How to Test PV Strings for Intermittent Ground Faults
By Will White, Fluke Senior Application Specialist, DER
Intermittent ground faults in photovoltaic (PV) systems are among the most elusive problems solar technicians face. Unlike hard, or active, faults, intermittent faults often only appear under specific conditions—wet weather, thermal expansion, or even tracking array movement. Left unresolved, they can trip inverters, create safety hazards, and lead to full-blown failures.
This guide provides a step-by-step method for safely testing energized PV strings to locate intermittent ground faults using reliable tools and procedures.
What Is an Intermittent Ground Fault?
An intermittent ground fault is a temporary electrical connection between a current-carrying conductor (like DC positive or negative) and a grounded metal component, such as the rack or module frame. Unlike a hard fault, the connection is not sustained.
These faults may only occur when:
- It rains or condensation forms
- The sun heats and expands materials
- The array is in a specific tracking position
- Conductors rub against edges under wind load
Because of their on-and-off nature, they often go undetected by standard ground fault protection.
Safety First: Use the Right PPE
Testing energized systems carries significant risk. Always follow your organization’s electrical safety protocols and the NFPA 70E® standard.
Depending on system size and voltage, PPE may include:
- Electrically insulated gloves
- Flame-resistant (FR) clothing
- Arc flash-rated face shield or suit
Ground faults can cause unpredictable current flow. Always assume the system is hazardous and non-current carrying metal parts are energized until verified otherwise.
Step-by-Step Guide to Testing Energized PV Strings for Intermittent Ground Faults
Step 1. Understand the Expected Voltage Values
Before testing, calculate the open circuit voltage (Voc) for each PV string. Use the module’s data sheet and multiply the module's Voc by the number of modules in the string.
For example:
- Module Voc = 53.8 VDC
- 16 modules in series
→ String Voc = 860.8 VDC
Knowing this baseline helps you identify abnormal readings.
Step 2. Use the System Monitoring Data (If Available)
If the system is monitored, check the inverter logs for:
- Ground fault error codes
- Time-of-day tripping
- Weather-related trends
This can help you pinpoint when and under what conditions the fault occurs.
Step 3. Lockout/Tagout (LOTO) and Visual Inspection
Although you’ll test energized strings, you must isolate and secure the area first.
- Open the load-break-rated disconnect for the section you’ll work on.
- Apply LOTO devices to the combiner box or disconnects.
- Label each LOTO device with:
- Technician name
- Phone number
- Date
- Work being performed
Next, visually inspect the array for:
- Burn marks or discoloration on modules
- Melted connectors
- Damaged or pinched wires
- Signs of water intrusion
Some intermittent faults are visible before you start testing.
Step 4. Verify No Current Is Flowing
Before opening any fuse holders or disconnecting conductors, use a DC clamp meter (e.g., Fluke 393 FC or 283 FC/PV) to check for current flow.
- Clamp around each positive and negative string conductor individually.
- Confirm zero current before proceeding.
Warning: Never open fuse holders or disconnect wires when current is flowing. This can result in a dangerous DC arc.
Step 5. Test PV String Voltage to Ground (Positive Side)
- Open all positive fuse holders to isolate the positive conductors.
- If possible, ensure all negative fuse holders are open or negative conductors are removed from the negative bus bar and isolated. (This is beneficial, but at this step, there is no need to isolate the negative conductor.)
- Connect:
- Meter positive lead to the string positive conductor
- Meter negative lead to the grounding bus bar
- Record the voltage.
- A reading of zero indicates no fault. A non-zero voltage may indicate an intermittent ground fault.
Continue testing each string individually, always protecting exposed wire ends with wire nuts or tape.
When testing with the negative conductors connected in parallel through the negative busbar, a ground fault will cause voltage to be measured on any positive conductor to ground until the negative parallel connections are removed and the negative end of the strings are isolated.
Step 6. Test PV String Voltage to Ground (Negative Side)
Now repeat the same process for the negative conductors:
- With all positive fuse holders open and the negative end of the conductor isolated from other conductors (open negative fuse holder if available, or disconnect the negative conductor from the bus bar), connect:
- Meter negative lead to the string negative conductor
- Meter positive lead to the grounding bus bar
- Record the voltage.
- Expected result: 0 VDC
Abnormal: Any non-zero voltage may indicate a fault. - After each test, protect exposed wire ends with wire nuts or tape or re-land the conductor safely.
Repeat this for each string in the combiner box.
Step 7. Test Under Wet or Fault-Recreating Conditions
Because intermittent faults often rely on external factors, try testing under similar conditions:
- Perform tests during early morning when condensation is present, after it has rained, or after spraying modules with distilled water (only if safe and permitted).
- If the system uses trackers, move the array to the position when the fault last occurred and retest.
Take extreme caution when testing in wet conditions, as additional safety hazards may be present. Ensure tools are rated for use in these environments.
Step 8. Identify Outliers
Once all strings are tested:
- Compare all ground-to-conductor voltage values.
- Look for outliers—strings that show abnormal readings.
- These are candidates for additional testing using insulation resistance or localized inspection.
For additional isolation methods, see: How to Use Voltage Readings to Locate Ground Faults in Solar PV Arrays
Recommended Tools for This Process
- Fluke 393 FC Solar Clamp Meter – For measuring DC current
- Fluke 283 FC/PV Solar Digital Multimeter and Wireless Current Clamp – For measuring DC current
- Fluke GFL-1500 ground fault location tool
- Fluke 1587 FC Insulation Multimeter – For insulation resistance testing (if needed)
- CAT III or IV rated multimeter – With voltage rating above the system’s Voc
What If No Fault Is Detected?
If you don’t find a fault under current conditions:
- Schedule retesting during adverse weather
- Use monitoring data to narrow the window
- Consider additional insulation resistance testing on de-energized circuits
→ How to Test De-Energized PV Circuits for Ground Faults
Summary
Intermittent ground faults can be difficult to catch, but they’re often the early warning sign of a more serious failure. With careful safety precautions and a step-by-step approach, you can isolate potential problem strings and keep the system running safely.
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.