Solar Inverter Isolation Resistance Faults: Diagnosis and Resolution

Isolation resistance faults are closely related to ground faults but represent a broader category of insulation breakdown in the DC circuit. When an inverter reports an isolation resistance fault, it means the insulation between the DC conductors and ground has degraded below a safe threshold. This guide explains how inverters detect isolation issues, what the fault codes mean, and how to systematically locate and resolve the problem.

How Inverters Measure Isolation Resistance

Grid-tied inverters continuously monitor the isolation resistance of the DC array to ground. They do this by injecting a small test signal and measuring the leakage current, or by measuring the voltage between each DC pole and ground. When the calculated isolation resistance drops below a threshold (typically defined by the inverter's safety standard, often around 1 MΩ for transformerless inverters), the inverter reports an isolation fault and disconnects.

Common Causes of Isolation Faults

• Degraded module backsheet allowing leakage between cells and the grounded frame
• Water ingress into DC connectors, combiner boxes, or junction boxes
• Pinched or damaged DC cable with insulation compromised at the pinch point
• Rodent damage to string wiring
• Aging cable insulation, particularly on arrays over 10 years old
• Thermal cycling loosening connector seals over time
• Lightning or surge damage to surge protection devices (SPDs) creating a leakage path
• Improper installation — cables resting on sharp edges or in contact with hot surfaces
• PID (potential-induced degradation) creating leakage paths in modules near the negative pole

Intermittent vs. Persistent Isolation Faults

Isolation faults can be intermittent or persistent, and the pattern provides important diagnostic clues:

• Intermittent, correlated with rain or dew: Strong indicator of water ingress into a connector, combiner box, or junction box. The fault clears when the system dries.
• Intermittent, correlated with temperature: Thermal expansion may be opening or closing a gap in degraded insulation. Often related to pinched cables or backsheet issues.
• Persistent: The insulation has completely failed and the fault will not clear. Requires immediate physical inspection and repair.
• Progressive — getting worse over time: Indicates ongoing degradation (backsheet breakdown, cable aging) that will eventually become persistent.

Step-by-Step Fault Location

1. 1Record the exact fault code, the reported isolation resistance value (if available), and weather conditions. This information helps narrow the search.
2. 2De-energize the system following lockout/tagout procedures. Remember that DC arrays are energized during daylight hours.
3. 3Open the combiner box and disconnect all strings. This allows you to test each string individually.
4. 4Perform a megger (insulation resistance) test on each string to ground. Use a test voltage appropriate for the system (500V for systems under 600V, 1000V for systems up to 1000V). Record the resistance value for each string.
5. 5Healthy strings typically read above 40 MΩ. A string reading below 1 MΩ has a significant insulation failure. Readings between 1 MΩ and 40 MΩ indicate degradation that should be monitored.
6. 6Once the faulted string is identified, inspect the full cable run for visible damage: pinched wires, chewed insulation, cracked connectors, or water intrusion points.
7. 7If no visible damage is found, use a multimeter to check each module's frame-to-conductor isolation. Start at the module closest to the negative pole (most likely to have PID-related leakage) and work outward.
8. 8If the fault is in the home-run wiring (between the combiner box and the inverter), test the home-run cable separately after disconnecting it from both ends.

Resolving the Fault

Once the fault location is identified, the resolution depends on the component:

• Damaged cable: Replace the affected cable run. Do not attempt to repair insulation with tape — this is a temporary fix that will fail.
• Failed connector: Cut and replace the connector using proper crimping tools. Do not mix connector brands — this is a common cause of water ingress.
• Degraded module: Replace the module if the backsheet is compromised. Check the manufacturer's warranty — backsheet degradation may be covered.
• Water-damaged combiner box: Dry the box, replace any corroded components, and improve the sealing. Consider relocating the box if water intrusion is recurring.
• PID-affected modules: If PID is the cause, some inverters offer a PID recovery function. If not available, module replacement may be necessary.

When to Call for Remote Support

Isolation faults can be challenging to locate, particularly intermittent faults that clear during dry conditions. If you have performed megger testing but cannot pinpoint the exact location, or if the fault is intermittent and difficult to catch, remote support can help by analyzing your fault logs, weather correlation, and test results to recommend the most efficient diagnostic approach.