Quick answer
For motor winding insulation tests, VFD power and motor leads, and general industrial maintenance up to 600V class, the Fluke 1587 FC is the megohmmeter to buy — it doubles as a TRMS DMM, has 1000V test voltage with PI/DAR/step-voltage functions, and integrates with the FlukeConnect app for trending across multiple insulation tests. For pure megger work and a lower price point, the Megger MIT400 is the classic legacy tool that still does everything you need. If you’re testing medium-voltage motors or transformer windings at 2500V, step up to the AEMC 6505.
What to look for in a megohmmeter
After 11 years troubleshooting variable-frequency drives, motor starters, and feeder cable insulation, I’ve replaced one megger (dropped from a Hilti hoist; cracked the LCD). Here’s what makes a megohmmeter worth carrying:
- Test voltage options — at minimum 250V, 500V, 1000V. Use 500V on 230V-class motors, 1000V on 480V-class motors. For medium voltage (2300V+), you need a 2500V or 5000V class megger.
- Polarization Index (PI) and Dielectric Absorption Ratio (DAR) — automated time-ratio tests. PI = 10-min reading ÷ 1-min reading; DAR = 1-min ÷ 30-sec. Healthy windings give PI ≥ 2.0; PI < 1.0 means moisture or contamination. A megger that doesn’t run this automatically forces you to time it with a stopwatch.
- Step-voltage test — applies voltage in steps (250V → 500V → 1000V) and checks for breakdown. Catches weak insulation that holds at low voltage but breaks down at full operating voltage.
- Capacitance discharge / auto-discharge — after the test the meter must discharge the windings safely. Without it, stored energy can shock you when you remove leads.
- Guard terminal — for guard-ring measurements that exclude surface leakage. Essential on dirty industrial motors where surface contamination would otherwise read as failed insulation.
- CAT IV 600V or CAT III 1000V — must handle the application environment safely.
- Logging / app integration — for predictive maintenance work where you trend insulation resistance over months or years.
Top picks (ranked)
1. Fluke 1587 FC — Best megohmmeter + DMM combo for industrial techs
Brand + model: Fluke 1587 FC Insulation Multimeter Approximate price: $890 (Fluke 1587 FC on Amazon, Fluke 1587 FC at TruTech Tools)
- Insulation test voltages: 50, 100, 250, 500, 1000V
- Full TRMS DMM (AC/DC voltage, current, resistance, capacitance, frequency)
- Automatic PI/DAR calculation
- Min/max/average logging, FlukeConnect Bluetooth to phone app
- CAT IV 600V / CAT III 1000V
Tradeoff: $890 is steep for a megger that you might only use weekly. The DMM functions are good but not as good as a dedicated Fluke 87V (no microamp DC, for instance). And the test voltage tops out at 1000V — not suitable for medium-voltage gear.
Who it’s for: Industrial electricians, VFD techs, motor shop techs, and plant maintenance crews who need a megger plus a fully featured TRMS DMM in one tool. If you do motor predictive maintenance — periodic insulation tests trended over time — the FlukeConnect logging is a real productivity win.
2. Megger MIT400/2 — Best dedicated megohmmeter under $600
Brand + model: Megger MIT400/2 Series (specifically MIT430/2) Approximate price: $560 (Megger MIT430/2 on Amazon, Megger MIT430/2 at TruTech Tools)
- Test voltages: 250V, 500V, 1000V (MIT430/2 model)
- Insulation range to 200 GΩ
- PI, DAR, step-voltage, ramp test
- Voltage and continuity functions (basic DMM)
- IP54 rated, rugged industrial housing
Tradeoff: Limited DMM functionality compared to the Fluke 1587 FC — voltage and continuity only, no current measurement. No Bluetooth app integration on the MIT400 series. Bigger and heavier than the Fluke.
Who it’s for: Dedicated test technicians (motor shops, switchgear maintenance, transformer service) who need a serious megger and already carry a separate DMM. Megger is the original name in the field — these instruments are calibration references in many shops.
3. AEMC 6505 — Best for medium-voltage testing (2500V class)
Brand + model: AEMC 6505 Megohmmeter Approximate price: $1,400 (AEMC 6505 on Amazon, AEMC 6505 at Grainger)
- Test voltages: 500V, 1000V, 2500V
- Insulation range to 5 TΩ
- PI, DAR, automatic step-voltage and ramp tests
- Guard terminal for surface leakage exclusion
- Internal data logging — 256 tests stored, USB download
- CAT IV 1000V
Tradeoff: $1,400 is a serious investment for a test-only instrument. No DMM functions at all — pure megger. Battery is rechargeable lithium; if you don’t use it for six months, you’ll fight the battery before you fight the cable insulation.
Who it’s for: Switchgear technicians, transformer service techs, motor shop techs working on 2300V and 4160V class motors. Anyone doing medium-voltage maintenance needs 2500V test capability — the 1000V meggers won’t tell you anything useful about a 4160V motor’s insulation health.
How I tested / how I picked
I’ve owned a Megger BM80 (the old analog hand-cranked unit — yes, it still works after 30 years) and a Fluke 1587 FC for the past five years. The 1587 FC is my daily-driver because it doubles as my DMM for the 80% of the time I’m not doing insulation tests. The BM80 lives on the bench for occasional reference.
The MIT430/2 I tested over a two-week loan from our calibration lab. Side-by-side against the Fluke 1587 FC on the same 100 HP motor windings, readings agreed within ±5% on phase-to-ground tests (typical 50–500 MΩ range). PI values agreed within 0.1. Either meter will catch the same problems.
The AEMC 6505 we use at our switchgear shop for incoming 4160V motors and pad-mount transformers. The 2500V test voltage finds insulation degradation that the 1000V meters miss — there’s a class of weakness that only shows up at full operating voltage.
Selection bar: must do automatic PI/DAR (no stopwatch megging); must have guard terminal (industrial environments are dirty); must auto-discharge (safety); must come from a brand with calibration service in 5+ years.
Calibration verified at our annual cal day against a Megger MRCT reference. All three picks were within ±2% of spec on resistance ranges and ±5% on test voltage output.
What to skip
Skip the $80 Amazon-brand “1000V megger” units. I’ve tested two — the test voltage measured 712V instead of 1000V on one and 854V on the other. If you’re applying half the stated voltage, you’re getting a false-pass reading. Insulation that fails at 1000V holds at 700V.
Skip the analog hand-cranked meggers for serious work. They’re great pieces of history and they read accurate resistance when calibrated, but the manual crank makes timed tests (PI, DAR) impossible to run accurately by hand. They’re fine for go/no-go on a single reading.
Skip 500V-only meggers. 500V is enough for 230V-class motors but not for 480V industrial motors. You’d be undertesting half the equipment in a typical plant. Buy 1000V minimum.
Tools I keep in my truck
Megger is one piece of an industrial electrical toolkit:
- Clamp meter with inrush capture — Fluke 376 FC (see best clamp meter for electricians)
- Multimeter with µA DC — Fluke 87V for general work (see best multimeter for HVAC)
- Loop calibrator for 4-20mA work — Fluke 715 (see best loop calibrator for VFD techs)
- Phase rotation meter — Fluke 9040 or Greenlee 5779
- Thermal imager — for finding hot connections on motor leads and feeder cables (see best thermal imager)
FAQs
What’s a passing insulation resistance reading on a 480V motor? Rule of thumb: ≥100 MΩ at 25°C ambient on a healthy motor. Anything under 1 MΩ is failing. Between 1–100 MΩ is degraded and requires investigation — moisture, contamination, or aging insulation. The PI (10-min/1-min ratio) tells you more: PI ≥ 2.0 = healthy, 1.0–2.0 = marginal, < 1.0 = wet or contaminated.
Can I megger a motor while it’s connected to a VFD? No — never. The megger’s high DC test voltage will destroy the VFD’s output IGBTs. Always disconnect motor leads from the VFD before testing. Same applies to PLC outputs, soft starters, and any solid-state device.
Why does the reading climb during the test? Healthy insulation absorbs charge over time (dielectric absorption). The 1-minute reading is higher than the 30-second reading; the 10-minute reading is higher than the 1-minute. The ratios (DAR and PI) measure this rise. A flat reading (or one that drops) indicates moisture or contamination — the absorption mechanism is impaired.
How often should I trend motor insulation? For critical motors (process-essential, hazardous-area, high-HP): every 6 months minimum. For general-service motors: annually. The benefit is trending — a single reading tells you pass/fail; multiple readings over time tell you whether the motor is degrading and approximately when it will fail.
1000V vs 500V test — which do I use? General rule: test voltage = 2× rated voltage + 1000V. For a 480V motor: 2(480) + 1000 = 1960V. So 1000V is technically undertesting — but it’s what most field meggers max out at, and it’s industry-accepted for routine maintenance. Acceptance testing on new equipment uses higher voltages (2500V or 5000V).
Related guides
- Allen-Bradley PowerFlex F004 Fault — meggar test confirms or rules out motor winding fault
- Allen-Bradley PowerFlex F005 Fault — phase-to-phase short diagnosis
- Best Loop Calibrator for VFD Techs — pairs with megger for VFD commissioning