By Will White, Fluke Senior Application Specialist, DER
Ground faults are one of the most common issues in solar photovoltaic (PV) systems, and they’re often the most dangerous. When a direct current (DC) conductor in a PV array makes unintended contact with grounded metal, it creates a DC ground fault that can lead to electrical fires, arc flashes, damaged equipment, and serious safety risks for personnel.
Despite their frequency, many DC ground faults go undetected, especially in large-scale or aging solar arrays. Understanding why these faults matter—and how to detect them early—can help you protect people, maximize uptime, and ensure system reliability.
How Does a DC Ground Fault Occur?
A DC ground fault occurs when a current-carrying conductor (like DC positive or negative) comes into contact with a grounded metal surface, such as a PV module frame, racking, conduit, or the equipment grounding conductor (EGC). This results in current flowing through unintended paths, outside of the designed electrical circuit.
Learn the fundamentals in What is a DC Ground Fault in a PV System?
While a single fault may not immediately shut down the system, it introduces leakage current that bypasses protective devices and increases the risk over time.
How DC Ground Faults Create Fire Hazards
The real danger of DC ground faults lies in the combination of undetected leakage current and the possibility of a second fault.
In a properly grounded system, the first ground fault creates a path to ground but may be too small to trip the ground fault protection (GFP) device. In fact, many faults are less than 1 amp, well below the detection threshold of legacy GFDIs (Ground Fault Detection Interrupters).
If a second fault develops on a different conductor, the two faults may create a parallel current path, bypassing the inverter’s internal protection and allowing large amounts of current to flow directly through metal surfaces. This can:
- Cause a DC arc fault, presenting a safety risk to personnel
- Melt insulation and conductors, damaging equipment
- Ignite surrounding materials, starting a fire
Case Study: The 2009 Bakersfield Fire
One of the most widely cited PV fire incidents caused by ground faults occurred in Bakersfield, California, in 2009.
A 383 kW rooftop system experienced a 2.5 amp ground fault on a 12 AWG conductor. The system continued operating because the current was too low to trigger the GFDI.
Later, a second fault occurred: an expansion joint separated on a 500 MCM conductor, causing a massive 311-amp fault. Instead of safely interrupting the circuit, current returned through the original small ground fault, rapidly overheating the conductor and starting a rooftop fire.
This tragic sequence highlights two critical points:
- Small faults are not less hazardous
- Undetected ground faults can escalate into catastrophic events
Why Ground Faults Are Hard to Detect
Most ground faults—especially intermittent or low-level ones—don’t produce enough current to trip standard ground fault protection, especially in older, transformer-based inverters.
Here’s why:
- GFDI fuse-based protection in transformer-based inverters often requires several amps to blow the fuse
- Residual current detectors (RCDs) in transformer-less inverters are more sensitive but still have thresholds (~300 mA or more)
- Environmental conditions (e.g., dry weather) can temporarily raise resistance and hide a fault
- Insulation breakdowns may only cause current to leak intermittently (e.g., during rain or tracking array movement)
That’s why proactive testing is essential. Even if a system is running, it may still be operating with hidden faults.
Learn how to test for hard and intermittent faults:
- How to Test PV Strings for Intermittent Ground Faults
- How to Test PV Strings for Hard Ground Faults
- How to Test De-Energized PV Circuits for Ground Faults
How Ground Faults Lead to Arc Faults
A ground fault can cause an arc fault when the damaged wire creates a high-resistance path that produces heat and sparking.
DC arcs are particularly dangerous because:
- They can persist indefinitely until power is interrupted
- They are harder to extinguish than AC arcs
- They can ignite surrounding materials (dust, plastic, insulation, grass, roof materials)
High voltage PV arrays—such as those operating at 1,000 VDC or 1,500 VDC—are especially susceptible to sustained arcs once a ground fault forms.
Arc Flashes and Personnel Safety
In addition to causing fires, DC ground faults can lead to arc flash incidents that pose life-threatening risks to technicians.
If a system contains a hidden ground fault, opening a fuse holder or pulling a conductor off a terminal can open a circuit with circulating current and potentially cause a flashover. In worst cases, this results in:
- Severe burns
- Hearing damage
- Blast injuries
- Equipment damage
This is why testing for current before opening a non-load break rated disconnect and using personal protective equipment (PPE) is essential when working on energized systems, even if the system appears to be functioning normally.
For safe testing, technicians must:
- Test for current in the circuit with a non-contact clamp meter before opening non-load break rated disconnecting devices like fuse holders or module interconnections
- Use proper personal protective equipment
- Electrically insulated gloves
- Flame-resistant clothing
- Arc flash-rated face shields or full suits, depending on system size
How Ground Faults Damage Equipment
Beyond safety, ground faults degrade equipment performance and reliability.
If not detected and repaired, they can:
- Trip inverters repeatedly, reducing uptime
- Cause false data in monitoring systems
- Lead to corrosion or long-term wear at the fault site
- Destroy PV modules, junction boxes, or home run conductors
Additionally, repeated tripping from undiagnosed faults can mask other issues and increase service costs over time.
The Importance of Proactive Detection
The best way to prevent ground faults from becoming dangerous is through early detection and isolation. Tools like the Fluke 1587 FC Insulation Multimeter, SMFT-1000, and GFL-1500 allow technicians to:
- Conduct insulation resistance tests
- Identify conductors with low resistance to ground
- Isolate problem strings or circuits before major failures occur
- Find the exact location of the ground fault
Learn how to find the source of a fault: How to Use Voltage Readings to Locate Ground Faults in Solar PV Arrays
And when repairs are needed: How to Repair Ground Faults in PV Systems
Designing for Safer Ground Fault Detection
Newer system architectures help reduce the risk of hidden faults:
- Transformer-less inverters use RCDs for better fault sensitivity
- Module-level power electronics (MLPEs) limit the fault to a single module
- Ungrounded or floating systems reduce the likelihood of unintentional ground paths
Even so, these technologies are not foolproof. Regular testing and documentation must be part of every maintenance protocol.
Final Thoughts
DC ground faults are not just a nuisance—they are a silent threat to PV systems. Left undetected, they can evolve into fire hazards, arc flashes, or complete system failures.
Understanding why ground faults are hazardous is the first step toward developing safer, more reliable solar power installations. The next step is to take action using the right testing procedures and tools.
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.
