How to Use Voltage Readings to Locate Ground Faults in Solar PV Arrays

By Will White, Fluke Senior Application Specialist, DER

After confirming a ground fault in a photovoltaic (PV) string, the next challenge is determining where it is. Is the fault inside a module? Along a wire run? In a connector? The key to locating the fault efficiently, without dismantling the entire array, is using voltage measurements and some basic math.

In this article, you'll learn how to calculate fault location in a PV string using step-by-step voltage testing. This method is especially useful in rooftop systems where removing every module is not practical.

Before You Begin: When to Use This Method

This voltage-based approach works best when:

• You've already identified a ground fault on a string or in a combiner
• The system or section of the system is turned off and locked out
• The PV string voltage is within the test tool's measurement range
• You know the modules Voc (open circuit voltage)

If the system has intermittent faults or if the circuit can be safely de-energized, first see:

• How to Test PV Strings for Intermittent Ground Faults
• How to Test De-Energized PV Circuits for Ground Faults

Step-by-Step Guide to Locate Ground Faults in Solar PV Arrays Using Voltage Readings

Step 1. Gather the Information You'll Need

Before taking any measurements, make sure you know:

• The number of modules in the string
• Each module's Voc (from the data sheet or label)
• Your expected string open circuit voltage (Voc × module count)
• Safety procedures for LOTO and meter ratings

Example:

• 16 modules × 53.82 VDC = 861.12 VDC expected string voltage

Step 2. Measure Between Positive and Negative Conductors

With the system isolated and safe for testing:

1. Use a CAT III-rated meter (like the Fluke 393 FC or Fluke 283 FC/PV) with a voltage rating equal to or greater than the maximum system voltage.
2. Connect probes to the positive and negative conductors of the string.
3. Record the measured voltage.

This confirms that the full Voc is present across the string.

Expected reading: Full string voltage (e.g., 861.12 VDC for 16 modules)
Abnormal reading: Lower or higher than expected may indicate multiple issues

Step 3: Isolate the Positive and Negative Ends of the String

1. Open the positive fuse holder.
2. Open the negative fuse holder or unwire the negative conductor from the negative bus bar.

For voltage testing to be accurate, both ends of the string must be isolated from any parallel connections.

Step 4. Measure Between Positive and Ground

1. Connect the positive lead of your meter to the positive string conductor.
2. Connect the negative lead to the system ground or grounding bus bar.
3. Record the voltage reading.

Expected: 0 VDC
Abnormal: A positive-to-ground voltage indicates a fault path between the positive conductor and ground.

Step 5. Measure Between Negative and Ground

1. Switch meter leads:
   • Negative lead to the negative conductor
   • Positive lead to ground
2. Record the voltage.

Expected: 0 VDC
Abnormal: A non-zero reading indicates a ground fault on the negative side of the circuit.

Step 6. Divide Voltage-to-Ground by Module Voc

This is where "by the numbers" comes in. By dividing the measured voltage to ground by an individual module's Voc, you can estimate the number of modules between the measurement point and the fault.

Example 1: Ground Fault Between Two Modules

Calculation:

• 645.84 ÷ 53.82 = 12 modules from the positive end to the fault
• 215.28 ÷ 53.82 = 4 modules from the negative end to the fault

Example 2: Ground Fault on a Home Run Conductor

Calculation:

• 0 ÷ 53.82 = 0 modules
• 861.12 ÷ 53.82 = 16 modules

These results suggest that the issue lies with the positive wire run between the test point and the first module in the string on the positive side. The fault could be in the inverter or combiner box, in the conduit, or in the array before the first module on the positive side of the string.

Step 7. Visually Inspect the Suspected Area

Now that you've narrowed it down, inspect:

• Wire harnesses at the estimated fault location
• Connectors between the two suspected modules
• Module frames or grounding lugs near the suspect spot
• The conduit and strain reliefs at homerun terminations

Burn marks, damaged insulation, melted connectors, or pinched wires are clear indicators of the fault location.

Step 8. Confirm with Insulation Resistance Testing (Optional)

To validate your voltage-based findings:

1. Disconnect the positive or negative conductor.
2. Use an insulation resistance tester (e.g., Fluke 1587 FC, 1537, or SMFT-1000).
3. Test the conductor resistance to ground using 500–1,500 VDC.
4. Record resistance.

A low resistance confirms that the conductor you suspect has compromised insulation or contact with grounded metal.

Step 9. Document and Prepare for Repair

Before any repair, document:

• Voltage readings between all points
• The suspected module or wire section
• Environmental conditions (e.g., wet, dry)
• Photographs, if damage is visible

This documentation is helpful for warranty claims, insurance reports, and future troubleshooting.

Ready to make repairs? Go to: How to Repair Ground Faults in PV Systems

Troubleshooting Tips

• Always round voltages to two decimal places for better accuracy
• If voltage readings seem off, retest using different leads or meter
• Watch out for ghost voltage—a slowly decaying signal that isn't real fault voltage
• Confirm modules are clean and dry when measuring; moisture can skew readings

Conclusion

Locating ground faults using voltage measurements is a smart, efficient way to pinpoint problems in PV strings—without dismantling large portions of the array. By using voltage measurements to ground and comparing them to module Voc, technicians can isolate fault locations to within one or two modules.

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.