Safety guide for reading megohmmeter results

What a megger actually measures

A megohmmeter forces a known DC voltage across insulation and reads the resulting leakage current as resistance. Higher reading means cleaner insulation. Lower reading means moisture, contamination, carbon tracking, or broken insulation is letting current pass where it should not.

Test voltage is not arbitrary. Match it to the system rating. IEEE 43 and most manufacturer specs land in this range for field testing:

• Up to 250V systems: test at 500V DC
• 480V to 600V systems: test at 1000V DC
• 2.4kV to 5kV systems: test at 2500V DC
• Above 5kV: test at 5000V DC or per OEM spec

Read the number, then read the trend

A single reading tells you pass or fail against a minimum. The trend tells you whether the insulation is dying. One megohm per kV of rating plus one megohm is the old rule of thumb minimum for motors and rotating gear. For building wire and feeders, most inspectors and commissioning specs want 100 megohms or better at 1000V DC on a new install.

Temperature changes everything. Insulation resistance roughly halves for every 10C rise. Always record ambient and winding temperature with the reading, then correct to 40C using the manufacturer curve or IEEE 43 Table 3. A 20 megohm reading at 15C is not the same circuit as 20 megohm at 45C.

If the reading climbs steadily for the first 60 seconds and settles, the insulation is charging normally. If it drops or stays flat and low, stop and investigate before you energize.

PI, DAR, and what they tell you

Polarization Index (PI) is the 10 minute reading divided by the 1 minute reading. Dielectric Absorption Ratio (DAR) is the 60 second reading divided by the 30 second reading. Both strip temperature out of the picture because you are comparing the circuit to itself.

Rule of thumb targets per IEEE 43 for thermoplastic and modern insulation systems:

1. PI above 2.0: insulation is good
2. PI between 1.0 and 2.0: questionable, schedule follow up
3. PI below 1.0: do not energize, insulation is wet or damaged
4. DAR above 1.4: acceptable
5. DAR below 1.0: insulation is saturated or contaminated

For motors and generators rated under 1kV with windings below 5 megohm total, PI is not reliable and you should not rely on it as a pass criterion. Fall back on absolute resistance and trend data from prior tests.

Safety before the leads touch copper

A megger puts out real voltage. A 5kV instrument on a capacitive cable run will store enough charge to hurt you well after the test ends. Treat every tested conductor as live until you have grounded it.

Lockout tagout per NFPA 70E applies the same way it does for any energized work. The circuit must be opened, locked, tested dead with a separate meter, then isolated from anything you are not testing. Disconnect neutrals from ground bars, lift surge protectors, pull electronic loads and VFD outputs. MOVs and capacitors will skew the reading and can be damaged by the applied voltage.

• Verify the megger is rated for the test voltage and CAT rating of the environment
• Confirm the instrument has a working discharge circuit and use it
• Short tested conductors to ground for at least four times the test duration after the reading
• Barricade and post warning signs per OSHA 1910.333 while the test is live

NEC 110.26 working space rules still apply. If you cannot stand clear of energized parts while testing, reschedule for a de-energized panel configuration.

Never megger across a closed breaker into a live bus. One slipped lead and your 1000V test instrument becomes a fault initiator on a 480V system with thousands of amps of available short circuit current behind it.

What low readings actually mean in the field

A failed megger test is a symptom, not a diagnosis. Before you condemn a cable run, work through the obvious causes. Moisture intrusion at a pulled condulet, a pinched jacket at a strut clamp, or a nicked conductor from a rough pull are far more common than bulk insulation failure.

Typical field findings by reading range at 1000V DC on a 600V feeder:

• Above 1000 megohm: clean, document and move on
• 100 to 1000 megohm: acceptable for service, trend on next PM
• 10 to 100 megohm: investigate before energizing critical loads
• 1 to 10 megohm: insulation compromised, locate and repair
• Below 1 megohm: do not energize, hard fault or saturation present

Reference NEC 590.4(D) for temporary wiring insulation requirements and NEC 300.5(B) for direct buried conductors, where water ingress is the failure mode you will see most often on recall work.

Documentation that holds up later

Commissioning reports, warranty claims, and arc flash studies all lean on megger data. Sloppy records lose arguments. Every test record should capture the instrument model and calibration date, test voltage, duration, ambient and conductor temperature, humidity, and the circuit identifier from the one line.

Log the 30 second, 60 second, and 10 minute readings even when the spec only requires one. The extra numbers cost you nothing on site and give the next tech a trend to work from. Save raw data, not just the pass or fail.