Insulation Resistance (Megger) Testing Basics
A megohmmeter tells you whether a conductor's insulation is still doing its job before you energize — catching a failing motor, a damaged run, or a wet cable before it faults. Here's what the test is, how to read it, and how to do it safely.
⚠️ Before you start
- A megohmmeter applies a high DC test voltage (typically 250–1000V for common work). The circuit must be fully de-energized, isolated, and verified dead before testing — never megger a live circuit.
- Disconnect or account for sensitive electronics, and discharge the conductor after testing, since the test can leave a stored charge on cables and windings.
- This is an educational overview; follow your equipment's instructions, your employer's procedures, and the relevant standards for test voltages and acceptance values.
🧰 Tools you'll need
- Insulation resistance tester (megohmmeter)
- Lockout/tagout gear
- A voltage tester to confirm dead
A megohmmeter — a "megger" — answers a question an ordinary multimeter can't: is this conductor's insulation still doing its job? It catches a failing motor winding, a damaged cable run, or a wet feeder before you energize it and it faults. Here's the working primer. This is an educational overview — follow your instrument's instructions, your employer's procedures, and the relevant standards.
What it measures
A megger applies a high DC voltage (commonly 250, 500, or 1000V for typical work) and reads the resulting insulation resistance in megohms (millions of ohms) — essentially, how well the insulation resists current trying to leak through it.
- Good insulation reads very high — often hundreds or thousands of megohms.
- Failing, wet, or damaged insulation reads low, warning you that current could leak or fault.
It's a go/no-go health check on insulation, and a predictive-maintenance workhorse for motors and feeders.
Doing it safely (non-negotiable)
Because it puts out high DC voltage, the safety steps come first:
- De-energize, isolate, and verify dead — full lockout/tagout. Never megger a live circuit.
- Disconnect or account for sensitive electronics — the test voltage can damage them.
- Test per the instrument's instructions.
- Discharge the conductor/winding afterward — cables and motor windings can hold a stored charge like a capacitor.
Reading the result
Higher is better, but there's no single universal "pass" number — acceptable values depend on the equipment, its voltage rating, and the standard you're working to.
- An old rule of thumb was roughly 1 megohm per 1000V of rating (with a 1 megohm floor), but modern standards and equipment specs are more nuanced.
- Trends matter as much as absolutes. A motor reading far below its baseline, or dropping over successive tests, is failing — even if today's number looks "okay." Always compare to the equipment spec and prior readings.
Some testers also do PI (Polarization Index) and DAR ratios — comparing readings over time (e.g., 1 minute vs. 10 minutes) to judge insulation quality and moisture, which is common on larger motors.
When to reach for it
- Before energizing a new install or a repaired circuit.
- Troubleshooting a motor or feeder you suspect is wet, pinched, or overheated.
- Commissioning equipment.
- Periodic/predictive maintenance on motors and important feeders.
If a run got soaked, crushed, or cooked, or a motor keeps tripping its protection, a megger tells you whether the insulation is compromised before you put it back in service.
Bottom line
Insulation resistance testing is how you verify the insulation, not just continuity — applying a high DC voltage and reading megohms to catch a failing conductor or winding before it faults. De-energize and verify dead first, discharge after, compare readings to the spec and the baseline trend rather than chasing one magic number, and use it before energizing and as part of motor/feeder maintenance. Done right, it's one of the best "catch it before it fails" tools in the trade.
📞 When to call a professional
This is professional reference material. Insulation testing should be done by trained personnel following proper de-energizing and safety procedures.
Frequently asked questions
What does a megger actually measure?
It measures insulation resistance — how well the insulation around a conductor resists current leaking through it — by applying a high DC voltage (commonly 500 or 1000V) and reading the resistance in megohms (millions of ohms). Good insulation reads very high (often hundreds or thousands of megohms); failing, wet, or damaged insulation reads low, warning you that current could leak or fault. It's a go/no-go health check on insulation before energizing.
What's a 'good' insulation resistance reading?
Higher is better, and acceptable values depend on the equipment, voltage, and the standard you're working to — there's no single universal number. A very old rule of thumb was roughly 1 megohm per 1000 volts of rating (with a 1 megohm minimum), but modern standards and equipment specs are more nuanced, and trends matter as much as absolute values: a motor that reads far lower than its baseline, or keeps dropping over time, is failing. Always compare to the equipment spec and prior readings.
When would I megger something?
Common cases: before energizing a new installation or a repaired circuit, troubleshooting a motor or feeder you suspect has damaged or wet insulation, commissioning equipment, and periodic maintenance/predictive testing on motors and important feeders. If a run got wet, pinched, or overheated, or a motor is tripping its protection, an insulation test tells you whether the insulation is compromised before you put it back in service.
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