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How to Detect Ground Faults in Your PV System
Introduction:
Ground faults in PV systems are one of the most common—yet most elusive—issues technicians face in the field. They often develop silently due to rodent damage, moisture ingress, UV-degraded insulation, or improperly routed cables. If left undetected, ground faults can lead to inverter shutdowns, loss of system yield, or even fire.
In this article, we’ll show you how to locate a ground fault in a solar PV string using only a multimixer, a basic understanding of voltage behaviour, and a method proven in real-world installations.
⚠️ Important Notice:
This test should only be performed by qualified personnel. DC systems can carry lethal voltages. Ensure proper PPE is used and that the system is safely isolated from the inverter before conducting any tests.
Understanding the Problem:
What Is a Ground Fault? A ground fault occurs when a conductor in the DC system unintentionally contacts ground. This creates a leakage path, which can trigger insulation resistance faults, trip monitoring systems, or compromise inverter safety protocols.
While modern inverters (like those from Solis) have insulation monitoring functions, they don't always pinpoint the location of the fault. For that, we need to turn to voltage measurements.
⚠️ Dangers & Safety Precautions
Before working on ground-fault detection, keep these crucial safety measures in mind:
⚡ 1. Risk of Lethal Electric Shock
- Even with the inverter off, PV modules remain live when exposed to light. Ground faults can make the array or structure live, creating an unseen yet deadly risk.
- Always disconnect the inverter and isolate the PV string(s) from all power sources before measuring
- Only handle cables by their insulation, never touch frames, module backsheets, mounting hardware, or metal structures.
? 2. Prevent Damage to Measuring Equipment
- Use DC multimeters rated ≥ 1000 V DC (or ≥ 600 V if specified) to prevent overvoltage damage .
- Overvoltage may damage the meter, energizing its enclosure and posing a shock hazard .
? 3. Personal Protective Equipment (PPE)
- Wear electrical PPE: insulated gloves, eye protection, face shield, flame-resistant clothing—and respiratory protection if cleaning or opening equipment
- Ensure unauthorized personnel are kept clear of the testing area
⏱️ 4. Wait After Disconnecting
- After shutting off and isolating a string, wait for ~5 minutes to allow capacitive discharge before touching system components.
? 5. Avoid Energizing Faulty Strings
- Do not reconnect PV strings with a ground fault to the inverter—even for testing
- Keep the inverter’s AC breaker turned off and ensure the DC switch (isolator) on the inverter remains in the OFF position until the ground fault has been fully located and repaired.
? 6. Structural & Electrical Bonding
- Ensure all module frames, support structures, and mounting hardware are properly earthed/grounded.
- Maintain over-voltage surge protection for any communication or network lines connected to the array.
? 7. Avoid Fire & Explosion Hazards
- Rare faults may generate explosive or toxic gases inside inverter enclosures. Do not operate DC switches or open the enclosure until cleared
- Use respiratory protection when there’s potential for contact with toxic by-products.
Step-by-Step: How to Detect and Locate a Ground Fault
Step 1:
Confirm That a Ground Fault Exists
Start by isolating the suspected string from the inverter. Measure the voltage from:
- Positive (+) to Ground
- Negative (−) to Ground
Example:
10 Modules (50V/Module) – Total Voc 500V
In a healthy floating system, voltage from either terminal to ground is usually under 30-40V.
If one side shows significant higher voltage, there is likely a ground fault.
This imbalance confirms leakage through an unintended path.
Step 2:
Estimate the Fault’s Location – Single Multimeter Method
Now, measure the total string voltage (Voc) from + to −. Then divide it by the number of modules to determine voltage per module.
Example:
- Total Voc = 500V
- 10 modules → 500 / 10 = 50 V/module
If + to Ground = 300V
→ 300 / 50 = 6 modules from the positive end
If − to Ground = 200V
→ 200 / 50 = 4 modules from the negative end
? This narrows your inspection zone to between modules 6–7.
Step 3:
Locate Fault Using Dual Multimeter Method (Voltage Divider Technique)
This method improves accuracy using two identical multimeters with the same internal resistance.
Procedure:
- Connect one voltmeter between positive and a common point.
- Connect a second voltmeter between negative and the same common point.
- Ground the common point (this balances the voltage between the meters).
- Read both voltages.
Example 2: Total string voltage: 548 V
- Left multimeter (− to PE): 295.6 V
- Right multimeter (+ to PE): 252.6 V
Now calculate:
Fault location = (295.6 / 548) × 18 modules ≈ 9.7
→ Start checking between module 9 and 10 from the negative end.
This method offers higher precision and is especially useful in longer strings.
Conclusion:
Ground faults can be frustrating—but with the right tools and approach, they can be pinpointed quickly and accurately. At Solis, we recommend this voltage-based method for all field technicians dealing with insulation or ground leakage faults.
If your inverter flags a PV-ISO error, don’t panic. Get your multimeter, isolate the string, and follow this guide step by step.
