How to Test Insulation Resistance (Megger Test) Step-by-Step

Insulation resistance testing — often called a Megger test — checks whether the insulation around conductors is still doing its job. It is an essential safety and maintenance check for cables, motors, and electrical installations. This guide explains how to test insulation resistance step by step, what readings mean, and how to choose the right tester from the MarMonix insulation testers range.

What Is an Insulation Resistance Test?

An insulation tester applies a high DC test voltage (commonly 250 V, 500 V, or 1000 V) between a conductor and earth, then measures the resulting leakage current to calculate resistance in meg-ohms (MΩ). Healthy insulation has very high resistance; degraded, damp, or damaged insulation reads low and warns you of a potential fault or shock risk before it fails.

Safety Before Testing

• Isolate and de-energise the circuit, and prove it is dead before connecting the tester.
• Discharge any stored capacitance after testing — the tester applies a high voltage.
• Disconnect sensitive electronics that could be damaged by the test voltage.
• Use the correct CAT-rated leads and follow site safety procedures.

How to Perform a Megger Test (Step by Step)

1. Isolate the circuit and confirm it is de-energised.
2. Select the correct test voltage for the equipment (e.g. 500 V for most 230/400 V installations).
3. Connect the leads between the conductor and earth (or between conductors).
4. Apply the test and hold for the recommended time to let the reading stabilise.
5. Record the MΩ value and compare it against the required minimum.
6. Discharge the circuit before disconnecting.

What Do the Readings Mean?

Higher is better. As a general guide, readings in the high meg-ohm or giga-ohm range indicate healthy insulation, while low values (for example below 1 MΩ on a typical installation) suggest moisture or deterioration that needs investigation. Always compare against the specific limit for your equipment and standard.

Choosing the Right Insulation Tester

Look for selectable test voltages, a wide resistance range, and a clear digital display. Compare models in the insulation testers category and choose one that offers the test voltages your work requires.

Frequently Asked Questions

What is a good insulation resistance reading?
It depends on the equipment and standard, but higher is always better; many installations require a minimum of 1 MΩ, with healthy systems reading far higher.

What test voltage should I use?
500 V is common for typical 230/400 V installations; use the voltage specified for the equipment you are testing.

Why is it called a Megger test?
“Megger” is a long-standing brand of insulation tester, and the name became shorthand for the insulation resistance test itself.