How to Test Generator Insulation Resistance with a Megger

What the test measures

The winding insulation in a generator alternator keeps current on its intended path through the copper and prevents leakage to the frame or to other windings. Heat, vibration, moisture, and contamination all degrade that insulation over time, and a failing winding can short out, overheat, or damage the rest of the machine.

A megohmmeter, often called a megger, measures insulation resistance in megohms or gigohms. Unlike a multimeter, it applies a relatively high DC test voltage to stress the insulation and reveal weak spots that a low-voltage reading would miss.

The result is a single number: the resistance between the winding under test and ground, which is the generator frame. Higher resistance means less leakage current and healthier insulation. Lower resistance means the winding is leaking and needs attention before the generator is put back into service.

When to test

Run the test before the first start of the season and again before storing the generator at the end of the season. It is also worth testing any time the unit has been exposed to rain, high humidity, or flooding, since moisture is the most common cause of a low reading.

Test after any repair that disturbs the alternator, such as brush, bearing, or rotor work, because the winding may have been contaminated or damaged during the job. See our guide on checking generator brushes and rings and on rotor winding testing.

If you are buying a used generator, an insulation test is one of the quickest ways to judge the condition of the alternator before you commit to the machine. For storage advice, see generator storage preparation.

Safety preparation

Work in a dry location and keep the generator itself dry. A wet winding will leak current and produce a misleadingly low reading, so if the unit has been out in the rain, give it time to dry before you test.

The engine must be off and the windings de-energized for the whole test. Disconnect all loads and make sure no one can start the generator while you have the test leads connected. Review the generator safety checklist for outdoor use before you begin.

Carbon monoxide is a related hazard. The CPSC warns that generator exhaust contains carbon monoxide, a colorless, odorless poison, and that one portable generator produces as much carbon monoxide as hundreds of cars. Never run a generator indoors or in a garage, basement, crawl space, or shed; the CPSC and CDC recommend operating it outside, more than 20 feet away from windows, doors, and vents. See our carbon monoxide safety guide for details.

Step-by-step measurement

Follow the manufacturer's service manual for your exact model. The general procedure below covers the most common portable and standby generators.

• Isolate the generator. Disconnect all loads, the transfer switch, and the utility supply so the windings are completely de-energized. See transfer switch types if you need a refresher.
• Reach the winding ends. Remove the covers on the alternator connection box and identify the winding leads you want to test.
• Discharge stored charge. Windings and any connected capacitors can hold voltage after shutdown. Follow the manufacturer's discharge procedure before you connect anything.
• Connect the megohmmeter. Attach one lead to the winding under test and the other to the generator frame, which is the ground reference. Keep the leads separated so they do not touch.
• Select the test voltage. Use the value stated in the generator's service manual. If the manual does not state a value, use the lowest test voltage available on the instrument.
• Apply the test voltage. Hold it for the period the manual specifies, long enough for the reading to stabilize, and note the value at the end of that period.
• Discharge again. After the test, discharge the winding before you disconnect the leads, then reconnect everything the way you found it.

Interpreting the reading

A healthy winding shows a high, stable reading. A low reading indicates leakage caused by moisture, conductive dirt, or damaged insulation. The correct pass or fail threshold comes from the manufacturer's specification for your generator model, not from a general number: winding voltage, insulation class, and machine size all change the requirement. Our article on insulation classes F vs H explains one part of that picture.

Track the trend. If you test regularly and the reading stays high and steady, the insulation is healthy. A reading that falls between tests, or drops sharply after rain, is a warning. A reading that climbs as the winding dries out usually points to moisture rather than permanent damage.

After the test: record and compare

Record the reading, the test voltage, the date, and the air or winding temperature in a maintenance log. Insulation resistance changes with temperature, so the same winding can show different values on a cold morning and a warm afternoon. Keeping notes lets you compare like for like next season.

If the reading was good, finish the rest of the pre-season checks: oil level, fuel, air filter, spark plug, and a load test to confirm the machine produces clean power under load.

If the reading was low, do not put the generator back into service. Dry the winding with warm moving air, clean the end windings if they are dirty, and retest. If the reading stays low after drying and cleaning, the insulation is probably damaged and the alternator may need professional repair or replacement.

What to pick for your maintenance routine

Questions

What is a megohmmeter?

A megohmmeter, or megger, applies a DC test voltage to a winding and measures the insulation resistance in megohms. It is the correct tool for this job; a standard multimeter does not apply enough stress to reveal weak insulation.

How often should I test insulation resistance?

Test at least once a year, before the first start after storage, and again before storing the unit. Test more often if the generator lives in a humid space or has been exposed to rain or flooding.

What does a low reading mean?

A low reading means current is leaking from the winding to the frame. Moisture, dust, oil film, or damaged insulation can cause it. Dry and clean the machine and retest before concluding the insulation itself is bad.

Can I test with the engine running?

No. The engine must be off and the windings de-energized. A running generator produces voltage in the winding, which makes the measurement meaningless and creates an electrical hazard.

Do I need to disconnect the voltage regulator before testing?

Usually yes. The automatic voltage regulator and other control electronics are not designed for the test voltage. Follow the manufacturer's procedure for isolating them before the test.

Should I test a new generator before its first run?

It is not required, but it gives you a baseline reading. Test the new machine once, record the value in your log, and compare future readings against it.