What Is a Megger?

A Megger, or megohmmeter, is a portable instrument used to measure high-value insulation resistance in electrical circuits and equipment. It applies a controlled DC voltage to an insulation barrier and reads the resulting resistance to detect leaks or degradation. This test ensures that wiring, cables, motors, and other components maintain safe insulation levels before commissioning or after maintenance.

How a Megger Works (Principle)

A Megger operates on the principle of electromagnetic attraction. Inside, a small DC generator (hand-cranked or battery-powered) produces high-voltage current. That current passes through two coils arranged at right angles; the balance of forces on those coils creates torque that deflects a pointer on a calibrated scale, indicating insulation resistance in megohms.

Types of Meggers

• Hand-Operated Megger
  • Voltage generated by cranking a hand lever
  • Analog display, no external power source needed
• Electronic (Battery-Powered) Megger

• DC voltage supplied by internal battery
• Digital display, selectable voltage ranges, often automatic test functions

Step-by-Step Testing Procedure

1. Safety and Isolation
   • De-energize and lock out the circuit under test.
   • Discharge stored capacitance by shorting conductors to ground.
   • Ensure test leads and terminals are clean and dry.

1. Use 500 V DC for motors, control wiring, and cables rated up to 440 V.
2. For higher-voltage cables, select a 1 000 V range if available.
3. Connect the Megger
   • Attach the positive lead to the conductor or winding under test.
   • Connect the negative lead to the equipment’s chassis or ground reference.
4. Perform the Test
   • For a hand-crank Megger, rotate the crank at the prescribed speed.
   • For an electronic Megger, press the TEST button.
   • Observe the pointer or digital readout as it stabilizes.
5. Record and Interpret Results

• A reading of “∞” (infinite) indicates good insulation.
• Low readings (toward zero) reveal moisture, contamination, or insulation breakdown.
• Compare against manufacturer or marine-industry minimums to determine acceptability.
• Typical Test Voltages and Readings
• Marine and industrial motors, cables, and control panels commonly use a 500 V DC test. Smaller circuits or delicate electronics may require 250 V. For equipment rated above 1 000 V, higher test voltages are used, but these must match insulation system capability. Readings above 1 MΩ generally indicate acceptable insulation; below 0.5 MΩ often necessitate cleaning, drying, or replacement.
• Best Practices and Safety Precautions
• Always verify the Megger’s calibration and battery level before testing.
• Keep hands clear of test leads and live conductors during measurement.
• Allow sufficient dwell time (typically 60 seconds) for readings to stabilize.
• Protect nearby personnel from contact by posting warning signs or barriers.
• Store the Megger and leads in a clean, dry case to prevent damage and ensure accuracy.

Here’s a clear graphic visualization of a Megger’s components and test setup:

Megger Graphic Description

In this illustration:

• Hand-Crank Megger (left):
  • Crank handle powers a small DC generator.
  • Generator shaft attached to rotor coil.
  • Fixed stator coil at right angle.
  • Pointer deflected by electromagnetic torque.
• Battery-Powered Megger (right):
  • Internal battery pack feeding DC generator circuit.
  • Selector switch for test voltages (250 V, 500 V, 1 kV).
  • Digital display or analog movement.
• Test Leads and Setup (bottom):
• Test Leads and Setup (bottom):
• Positive (red) lead connected to motor winding.
• Negative (black) lead grounded to motor frame.
• Insulation barrier between winding and frame.
• Ohmmeter scale showing megohms (MΩ) reading.
• This layout highlights both device types, their core internal mechanisms, and the field connection points you’ll use when testing insulation resistance.

ASCII Graphic of a Megger and Its Test Setup

Below is a simplified ASCII diagram showing both the core mechanism inside a hand-crank Megger and how you connect it to a motor winding for an insulation test.

┌────────────────────────────────────────────────┐ │ Hand-Crank Megger │ │ │ │ ┌─────────────┐ ┌─────────────┐ │ │ │ Rotor │ │ Stator │ │ │ │ Coil │ │ Coil │ │ │ └─────┬───────┘ └────┬────────┘ │ │ │ │ │ │ ┌──▼───┐ ┌──▼───┐ │ │ │ DC │ ← Crank │ DC │ │ │ │Gen. │ │Gen. │ │ │ └──┬───┘ └──┬───┘ │ │ │ │ │ │ ┌─▼─────────┐ ┌────────▼───┐ │ │ │ Pointer │ │ Calibrated │ │ │ │ Movement │ │ Scale (MΩ) │ │ │ └───────────┘ └────────────┘ │ └────────────────────────────────────────────────┘ Test Leads and DUT (Device Under Test) ┌────────────────────────────────────────────────────────────┐ │ │ │ +─────────+ Frame │ │ │ Megger │ +–––[Motor Winding]–––┐ ┌───────┐ │ │ │ Output │──┤ └───▶│Motor │ │ │ │ (Red) │ └─[Insulation Layer] │Frame │◀┘ │ │ │ │ (Dielectric) └───────┘ │ │ │ Return │─── Ground/Chassis (Black) │ │ └─────────┘ │ └────────────────────────────────────────────────────────────┘ 

How to Read This Diagram

• The hand-crank mechanism drives a small generator whose output powers two coils (rotor and stator) at right angles. Their interaction moves the pointer across the megohm scale.
• Pointer movement against a calibrated scale gives you the insulation resistance reading in megohms.
• The test leads connect the red (positive) output to the winding under test and the black (negative) lead to the metal frame or ground reference.
• The insulation barrier between winding and frame blocks most current. Any leakage current causes a pointer deflection proportional to the insulation’s resistance.