What is the Difference Between a Hard and Intermittent Ground Fault?
By Will White, Fluke Senior Application Specialist, DER
Ground faults are a leading cause of safety hazards, shutdowns, and fire risks in photovoltaic (PV) systems, but not all ground faults behave the same. Some are continuous and easy to identify. Others are unpredictable, appearing only in certain conditions.
To maintain safety and performance in PV arrays, it's critical to understand the difference between hard ground faults and intermittent ground faults, how each develops, and how to test for them effectively.
When Does a Ground Fault Occur?
A ground fault occurs when a current-carrying conductor (DC positive or negative; AC L1, L2, or L3) comes into contact with a grounded metal surface, such as module frames, conduit, or racking. This creates an unintended path for current to flow to ground, bypassing the inverter or other protective equipment.
For a full overview, see: What is a DC Ground Fault in a PV System?
Hard vs. Intermittent Ground Faults: The Basics
| Feature | Hard Ground Fault | Intermittent Ground Fault |
|---|---|---|
| Connection Type | Continuous and low resistance | Appears and disappears based on conditions |
| Detection Difficulty | Relatively easy to identify | Often hard to detect |
| GFDI/Inverter Reaction | May trip immediately | May not trip at all or trip sporadically |
| Common Triggers | Crushed or abraded wires, melted insulation | Moisture, movement, thermal expansion |
| Testing Method | Standard voltage/insulation resistance testing | Requires voltage/insulation resistance testing under specific conditions |
| Repair Urgency | Immediate safety hazard | Must be fixed before it worsens |
What Is a Hard Ground Fault?
A hard ground fault is a persistent, low-resistance connection between a PV system conductor and a grounded metal surface. This type of fault remains present under all operating conditions.
Common Causes:
- Pinched wires from poor cable management
- Melted insulation from overcurrent or arcing
- Abrasion due to vibration, rubbing on racking, thermal expansion/contraction
- Incorrect grounding or termination
How to Detect It:
- Measure the voltage between the DC positive/negative and ground
- Use a multimeter, clamp meter, or the analyze function of the GFL-1500 ground fault location tool
- Use an insulation resistance tester (e.g., Fluke 1587 FC, 1537, SMFT-1000)
- Expect a non-zero voltage to ground and/or low insulation resistance (< 1 MΩ)
Learn more: How to Test De-Energized PV Circuits for Ground Faults
Why It's Dangerous:
Hard ground faults are often the triggering fault in inverter shutdowns. If a second fault occurs on the opposite polarity, it can create a parallel current path, leading to fires or arc flash events.
What Is an Intermittent Ground Fault?
An intermittent ground fault is a temporary or conditional connection between a conductor and ground. These faults may only appear when:
- The system is wet (rain, condensation, humidity)
- Modules or racking expand/contract due to temperature
- Trackers move the array into a fault-prone position
- Wires sag or rub during wind or snow load
Because these faults are not constant, they can be difficult to detect during routine maintenance or dry weather.
Why Intermittent Faults Are Risky
Intermittent faults are dangerous because:
- They often evade detection by standard ground fault protection devices
- They can evolve into hard faults over time
- They cause repeated inverter tripping, resulting in downtime
- They can arc, which may lead to a fire
- If ignored, they may become part of a high-current fault loop if a second issue arises
Learn how to isolate them: How to Test PV Strings for Intermittent Ground Faults
Signs You're Dealing with an Intermittent Ground Fault
You might be facing an intermittent fault if:
- The inverter shows sporadic ground fault errors
- The system trips only during rain or early mornings
- No fault is detected during dry or mid-day conditions
- Faults occur at a specific time of day when a tracking array moves into a specific position.
- Insulation resistance tests show borderline values
How to Test for Each Fault Type
Testing for Hard Ground Faults
- Isolate and de-energize the circuit
- Perform voltage-to-ground tests
- Use an insulation resistance tester
- Expect consistent fault readings
More: How to Test De-Energized PV Circuits for Ground Faults
Testing for Intermittent Ground Faults
- Perform measurements during or after rain, dew, or temperature shifts
- Use a clamp meter to verify no current is flowing before opening any circuits
- Watch for sudden voltage spikes to ground during array motion
- Look for insulation resistance values that may be considered passing but are lower than the resistance of other similar circuits under the same environmental conditions.
More: How to Test PV Strings for Intermittent Ground Faults
Why Ground Fault Type Matters
Understanding the fault type affects everything from diagnosis to prevention:
| Task | Hard Faults | Intermittent Faults |
|---|---|---|
| Repair timeline | Immediate | Conditional on fault recurrence |
| Safety risk | High | Moderate-to-high |
| Detection | Reliable, but time intensive, with basic tools | Requires more environmental context |
| Prevention | Physical protection, quality wiring | Physical protection, quality wiring, flexible cable support |
| Monitoring systems | Likely to catch | Often miss until the inverter trips |
Repair Strategies for Each Type
For Hard Faults:
- Remove damaged conductors and replace with new wiring
- Install field-installable connectors where necessary
- Replace burned modules or junction boxes
- Retest insulation and voltage-to-ground
More: How to Repair Ground Faults in PV Systems
For Intermittent Faults:
- Replicate conditions that trigger the fault (wet modules, movement)
- Test and replace suspect connectors or junction boxes
- Inspect conductor bends, racking edges, and tracker alignment
- Use UV-resistant wire management hardware and improved cable management to prevent recurrence
Summary
Hard and intermittent ground faults may start the same way—damaged wire insulation, a pinched conductor—but their behavior and detection require different strategies. By understanding the difference, technicians can more accurately troubleshoot, prevent, and repair faults in solar PV systems.
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.