By Will White, Fluke Senior Application Specialist, DER

There are different types of ground faults, and some are easier to diagnose and locate than others. Several test techniques can be used to identify and locate faults. It can often be challenging to determine which test will provide the fastest and most accurate results. Ground faults are one of the most common issues in photovoltaic installations, regardless of system type or size. 

<>

When a current-carrying conductor contacts grounded metal parts, several problems can happen. 

• Non-current-carrying metal parts, such as a module frame, may be energized, which can create a significant safety hazard.
• Ground faults may cause arching, which results in heat and sparks that can cause fires.
• Current flowing to ground through the ground fault isn’t going to the inverter, which reduces system performance and financial return. 

In this article, we’ll briefly discuss the types of ground faults, we’ll look at the different tests that can be used to identify and locate ground faults, and we’ll discuss when to use the different tests to get the fastest results. 

The Two Types of DC Ground Faults

There are two types of ground faults: hard ground faults and intermittent ground faults.

1. Hard ground faults are low-resistance connections between a current-carrying conductor and a grounded metal part.
   • They are always present and will trip the ground fault detection system in the inverter.
   • There are many causes of hard ground faults, but most of them stem from poor execution of work.
   • Common installation errors include pinching a wire, damaging it during installation, or poor wire management techniques.
   • Because these ground faults are always present, they are relatively easy to find with any of the test methods described below, however, some test methods can take days due to their process requirements or other limitations.
   • Intermittent ground faults are higher-resistance faults that may occur and disappear depending on environmental or mechanical conditions.
   • They are often caused by wires being pinched by tracking arrays.
   • They may also appear when the system is exposed to moisture, such as during rain or in the morning when dew is present.
   • Because intermittent ground faults can be high resistance and have a transient nature, they can be complex to identify and even more challenging to locate.
   • These types of faults may only trigger the inverters' ground fault protection under specific conditions. 

Looking for more in-depth information about these differences? Read the What Is the Difference Between Hard and Intermittent Ground Faults article next.

Test Methods for Locating Ground Faults

Insulation Resistance Testing

Fluke Tools: 1507, 1587 FC, 1537, SMFT-1000

What It Is / How It Works:

• Definition: Insulation resistance testing (IRT) is a method used to detect conductors with low resistance to ground.
• Mechanism: It sends a high-voltage, low-current signal through a conductor and measures the leakage current to ground, which is expressed in ohms.
• Interpretation: Low resistance indicates a potential ground fault.

Use Cases:

• Unknown Fault Location: Use when it’s unclear which circuit contains a ground fault.
• Systematic Isolation:
  • Test entire combiners to narrow down the affected area.
  • Then test individual strings within the affected combiner.
• Intermittent Faults:
  • Can help detect some intermittent ground faults.
  • Best used by comparing similar strings under identical environmental conditions.

Best For:

• Hard ground faults (clear, low-resistance paths to ground).
• Initial fault isolation in large PV systems.
• Comparative testing among similar PV strings to find subtle issues.

Limitations:

• Fault Location Precision: IRT won’t pinpoint the exact location of a ground fault.
• Physical Disconnection Required: Strings often need to be isolated (e.g., disconnecting the negative side from the bus bar), which:
  • Is time-consuming
  • Can cause wear or damage if done repeatedly
• Intermittent Fault Detection:
  • May miss faults that only appear under certain conditions.
  • Requires careful comparison between similar circuits to infer issues.
• Not Conclusive Alone: Can be used alongside the Fluke GFL-1500 when results are ambiguous (e.g., high resistance and capacitance). 

Want to learn how to use insulation resistance testing to locate intermittent ground faults? Read this article next: How to Test PV Strings for Intermittent Ground Faults

Voltage to Ground Testing

Suggestions: 283 FC/PV, 393 FC

What It Is / How It Works:

• Definition: A diagnostic method where voltage is measured between the positive or negative DC conductors and ground.
• Normal Condition: When the inverter is off, there should be no voltage between either DC conductor and ground (i.e., readings close to zero).
• Fault Condition: A ground fault introduces a connection between a conductor (positive or negative) and ground, resulting in measurable voltage potential between that conductor and ground. 

Application: 

• Helps identify which PV string has the fault.
• With basic calculations, can estimate the approximate location of the fault along the string. 

Use Cases:

• Finding Faulted Strings: Detects which string has a ground fault.
• Fault Location Estimation: Allows for approximate fault location using voltage readings and simple math.
• Intermittent Fault Investigation: Can detect intermittent faults if conditions causing the fault are recreated, such as:
  • Moving a tracker
  • Testing during wet conditions

Best For:

• Hard ground faults (direct, low-resistance faults).
• Situations where you need a rough estimate of fault position.
• Arrays with variable conditions that allow fault reproduction.

Limitations:

• Isolation Requited:
  • Must disconnect the negative side of the PV string from other strings or the inverter.
  • This is time-consuming, poses a safety risk, and can damage terminals if done frequently.
• Not Always Effective for Intermittent Faults:
  • Requires the ability to recreate fault conditions reliably.
• Rought Estimation Only:
  • Provides approximate, not precise, fault location.

For more details on this procedure, see: How to Use Voltage Readings to Locate Ground Faults in Solar PV Arrays

Advanced Ground Fault Location Testing

GFL-1500

What It Is / How It Works:

• Definition: A specialized tool for quickly identifying and locating ground faults in solar PV systems.
• Analyze Mode:
  • Measures open-circuit voltage, positive-to-ground, and negative-to-ground voltages.
  • Estimates fault resistance (in ohms) and returns one of four results:

1. Hard fault (low resistance, voltage to ground)
2. No fault detected
3. High resistance fault (voltage present, but resistance is high)
4. High capacitance (inconclusive results due to system characteristics)

• Fault Mode:
  • Injects a traceable signal that can be tracked using a receiver (with audio alarm) or current clamp to pinpoint the exact fault location. 

Use Cases:

• Hard Ground Fault Detection and Location
  • Quickly detects whether a ground fault exists.
  • Pinpoints exact fault location.
• Break Identification:
  • Can find open circuit breaks, such as a failed interconnection between modules.
• Circuit Mapping:
  • Helps map circuits in systems without accurate wiring documentation.

Best For:

• Technicians who need speed and precision in troubleshooting.
• Hard ground faults, and many higher-resistant faults.
• Situations where manual testing is too slow or inconclusive.
• Complex or undocumented arrays, where mapping is beneficial.

Limitations:

• Training Required:
  • Initial learning curve; users must be familiar with device operation to use effectively.
• Not Foolproof on All Faults:
  • May return inconclusive results on high capacitance systems.
• Device Dependency:
  • Relies on correct input (e.g., module count) for fault location accuracy. 

How to Test PV Strings for Hard Ground Faults

Which Ground Fault Test Method Should You Use—and When

Ground faults don’t always present in the same way, and that’s why no single testing method is sufficient on its own. Hard faults and intermittent faults can occur independently—or simultaneously—in any PV system, regardless of design or quality. To troubleshoot efficiently and safely, technicians should always be prepared with multiple diagnostic tools, so they can quickly confirm a ground fault is present, pinpoint its location, and get the system back online as soon as possible.

Use the following table to compare the strengths and limitations of each test method, and choose the right one based on the fault type and site conditions:

Regardless of the method you begin with, having all three diagnostic approaches ready to deploy will give you the best chance of quickly identifying and resolving a ground fault—without trial and error or unnecessary downtime.

Conclusion: Safety, Speed, and Precision Start with the Right Tools

Solar technicians are often under pressure to restore systems safely and quickly. Ground faults, whether persistent or intermittent, can put both people and power output at risk. That’s why the core objective is simple:

1. Identify the fault
2. Locate it precisely
3. Fix it efficiently and safely

To do this, techs need to match the right method to the specific fault they’re dealing with. A hard fault might be found fast with voltage testing or insulation resistance, but locating it exactly takes more precision. An intermittent ground fault could go undetected unless the right conditions and tools are in place. And in more complex or high-stakes systems, advanced diagnostic tools like the Fluke GFL-1500 Ground Fault Locator can drastically cut troubleshooting time and effort.

In short, if you want to keep PV systems safe, productive, and profitable, equip yourself with the full toolkit and the knowledge to use it wisely. That’s how you match the method to the fault—and fix it right the first time.

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.