in 1903. The basic purpose remains the same, but modern insulation resistance testers can add regulated electronic test voltages, digital displays, automatic discharge, timers and stored results.

What is Megger?

An insulation resistance tester applies a specified DC test voltage and measures the very small current through the insulation. It then calculates insulation resistance (IR). Heat, moisture, contamination, ageing and mechanical or electrical stress can reduce IR. Periodic tests can reveal a downward trend before the insulation fails, provided the results are compared under equivalent conditions.

Uses of Megger

The tester applies a DC test voltage to cables, motors, generators, transformers and other isolated equipment. It measures the electric current that flows through and over the insulation. The result indicates overall insulation resistance, not the exact location or cause of a defect. Temperature, test duration, surface contamination and leakage current can all affect the reading.

Types of Megger

Portable insulation resistance testers are commonly grouped by how they generate the test voltage:

1. Electronic Type (Battery Operated)
2. Manual Type (Hand Operated)

A third, less common megger design is motor operated. Instead of a hand crank or battery, it uses an external supply to produce the test voltage. The supply runs an electrical motor that drives the generator.

Electronic Type Megger

A typical electronic tester has the following parts:

1. Digital display: Shows the measured IR value, test voltage and relevant status information.
2. Test leads: Connect the megger to the isolated equipment under test.
3. Selection controls: Select the test voltage, resistance range or test mode.
4. Indicators: Show power, active high voltage, hold, battery and warning states, depending on the model.

Note: Controls and features vary by manufacturer and model. Always follow the tester and equipment instructions.

Advantages of Electronic Type Megger

• Regulated test voltage and direct digital measurement improve repeatability.
• The digital IR value is easy to read.
• One person can start, time and record a test.
• Compact models can be used where access is limited.
• Many models discharge the test object automatically. Other features may include a live-circuit warning and timed tests.

Disadvantages of Electronic Type Megger

• It requires charged batteries or another specified power source.
• It is generally more expensive than a basic hand-cranked tester.

Hand Operated Megger

Important parts:
Analogue display: Shows the insulation resistance on a graduated scale.
Hand crank: Drives the internal generator at the speed needed to produce the test voltage.
Test leads: Connect the tester to the isolated equipment.

Advantages of Hand Operated Megger

1. It remains useful for basic IR testing where battery power is unavailable.
2. It needs no battery or external power supply.
3. Basic models can cost less than electronic testers.

Disadvantages of Hand Operated Megger

1. The operator must maintain crank speed while observing the reading.
2. An unsteady crank speed can make the test voltage and reading unstable.
3. The tester needs stable placement for safe cranking and reading.
4. Movement or poor lead contact can disturb the result.
5. The analogue scale requires interpolation and manual recording.
6. It requires the same high-voltage test precautions as an electronic tester.

Construction of Megger

A traditional ratio-meter construction includes the following parts:

1. Deflecting and control coils: The current coil is in series with the insulation under test, while the control or pressure coil is connected across the generator. Their opposing torques set the pointer position.
2. Permanent magnet: Produces the magnetic field in which the coil assembly turns.
3. Pointer: Moves with the coil assembly and indicates resistance on the scale.
4. Scale: Runs from zero resistance at one end to infinity at the other and is non-linear between them.
5. DC generator or battery supply: The test voltage comes from a hand-operated DC generator in a manual tester. An electronic tester uses its battery and converter for the same purpose.
6. Pressure-coil and current-coil resistors: Limit current and protect the movement if the external resistance is low.

Working Principle of Megger

• Before connecting the tester, isolate the equipment, prove it is de-energised and discharge stored energy. The hand crank or electronic converter then produces the DC test voltage.
• Select the test voltage from the equipment manufacturer’s instructions and the applicable test standard. A blanket 500 V rule is not safe for every low-voltage device.
• Testers may offer 1,000 V to 5,000 V or more for suitable high-voltage assets, but only when the equipment rating and procedure permit it.
• The deflecting or current coil is in series with the insulation under test and carries its leakage current.
• The control coil, also called the pressure coil, is connected across the test source.
• A current-limiting resistor in each coil circuit protects the movement if the external resistance is very low.
• In a hand-operated tester, electromagnetic induction in the generator produces the test voltage.
• In an electronic tester, a battery-powered converter produces and regulates the test voltage.
• The pointer position depends on the opposing torques produced by the pressure-coil and current-coil currents.
• The ratio of these coil effects represents V/I and therefore the insulation resistance.
• When the electrical circuit under test is open, current-coil current is nearly zero and the pointer moves towards infinity.
• With a short circuit, high current-coil current moves the pointer towards zero resistance.

The traditional instrument works as a ratio meter based on Ohm’s Law.
The pressure-coil effect represents applied voltage, while the current-coil effect represents insulation current. Their ratio corresponds to R = V/I. The insulation under test is connected across the generator and in series with the current coil.
The opposing coil torques settle the pointer at the resistance value. After the test, keep the leads connected until the tester indicates that stored voltage has discharged, then verify the equipment is safe before touching conductors.

1. High resistance, very low current: Little current flows through the deflecting coil, so the pointer moves towards infinity.
2. Low resistance, high current: More current flows through the deflecting coil, so the pointer moves towards zero.
3. Intermediate resistance: The opposing torques balance at a point between zero and infinity.

Connection Diagram of Megger for Testing