PV Isolation Protection: Key Causes, Troubleshooting, and Preventative Measures (PV ISO PRO 01/02 Alarm)

PV Solar Site – Megger Test Procedure

(Insulation Resistance Test)

Purpose: Verify insulation integrity of PV strings, homeruns, combiners, and DC feeders to ensure there

are no ground faults, moisture intrusion, damaged conductors, or degraded insulation.

Standards Referenced:

    • IEC 62446-1 (PV system testing & documentation)

    • NEC 690.41–690.43 (grounding & fault protection)

    • NFPA 70E (electrical safety)

    • Manufacturer-specific inverter and module requirements

1. Safety Requirements

    1.1 PPE

        • Arc-rated FR clothing

        •Class 0 or higher electrical gloves

        • Safety glasses & insulated tools

        • Lockout/tagout equipment

1.2 Safety Precautions

        • Ensure all DC disconnects, combiner disconnects, and inverter DC switches are in the OFF

        position.

        • Confirm no backfeed from other sources (batteries, hybrid inverters, energized strings).

        • Verify that strings are open-circuited and safe to test.

        • Always follow LOTO procedures.2. Equipment Needed

        • Insulation Resistance Tester (Megger) rated to:

        o 500 V

        o 1000 V

        o 1500 V (required for utility-scale 1000–1500 VDC PV arrays)

        • PV test leads & MC4 adapters

        • Multimeter for polarity and voltage confirmation

3. Pre-Test Checks

        3.1 Verify Open-Circuit Voltage

            Using a multimeter:

                • Measure Voc of each string.

                • Confirm it is < 1000 V or < 1500 V (system rating).

                • Never megger a live or connected string.

        3.2 Verify Polarity

            Identify the string positive (+) and string negative (–) conductors.

        3.3 Disconnect All Equipment

                • Disconnect strings from combiners.

                • Disconnect combiners from DC feeders.

                • Disconnect feeders from the inverter.

            Megger testing must NEVER be performed into an inverter or electronic device.4. Megger Test Voltage Selection

            Use test voltage according to system class:

                PV System Rating Insulation Test Voltage

                    600 V DC system 500 V DC

                    1000 V DC system 1000 V DC

                    1500 V DC system 1000–1500 V DC (per manufacturer)

                Most utility-scale sites test at 1000 VDC even for 1500 V systems unless module manufacturer requires

                1500 V.

5. Test Procedure

        5.1 Megger Test – PV Strings

            1. Ensure string is disconnected from all equipment.

            2. Connect Megger positive lead to string positive conductor.

            3. Connect Megger negative lead to equipment ground.

            4. Select appropriate test voltage.

            5. Press “TEST” and maintain for 1 minute (IEC 62446-1 requirement).

            6. Record insulation resistance (MΩ).

            7. Repeat test:

        o Positive → Ground

        o Negative → Ground

        o (Optional) Positive → Negative (only when verified safe; rarely needed)

            Expected Result:• 1 MΩ minimum (IEC 62446-1)

                • 5 MΩ typical acceptable utility-scale threshold

                • 20 MΩ common in new installations

                If insulation resistance drops while voltage is applied, it indicates moisture or damaged insulation.

        5.2 Megger Test – Combiner Box

            Perform with all string inputs disconnected.

            1. Connect Megger positive to the incoming combiner positive bus.

            2. Connect negative to ground bar inside the combiner.

            3. Apply test voltage for 1 minute.

            4. Record insulation resistance.

5. Repeat for the negative bus.

        5.3 Megger Test – DC Feeders (Trunk Lines)

            1. Ensure feeders are isolated at both ends (combiners and inverter).

            2. Test:

        o Positive → Ground

        o Negative → Ground

            3. Hold for 1 minute.

            4. Record values.

6. Interpreting Test ResultsInsulation Resistance Value Interpretation

    > 20 MΩ Excellent, typical new system condition

    5–20 MΩ Acceptable, but monitor

    1–5 MΩ Marginal – investigate for moisture or minor insulation damage

    < 1 MΩ Fail – ground fault or damaged wiring likely

    Resistance drops during test Moisture ingress (combiners, junction boxes, connectors)

7. Common Causes of Low Megger Readings

    • Damaged PV wire insulation

    • Water intrusion in junction boxes

    • Improperly crimped MC4 connectors

    • Grounded metal touching live conductor

    • Rodent damage

    • Failed harness connectors

    • Wet combiner boxes

    • Pinched conductors under modules

8. Documentation Required

    Each test record should include:

    • Project name

    • Location / array number

    • Date & tester name• String or feeder ID

    • Test voltage

    • Measured resistance

    • Pass/Fail

    • Weather conditions (moisture affects readings)

9. Final Steps

    • Reconnect all strings, combiners, and feeders.

    • Remove lockout/tagout.

    • Restore inverter and PV system to normal operation.

    • Keep records for commissioning/ownership turnover.

For any further questions please contact:

Solis Service Support

US/Canada Toll-Free Service Line: +1.866.438.8408

usservice@solisinverters.com