Insulation Resistance Testing: Catching Motor Failures Before They Trip

Insulation resistance testing, often called megger testing after the classic instrument brand, measures how well a motor or cable resists current leakage through its insulation by applying a DC test voltage and reading the resistance in megohms. Winding insulation does not fail suddenly; it degrades over years under heat, moisture, contamination, and vibration. IR testing turns that slow decline into a number you can trend, so rewinds and replacements happen on your schedule instead of the machine’s.

How the test works

With the equipment de-energized, isolated, and locked out, the tester applies a DC voltage, commonly 500 V or 1,000 V for low-voltage motors, between windings and ground. Healthy insulation lets almost no current pass, reading hundreds or thousands of megohms. Moist, contaminated, or aged insulation leaks more, reading low. Readings are temperature-sensitive, so serious programs correct values to a reference temperature before trending.

What counts as a good reading

The long-standing floor is the one-megohm rule: minimum acceptable IR of one megohm per kilovolt of rating plus one. In practice, modern low-voltage motors in decent condition read far higher, and the trend matters more than any single value: a motor that read 2,000 megohms last year and 200 this year deserves attention even though 200 still passes. Standards such as IEEE 43 define the details, including temperature correction and minimum values by machine class.

Polarization index: the second number

The polarization index (PI) is the ratio of the 10-minute IR reading to the 1-minute reading. Dry, healthy insulation keeps absorbing charge and the ratio typically lands at 2 or above; wet or contaminated insulation flattens out near 1. PI is especially useful because it is largely self-correcting for temperature, making it a robust companion to spot IR values.

A worked example: trending beats testing

A 75 kW compressor motor gets an annual IR test, corrected to 40 degrees: 1,800, then 1,500, then 700, then 260 megohms over four years. Every single reading passes the minimum. The trend, however, is a textbook moisture-ingress curve, each year losing more than half its resistance. The team schedules a rewind decision at the next shutdown, swaps in the spare, and finds a failed shaft seal letting washdown water into the winding. Total cost: a planned motor swap and a seal. The alternative timeline ends with a winding-to-ground fault mid-shift, a tripped line, and an emergency rewind at premium price. Same motor, same physics, different outcome purely because the numbers lived in a trend.

Where IR testing fits among electrical tests

IR testing is the static, offline entry point of motor diagnostics. It complements rather than replaces other views: motor current signature analysis watches the running motor for rotor and load issues, thermography spots hot connections, and surge testing probes turn-to-turn insulation that IR cannot see. A practical program uses IR plus PI on a schedule, with the other techniques layered on critical drives.

Making it a program, not an event

• Test at consistent points: annually for critical motors, after any flood or washdown incident, at receipt of new and rewound motors, and before energizing anything long-stored.
• Record voltage, duration, temperature, humidity, and corrected values every time; an uncorrected number is barely evidence.
• Set trend-based alarms, a halving year over year, not just absolute floors.
• Tie failed or declining tests to planned work: dry-out, cleaning, seal repair, or scheduled rewind.

Where Fabrico fits

Fabrico does not measure megohms; your instruments do. Fabrico is where the readings become a program: test values recorded on work orders build the trend in the asset history, preventive schedules trigger the annual and post-incident tests, declining trends generate planned corrective work with the spare reserved, and real-time OEE shows the downtime that disciplined electrical maintenance is preventing, evidence that supports metrics like MTBF actually improving. EU-built, with EU data residency.

Frequently Asked Questions

Is a megger test safe for the motor?

Yes when done correctly: right test voltage for the winding rating, equipment isolated and discharged before and after, and electronics such as drives disconnected from the circuit under test. The DC test current is tiny; the discipline is in isolation and discharge.

How often should motors be IR tested?

Annually is a common baseline for critical motors, plus event-driven tests after moisture incidents, before energizing stored equipment, and at acceptance of new or rewound motors. Non-critical, easily replaced motors may justify only event-driven testing.

What makes IR readings drop?

Moisture is the leading cause, followed by conductive contamination (dust, oil, chemical films), thermal aging that embrittles insulation, and physical damage. Many low readings recover after cleaning and a controlled dry-out, which is why a low reading triggers investigation, not automatically a rewind.

Want every megger reading, trend, and follow-up work order on one asset timeline? Book a Fabrico demo to see how a field-ready CMMS turns electrical testing into planned reliability.