The megohmmeter, often referred to as a “megger” or “hand-cranked insulation tester,” is a compact, portable instrument engineered for the measurement of high electrical resistance. Its primary application lies in evaluating the insulation resistance of electrical circuits and equipment. With its lightweight construction and ease of transport, the megohmmeter remains a critical diagnostic tool in industrial maintenance, commissioning, and safety assurance.

Selection Criteria for Megohmmeters

The core component of a traditional megohmmeter is a hand-cranked DC generator capable of producing elevated voltages necessary for insulation testing. Megohmmeters are categorized based on their rated output voltage—typically 100V, 250V, 500V, 1000V, and 2500V.

Selecting the appropriate instrument requires aligning the megohmmeter's rated voltage with the nominal voltage of the equipment under test. Employing a megohmmeter with an excessively high test voltage risks overstressing or damaging the insulation of low-voltage apparatus.

As a general rule:

• For circuits and equipment rated below 500V, use a 500V or 1000V megohmmeter.

• For systems above 500V, opt for a 1000V to 2500V instrument, depending on the operational voltage and insulation standards of the equipment.

Choosing the correct test voltage ensures measurement accuracy and safeguards equipment integrity.

Operating Principles and Connection Methodology

A megohmmeter typically features three external terminals:

• E (Earth or Ground)

• L (Line or Live terminal)

• G (Guard terminal)

For conventional insulation resistance measurements, the L and E terminals are predominantly utilized. The G terminal comes into play during specific tests, such as when isolating leakage paths across insulation layers, particularly in multi-layered cables.

1. Testing Insulation Resistance to Earth in Lighting and Power Circuits

In the standard configuration:

• Connect terminal E to a reliable ground.

• Connect terminal L to the circuit line under test.

Crank the generator handle clockwise, gradually increasing the speed until it stabilizes at approximately 120 revolutions per minute. Allow 60 seconds for stabilization. The pointer on the dial will then settle, and the indicated resistance value corresponds to the insulation resistance between the conductor and earth. A high, stable reading denotes effective insulation, while a low or fluctuating value suggests potential leakage or insulation degradation.

2. Testing Insulation Resistance of Electric Motors

For motors:

• Connect E to the metallic motor frame or grounding screw—avoid painted surfaces.

• Connect L to one terminal of the motor winding.

Operate the generator until it reaches rated speed (120r/min), then note the pointer position. This yields the resistance between the winding and ground. For inter-winding resistance measurements, disconnect the star or delta links within the motor terminals. Then use L and E across two separate windings. Stabilize the generator and record the resistance value. This indicates the insulation resistance between windings—critical for assessing potential phase-to-phase insulation failure.

3. Testing Insulation Resistance of Power Cables

When measuring cable insulation:

• Connect E to the metallic sheath of the cable.

• Connect L to the conductor (core).

• Connect G to the insulation layer between the core and sheath.

Begin cranking, maintain generator speed, and read the stabilized pointer. The displayed value reflects the resistance across the insulation between the core and outer sheath. This method ensures that surface leakage or environmental interference is effectively bypassed using the guard terminal.

Precautions During Megohmmeter Use

1. Safety First

Prior to any insulation resistance test, ensure that the equipment under test is completely de-energized and discharged to ground. Failure to do so may result in severe injury, instrument damage, or erroneous readings due to residual voltage.

2. Pre-Test Instrument Verification

Before actual testing:

• Place the megohmmeter on a horizontal surface.

• With no external connection, crank to rated speed. The pointer should indicate “∞” (infinity).

• Then short terminals L and E, and crank gently. The pointer should swing to “0.”
  These checks confirm operational integrity.

3. Use of Appropriate Test Leads

Leads must be flexible, multi-stranded, and properly insulated. Avoid twisting or tangling them together, as this may induce capacitive coupling or measurement inaccuracies.

4. Proper Positioning and Environmental Considerations

The megohmmeter must be placed on a stable, flat surface, away from high-current conductors and magnetic fields. Ambient electromagnetic interference can cause needle oscillations and incorrect resistance readings. When testing energy storage devices like capacitors, disconnect the L lead first, allow the generator to coast down, and then proceed to disconnect other terminals—this prevents reverse discharge through the movement, protecting the instrument.

5. Post-Test Discharge

Immediately after testing, discharge the equipment under test. Do not touch any terminals, test leads, or the device itself until full discharge is confirmed. This minimizes the risk of electric shock and protects both personnel and the megohmmeter from residual energy damage.

Conclusion

The megohmmeter remains an indispensable tool for diagnosing insulation health in electrical equipment and infrastructure. Its ability to unveil insulation degradation, latent faults, or moisture ingress offers engineers and technicians a proactive pathway to system reliability and preventive maintenance. Mastery of its selection, operation, and safety precautions ensures the integrity of both personnel and equipment in high-voltage environments.