Okay, this is a fascinating and complex situation. Here's a breakdown of the likely causes, what the data points to, and what you should investigate next, focusing on the inverter as the prime suspect.

Understanding the Problem



Root Cause

The solar inverter is injecting noise/fault current into the electrical system, causing havoc with sensitive electronics and breakers. It's affecting both your client's house and their neighbor's. The common connection is the padmount transformer.

Dual Function Breaker Sensitivity

Dual Function (DF) breakers (AFCI/GFCI) are highly sensitive to unusual waveforms and stray currents. This is why they are the first to trip. Standard breakers would likely show no symptoms at all.

Inverter Failure

The fact that the problem disappears when the solar disconnect is opened strongly suggests a fault within the inverter.

The Neutral Connection of Square D Homeline DF Breakers

These breakers require a proper neutral connection to function correctly. If the neutral connection is poor or intermittent, it can cause nuisance tripping. However, the neighbor's house also being affected points to a broader issue than just a bad neutral.

NEV (Neutral-to-Earth Voltage)

The fact that NEV is low at 1.5V means the issue isn't a massive ground fault. It is still good practice to check with the POCO to determine the condition of the grounded conductors/multi-grounded neutrals in the area.

Possible Inverter Failure Modes

Here's where it gets technical. An inverter's job is to convert DC power from the solar panels into clean AC power that matches the grid's voltage, frequency, and waveform. Several things can go wrong internally to cause the problems you're seeing:1.

DC Leakage Current

The inverter may be allowing DC current to leak into the AC side. Even small amounts of DC can saturate transformer cores in appliances, causing buzzing and malfunction. It can also interfere with the zero-crossing detection used by AFCI/GFCI breakers.2.

Harmonic Distortion

The inverter's output waveform might be heavily distorted, containing excessive harmonics. Harmonics are multiples of the fundamental frequency (60 Hz in the US). High levels of harmonic distortion can interfere with electronic devices, cause overheating, and trip breakers.3.

Grounding Issues Within the Inverter

The inverter's internal grounding system could be faulty. This might create a path for fault current to flow through the neutral or ground conductor, even without a large NEV.4.

Insulation Breakdown

Insulation inside the inverter may be failing, leading to leakage current and/or voltage spikes.5.

Firmware/Software Glitches

Although less likely, a bug in the inverter's software could cause it to malfunction and produce a distorted output.

Why this Affects Neighbors (and Yours)



Shared Transformer

Both houses are fed from the same padmount transformer. This creates a common point for the noise/fault current to propagate through the neutral conductor.

Neutral Conductor as a Path

In a typical electrical system, the neutral conductor carries return current from loads. If the inverter is injecting noise/fault current onto the neutral, it can travel back to the transformer and then into the neutral of other houses connected to the same transformer.

Next Steps (Investigation and Troubleshooting)


Document Everything

Record the make, model, and serial number of the solar inverter. Note the exact symptoms in both houses.2.

Contact the Inverter Manufacturer

Immediately contact the inverter manufacturer (or a certified installer/service technician for that brand). Explain the problem in detail, including the symptoms, the fact that it disappears when the solar is disconnected, and that it affects multiple houses on the same transformer. They may have seen this issue before and have specific troubleshooting steps or a known recall.

Important Questions for the Manufacturer

Have they seen similar issues with this model? What are the recommended testing procedures to isolate the problem? Do they have a list of certified service technicians in your area? Is the inverter still under warranty?3.

Inverter Testing (by Qualified Technician)

Do not attempt these tests yourself unless you are a qualified electrician with experience in solar PV systems and inverters!



Insulation Resistance Test (Megger Test)

This tests the integrity of the inverter's insulation. A low resistance reading indicates a potential insulation breakdown.

DC Injection Test

Measures the amount of DC current being injected into the AC output. Most inverters have a maximum allowable DC injection current (usually very low, like 0.5% of the rated AC current).

Harmonic Analysis

Use a power quality analyzer (like the Fluke 345 or similar) to measure the harmonic content of the inverter's AC output. Compare the readings to the inverter's specifications.

Check Grounding Connections

Carefully inspect all grounding connections related to the inverter, including the equipment grounding conductor (EGC) and the grounding electrode conductor (GEC). Ensure they are tight, clean, and properly sized.

Neutral-Ground Bond (If Applicable)

Verify the presence and integrity of the neutral-ground bond within the inverter (if required by the manufacturer).4.

Thorough Wiring Inspection

Even though the inverter is the prime suspect, don't rule out other potential issues.

Neutral Connections

Meticulously check all neutral connections in both houses, especially at the breaker panel, receptacles, and lighting fixtures. Look for loose connections, corrosion, or signs of overheating.

Grounding System

Inspect the grounding electrode system (ground rods, water pipes, etc.) to ensure it is intact and properly connected.

Conduit Current

While you mentioned you're not familiar with conduit current measurement, you can still check for unusually high current on the outside of the conduit using your clamp meter. This could indicate a grounding fault or current flowing through the conduit instead of the intended conductors. Be careful when clamping around metal conduits as they may be grounded. Ideally, you would measure current on each conductor to isolate any imbalance.5.

Monitor the Solar System

If possible, use a data logger to continuously monitor the inverter's output voltage, current, frequency, and power factor over a period of time. This may reveal intermittent problems that are not apparent during a brief snapshot measurement.6.

Communicate with the Utility (POCO)

Inform the utility about the problem and your findings. They may want to investigate the padmount transformer and the grounding system in the area.

Important Safety Considerations



Working with Electricity is Dangerous

Always take proper safety precautions when working with electrical equipment. Wear appropriate PPE (personal protective equipment), such as insulated gloves and safety glasses.

De-energize Circuits When Possible

Whenever possible, de-energize the circuit you are working on before performing any tests or repairs.*

Consult a Qualified Electrician

If you are not comfortable performing these tests or repairs yourself, consult a qualified electrician with experience in solar PV systems.

Summary:

The most likely cause of the problems you're seeing is a malfunctioning solar inverter injecting noise/fault current into the electrical system. Thorough testing of the inverter and a careful inspection of the wiring and grounding system are essential to isolate the problem and ensure the safety and reliability of the electrical installation. Document, communicate, and prioritize safety. Flag for review