Okay, you're facing a common and frustrating issue: intermittent or hard-to-find ground faults in a solar PV array. Let's break down the problems with your current approach and discuss more effective methods.

Problems with the Existing Voltage Drop Test



Limited Reliability

As you've experienced, this test can be inaccurate, leading to false positives (thinking the fault is in the field when it's in the inverter) and false negatives (missing a fault altogether).

Unclear Explanation

The "2 volts per second" drop is not a standardized or universally understood test. It's likely based on a specific system characteristic (capacitance, leakage resistance) that may not be consistent across all installations or even over time within the same installation. It's essentially relying on a very specific leakage current characteristic.

Voltage vs. Current

A ground fault is, at its core, an unintended current path to ground. Measuring voltage potential to ground is less direct than measuring the actual current flowing through the ground path.

Safety Concerns

Working with high DC voltages carries inherent risks. Minimizing direct voltage measurements (especially with open circuits) can improve safety.

Why an Ammeter on the Chassis Ground is a Better Idea (Potentially)



Direct Current Measurement

This is a more direct measure of the problem. If a ground fault exists, current will flow through the grounding conductor.

Easier Quantification

You can see how close you are to the inverter's GFDI trip point (5 amps).

Safer (Potentially)

You are measuring current in a grounded conductor rather than measuring voltage referenced to ground with an open circuit and floating potential.

However, There Are Caveats



GFDI Trip Point

A small amount of leakage current can be normal in PV systems. The 5A trip point of the GFDI is the determining factor in the systems tolerance.

AC Interference

Ensure your clamp meter is designed to read DC current accurately. AC current from nearby sources can give false readings.

A Systematic Troubleshooting Approach (Recommended)

Here's a comprehensive method for finding ground faults in a solar array, combining elements of both your ideas and adding essential steps:1.

Safety First

Always follow proper lockout/tagout procedures. Use appropriate PPE (insulated gloves, safety glasses, etc.). Work with a qualified electrician or PV technician.2.

Initial Assessment



Record Data

Note the exact conditions when the GFDI tripped (time of day, weather, system production). This can provide clues (e.g., only trips in the morning after dew).

Visual Inspection

Carefully inspect the entire array for obvious issues: Damaged modules (cracks, delamination, signs of water ingress). Damaged or improperly installed wiring (pinched, cut, exposed conductors). Compromised conduit or junction boxes. Evidence of rodent damage. Loose connectors. Water pooling in or around components.

Inverter Health

Check the inverter's error logs and diagnostics. Are there any specific fault codes related to ground faults or other issues? Consult the inverter manual for interpretation.3.

Isolation and Segmentation



Isolate the Issue

Disconnect the array from the inverter completely (both positive and negative conductors). Terminate all sources. Use appropriate safe practices for working with DC voltage.

Open all Combiner Boxes

Open each combiner box in the array (if applicable).

Sequential Testing

The goal is to narrow down the problem to a specific section of the array.4.

Testing Methods (Use in Combination)



Option 1: Ammeter on Chassis Ground with Inverter Connected

Reconnect the array to the inverter, but leave the GFDI fuse out. Place a DC clamp meter around the inverter chassis ground. Monitor the current. If you see a significant current flow (approaching the GFDI trip point), you likely have a ground fault somewhere in the connected array.

Important

Observe the polarity of the DC current reading. This might indicate whether the ground fault is on the positive or negative side of the array. But do not rely on this solely. If the current reading is low, the fault might be intermittent or only occur under specific conditions (e.g., when modules are wet).

Option 2: Resistance Testing to Ground (Preferred After Isolation)



Disconnect

Disconnect the array from the inverter completely.

Module Level

Disconnect each module from the string.

Resistance Measurement

Using a megohmmeter (insulation resistance tester) set to an appropriate voltage (e.g., 500V or 1000V DC), measure the resistance: From the positive (+) conductor of each module to ground (module frame). From the negative (-) conductor of each module to ground (module frame).

Interpretation

A very high resistance (ideally in the megaohms range) indicates good insulation. A low resistance (anything significantly below the megaohm range) suggests a ground fault. The lower the resistance, the more severe the fault.

Repeat for Each Module

Test each module individually.

String Level Resistance Test (after individual modules are verified good)

After testing each module, reconnect them into strings. Then perform the same test from the positive and negative string conductor to ground.

Option 3: Voltage Testing (Use with Caution and After Isolation)



Disconnect

Disconnect the array from the inverter.

String Level



Voltage Measurement

With your multimeter, measure the DC voltage: From the positive (+) conductor of each string to ground. From the negative (-) conductor of each string to ground.

Interpretation

In a healthy system, you would expect to see roughly half the open-circuit voltage of the string on both the positive and negative conductors. If you see a significantly lower voltage on one side, it could indicate a ground fault on that side.

Important Safety Note

The "voltage drop per second" test you described relies on the capacitance and leakage characteristics of the array. The voltage test above (referenced from each pole to ground) relies on a resistive path to ground.5.

Pinpointing the Fault



If you find a low resistance or abnormal voltage on a specific module or string

Carefully inspect that section of the array again. Pay close attention to connectors, wiring, and module frames.

Consider Environmental Factors

If the fault only appears when the modules are wet, try spraying individual modules with water while monitoring the resistance or current to ground. This can help isolate the problem.

Connector Issues

Disconnect and re-mate connectors. Check for corrosion or water ingress. Use dielectric grease if appropriate.

Wiring Issues

Replace suspect wiring. Ensure proper strain relief.6.

Repair and Verification



Repair

Fix the identified issue (replace damaged modules or wiring, repair connectors, etc.).

Re-test

After the repair, repeat the resistance test (megohmmeter) to confirm that the ground fault is resolved.

Monitor

After reconnecting the array to the inverter, monitor system performance to ensure the GFDI does not trip again.

Key Considerations



Megohmmeter (Insulation Resistance Tester)

This is an essential tool for troubleshooting ground faults in PV systems. Do not rely solely on a multimeter.

Intermittent Faults

These are the most challenging. Sometimes, you'll need to use a combination of techniques and be patient. Thorough visual inspections are crucial.

Documentation

Keep detailed records of all tests, measurements, and repairs. This will help you track down future issues more efficiently.

Training

Ensure your technicians are properly trained in safe work practices for DC voltage and the use of test equipment.

Firmware

As you found with the inverter ground fault, its a good idea to check on inverter firmware.

In Summary:

The most reliable approach is a systematic one, starting with isolation, visual inspection, and then using a megohmmeter to measure insulation resistance to ground. The ammeter on the chassis ground can be a useful initial indicator, but it should be used in conjunction with other testing methods. Ditch the voltage drop test; it's too unreliable.By following these steps, you'll be better equipped to find and fix ground faults in your solar array quickly and safely. Flag for review