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How to perform a Megger test

The quality of insulation resistance is known to degrade with time due to exposure to their environmental conditions. Temperature, humidity, moisture, and dust particles negatively impact these cables inducing electrical and mechanical stress.

For any developing or existing site, insulation resistance tests, also known as Megger tests, are conducted on power cables at constant and regular intervals to check the health of their electrical systems.

Megger test procedure

What is a Megger test

The Megger test is a method of testing that assesses the insulation resistance, which relays the integrity of electrical insulation between core conductors and a ground reference. A low insulation resistance indicates voltage leakage, damage to the trace heater, and possible insulation faults. Failure to regularly conduct a Megger test could result in tripping of circuit breakers, electrical shock, or even a fire if the damage to an electrical system remains undetected for too long.

Megger test for electric heat trace cable installation

For heat trace cable installations, it's recommended to conduct the Megger test a minimum of four times throughout the installation phases. The preliminary test begins at the receipt of the cable (while still on the reel). This is in order to ensure that the cable was not damaged during transport or storage. Then comes the acceptance test (after the trace heater installation and before thermal insulation), which is carried out to check for possible damage occurring during installation. The final inspection test must be completed immediately after the thermal insulation installation to make certain no humidity or dust has negatively affected the cable. Lastly, test the insulation resistance of the cable before energizing the system. Branch circuit wiring can be included in the Megger test procedure by performing the test at the de-energized panel as well.

If a cable reel has been stored for an extended period, it is recommended to conduct another Megger test before the cable is installed on any construction site, as humidity and temperature may have affected the cable during storage.

Steps to perform a Megger test

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Only properly trained personnel should perform the insulation resistance test.

In the video above, we at BARTEC conduct a Megger test on BARTEC's self-regulating HSB heating cable.

In preparation for the measurement, use the appropriate personal protective equipment (PPE), follow all safety guidelines, and, if applicable, de-energize the heating circuit.

We will start the preliminary test with a cable still on the reel, which consists of four steps:

  • Step 1: Safely strip the cable exposing the conductors.
  • Step 2: Connect one of the test leads to the grounding braid and the other test lead to both bus wires.
  • Step 3: Perform the insulation resistance test for one minute with a voltage of at least 500 VDC. We recommend testing at higher voltages such as 1000 VDC or 2500 VDC. A passing test should be at a minimum of 20 MΩ. We recommend values higher than 1000 MΩ. Rapid fluctuations of the Megohm value during the Megger test procedure may indicate an issue with the electrical circuit, e.g. the breakdown of the insulation.
  • Step 4: Record the test values. This cable is now ready to be installed.

 

After installation, the acceptance test requires six steps:

  • Step 1: Open the power termination kit cover and disconnect wires from the terminals with the appropriate tools. If installed, also disconnect any thermostat or controller.
  • Step 2: Connect one of the test leads to the grounding braid and the other test lead to both bus wires.
  • Step 3: Perform the insulation resistance test for one minute with a voltage of at least 500 VDC. We recommend testing at higher voltages such as 1000 VDC or 2500 VDC. A passing test should be at a minimum of 20 MΩ. We recommend values higher than 1000 MΩ. Rapid fluctuations of the Megohm value during the Megger test procedure may indicate an issue with the electrical circuit, e.g. the breakdown of the insulation.
  • Step 4: Repeat the insulation resistance test between the grounding braid and the grounded workpiece. The reading should be similar to the one measured between the grounding braid and the bus wires.
  • Step 5: Record the test values.
  • Step 6: Re-connect the wires into terminals and close the cover on the power termination kit. If installed, reconnect any thermostats or controllers. Repeat steps one through five for your final inspection test and before energizing the system.

Click on the link to find a copy of our cable inspection and Megger test form

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Connection of heating cables to megger
Connection of self-regulating heating cables to the Megger

Typical results of Meg test

High resistance (hundreds of MΩ or more): Good insulation.

Low resistance (below manufacturer’s recommended threshold): Insulation is deteriorating or faulty.

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Frequently asked questions (FAQ)

A meg test (short for megger test) is a type of electrical insulation resistance test.  It uses a device called a megohmmeter or "megger" to apply a high DC voltage (typically 250V, 500V, 1000V, or higher depending on equipment rating) across the insulation of a conductor, motor winding, cable, or other electrical equipment. The tester then measures the resistance in megohms (MΩ).

Megger test is necessary to check the quality of insulation and ensure it hasn’t deteriorated due to heat, moisture, dirt, mechanical damage, or aging. Additionally, it helps to identify potential faults such as leakage currents, short circuits, or breakdowns before they cause equipment failure or safety hazards.

How often should Megger test be performed?

The frequency of Megger (insulation resistance test) testing depends on the type of equipment, its operating environment, and industry standards:

  • Critical equipment (motors, transformers, switchgear): Usually tested annually or during scheduled shutdowns.
  • Harsh environments (humidity, dust, chemicals): More frequent testing may be required, sometimes every 3–6 months.
  • New installations: Always tested before energizing to verify proper insulation.
  • Preventive maintenance programs: Often include insulation testing as part of routine checks.

Most facilities perform Megger testing once per year unless conditions demand more frequent monitoring.

The actual test itself is quick, usually taking only a few minutes per piece of equipment.

On average, a single test can take 5–10 minutes total, but testing large systems with many circuits can extend the process:

  • Setup time: 2–5 minutes (connecting the Megger and ensuring the system is safely de-energized).
  • Testing time: 1–2 minutes to apply voltage and record readings.
  • Documentation and safety checks: Another 2–5 minutes.

There are several types of Megger tests, depending on the application and method used:

  1. Insulation Resistance Test – The standard Megger test, measuring resistance of insulation in MΩ.
  2. Polarization Index (PI) Test – Measures insulation resistance over 10 minutes compared to 1 minute, useful for assessing insulation aging and moisture absorption.
  3. Dielectric Absorption Ratio (DAR) Test – Similar to PI but compares 60-second to 30-second readings, used for quick diagnostics.
  4. Step Voltage Test – Applies gradually increasing voltages to check insulation behavior under stress.
  5. Dielectric Discharge Test – Measures how insulation discharges after being energized, useful for identifying trapped charges and insulation condition.

The insulation resistance test is the most common, but for critical equipment, PI and step voltage tests provide a deeper assessment of insulation health.

 

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