How to Test De-Energized PV Circuits for Ground Faults

By Will White, Fluke Senior Application Specialist, DER

Not all ground faults occur in energized circuits. In photovoltaic (PV) systems, faults can develop in long homerun conductors, recombiners, or on the AC side—where the system may be shut down for maintenance or isolation. In these cases, technicians need to test de-energized circuits for ground faults using safe, proven methods.

This step-by-step guide explains how to isolate, test, and identify faults in de-energized PV circuits while protecting both equipment and personnel.

When Should You Test De-Energized Circuits?

De-energized testing is available when:

• A ground fault is suspected in the wiring between the combiner and the inverter
• You're testing on the AC side of the system, such as from the inverter output to the point of interconnection
• Module-level electronics are in place, and circuits can be disconnected before testing

In these cases, insulation resistance testing becomes the primary method for fault detection.

Learn about energized testing: How to Test PV Strings for Intermittent Ground Faults

Step-by-Step Guide to Testing De-Energized PV Circuits for Ground Faults

Step 1. Isolate and Lockout the Circuit

Before you begin any testing, you must ensure the conductors are completely de-energized and cannot be accidentally re-energized.

• Open all load-break rated disconnects at both ends of the circuit.
• Apply lockout/tagout (LOTO) devices at each disconnect.
• Label each LOTO with:
  • Technician name
  • Phone number
  • Date and time
  • Work being performed

Never assume a conductor is de-energized until verified.

Step 2. Perform a Live-Dead-Live Test

This essential safety step confirms that the circuit is truly off—and that your test equipment is functioning properly.

1. Live – Test your meter or tester on a known live source (e.g., the Fluke PRV240 Proving Unit).
2. Dead – Test the PV circuit. It should read 0 volts.
3. Live again – Re-test on the known live source to verify meter accuracy.

Skipping this test could expose you to serious arc flash risk if the circuit is still energized.

Step 3. Unland and Isolate the Conductors

Remove the conductors from their terminals at both ends of the circuit. This ensures you isolate:

• All conductors from one another
• All conductors from any electronics, surge protection devices, or internal resistance
• Any conductor bonded to ground (e.g., AC neutral) from its bus bar

Important:

• Do not test insulation resistance through electronics or surge protection devices. Disconnect them first or risk permanent damage.
• Use wire nuts or tape to protect exposed ends during testing (end of the conductor opposite of where the test lead is attached).

Step 4. Inspect Visually Before Testing

Look for physical signs of damage that could indicate a ground fault:

• Burn marks, insulation bubbles, or discoloration
• Wire abrasion where conductors enter the conduit
• Signs of corrosion or moisture in junction boxes
• Loose or poorly terminated connections

Fixing visible damage upfront may save time during testing.

Step 5. Perform Insulation Resistance Testing

You'll now test each conductor for insulation breakdown by applying a high test voltage and measuring resistance to ground.

Recommended Tool: Fluke 1587 FC Insulation Multimeter, 1537 Insulation Resistance Tester, or SMFT-1000 PV Tester

1. Set the tester to an appropriate test voltage (commonly 500V, 1,000V, or 1,500V DC for PV).
2. Connect:
   • Red test lead to one end of the conductor
     1. Ensure the other end of the conductor is isolated with a wire nut or electrical tape if not landed in a terminal at a disconnect.
   • Black test lead to ground (metal enclosure, GEC, or frame)
3. Record the insulation resistance (measured in megohms).

Pass criteria (typical, but always consult the manufacturer or required standard like NFPA 70B or ANSI/NETA MTS):

• 1,000 MΩ = excellent insulation
• < 20 MΩ = investigate further
• < 1 MΩ = likely ground fault

Never perform this test on a grounded conductor (like AC neutral) without isolating it from the ground connection.

Step 6. Compare Results and Identify Outliers

Test each conductor in the circuit individually. Look for:

• Resistance values that are significantly lower than others
• A value that is stable and repeatable (not ghost voltage)
• Consistency across multiple tests

The conductor with the lowest resistance to ground is your likely fault path.

To confirm:

• Swap leads and repeat the test
• Inspect the full conductor run for visible faults

If possible, divide the conductor into sections and repeat the process to narrow down the location.

Step 7. Test Between Conductors (Optional)

In addition to testing from conductor to ground, you can also test between conductors:

• Positive to negative
• L1 to L2, L2 to L3, L1 to L3
• Neutral to L1, L2, and L3

While not required for most faults, this can help detect parallel arc faults between conductors or insulation breakdown that hasn't yet reached ground.

Step 8. Document All Results

Before making repairs, record:

• The test voltage and results for each conductor
• Conditions during testing (e.g., dry, wet, ambient temperature, humidity)
• Visual findings
• LOTO records

This documentation helps with warranty validation, future maintenance, and insurance audits.

After repairs, repeat the insulation resistance test to confirm the fault is resolved.

What If No Fault Is Found?

If all insulation resistance values are high and no visible damage is present:

• Try testing under wet conditions, when intermittent faults are more likely to appear
• Reconnect and monitor for future GFDI or inverter trips
• Test upstream (toward the array) or downstream (toward the point of interconnection)

Sometimes, faults occur in a different section than initially suspected. Use test data and system logs to guide your next steps.

Summary

Testing de-energized circuits for ground faults requires careful planning, proper isolation, and the right tools. By following a methodical process, solar technicians can detect faults that might otherwise remain undetected, thereby protecting both people and the performance of the PV system.

About the Author

Will White began working in solar in 2005 for a small integrator. After starting as an installer, he worked in sales, design, and project management, and he eventually became the Director of Operations. In 2016, he joined the curriculum team at Solar Energy International (SEI), where he focused on developing course content and teaching solar classes. In 2022, Will moved into a solar application specialist role at Fluke, where he supports their renewable energy testing equipment like IV-curve tracers, electrical meters, and thermal imaging cameras.

Will has experience in wind power, solar thermal, energy storage, and all scales of PV. He is passionate about implementing high-quality, code-compliant installation techniques. Will has been a NABCEP Certified PV Installation Professional since 2006 and was previously a NABCEP Certified Solar Heating Installer. He has a B.A. in business management from Columbia College Chicago and an MBA from the University of Nebraska-Lincoln. In his free time, he can be found working with his wife and daughter on their homestead in central Vermont, which features an off-grid straw-bale house.

Connect with Will on LinkedIn.