What Is a Megger?

A megger (megohmmeter) is a specialized test instrument that measures insulation resistance by applying a high DC test voltage, typically 250V, 500V, or 1000V, across a conductor's insulation and measuring the tiny leakage current that results. The reading is displayed in megohms, hence the name, and a healthy insulation reading is usually in the hundreds or thousands of megohms.

Why Insulation Resistance Testing Matters

• Detects insulation breakdown from age, moisture, heat, or physical damage before it causes a short circuit or shock hazard.
• Required before energizing new wiring installations, motors, transformers, and cables to confirm they're safe.
• Helps diagnose intermittent earth faults that a standard multimeter or continuity tester cannot reliably find.
• Used in routine preventive maintenance for industrial motors and large electrical systems.

How to Use a Megger Safely

• Always de-energize and isolate the circuit or equipment completely before testing; never megger a live circuit.
• Discharge any capacitance in cables or motor windings after the test before touching the leads.
• Connect the test leads to the conductor and to earth, then apply the test voltage for the recommended duration, usually around 60 seconds for a proper reading.
• Compare the reading against manufacturer specifications and always follow proper safety procedures and applicable electrical codes.

Typical Insulation Resistance Guidelines

As a rough general reference, low-voltage wiring is often expected to read above 1 megohm at minimum, though most healthy new installations read far higher, in the hundreds of megohms. Motors and older installations may have lower acceptable thresholds depending on their voltage rating, so always check the applicable standard for your specific equipment rather than relying on generic numbers.

FAQs

Is a megger the same as a multimeter?

No. A multimeter measures voltage, current, and resistance at low test voltages, while a megger applies a much higher DC voltage specifically to stress-test insulation and reveal weaknesses a multimeter can't detect.

How often should insulation resistance testing be done?

For most home wiring, testing is typically done at installation and after any major fault or renovation. Industrial and commercial equipment is often tested on a routine annual or biannual maintenance schedule.

Final Thoughts

A megger is an essential tool for anyone serious about electrical safety, whether you're a professional electrician or a serious DIYer working on your home's wiring. AJ Electric stocks a range of testing tools