Megger / Insulation Resistance Basics
General field practice for qualified electricians. The equipment manufacturer's instructions, your adopted code edition, and the AHJ govern where they differ.
1 · What it measures
A megohmmeter applies a DC test voltage between a conductor and ground, or between conductors, and reads the leakage resistance in MΩ. Good insulation reads high and steady. Moisture, contamination, damage, and aging all lower the reading.
2 · Before you test — isolate electronics
- Remove or disconnect: AFCI, GFCI and dual-function breakers (manufacturers prohibit meggering through them), surge protective devices, VFDs, soft starters, electronic ballasts and LED drivers, smart switches, dimmers, timers, occupancy sensors, controls, meters, PV inverters and optimizers, and anything plugged in.
- Open both ends of the circuit. Note that a neutral connected at the bar will make conductor-to-ground readings meaningless, so lift it to test it.
3 · Test voltage and minimum readings
For acceptance testing of new equipment, ANSI/NETA ATS Table 100.1 lists minimum test voltages and minimum insulation resistance by equipment rating.[1] Common low-voltage rows:
| Equipment rating | Minimum test voltage | Minimum IR |
|---|---|---|
| 250 V | 500 V DC | 25 MΩ |
| 600 V | 1,000 V DC | 100 MΩ |
Check the current NETA edition and the equipment manufacturer's values, which govern. Megger's field rule of thumb for in-service equipment is at least 1 MΩ per 1,000 V of operating voltage, with a 1 MΩ minimum.[2] Trends matter more than a single reading.
4 · Doing the test
- Lock out, verify absence of voltage, and isolate electronics (section 2).
- Check the tester: short the leads (about 0 Ω), then open them (above range).
- Test each conductor to ground (EGC or raceway), then conductor to conductor.
- Apply the test voltage for 1 minute and record the reading. Insulation charges up, so the reading should rise and then settle.
- Record the temperature and humidity. Insulation resistance drops as temperature rises, so correct to a common base (often 20 °C or 40 °C) when you trend readings.
- For motors and transformers, a polarization index (PI = 10-min reading ÷ 1-min reading) shows insulation condition. Megger's guide rates below 1 as dangerous, 1–2 questionable, 2–4 good, and above 4 excellent.[2]
- Let the tester discharge the circuit, reconnect, and restore.
Sources
- NETA tables (ATS/MTS Table 100.1, insulation resistance test values), government contract attachment copy — https://govtribe.com/file/government-file/75n99021r00025-sow-rmlcampus-electrical-maintenance-attachment-b-neta-tables-june-2021-dot-pdf
- Megger, "A Stitch in Time" — The Complete Guide to Electrical Insulation Testing — https://assets.tequipment.net/assets/3/7/A-Stitch-In-Time.pdf