Ground Fault Clears When One String Is Isolated

Why this matters

When an inverter's ground-fault (GFDI / Riso / isolation) fault clears the moment you disconnect one string, you have already done the hard half of the diagnosis: the fault is in that string. The remaining work is to localize WHERE in that string the conductor insulation has failed to ground and to fix it without leaving a latent fault that re-arms intermittently in wet weather. A PV ground fault is a real shock and fire hazard - a faulted conductor energizes equipment and roof structure, and a second fault on the opposite polarity can create a fire-igniting fault current the GFDI may not catch. This tree localizes the fault methodically.

Symptom presentation

The inverter logs a ground fault, isolation (Riso) fault, or GFDI trip and stops producing. Disconnecting one string at the combiner or inverter input clears the fault and the inverter resumes (often on the remaining strings). Reconnecting the suspect string re-trips it. The fault may be intermittent - present in the morning dew or after rain and absent when the array dries - which points to moisture bridging a compromised insulation point.

Quick checks

With the inverter off and the array safely de-energized per rapid shutdown, confirm the suspect string and gather isolation data:

• Re-add strings one at a time: the string that re-trips the GFDI is the faulted string. This confirms the string but not the location.
• Measure insulation resistance (megohmmeter / Riso) from each conductor of the suspect string to the equipment-grounding conductor. A healthy PV string reads in the megohm-to-gigohm range; a fault reads low (kilohms or less). A low reading on positive-to-ground vs negative-to-ground tells you which polarity conductor is faulted.
• Note whether the fault is wet-weather only. Intermittent moisture-driven faults read marginal when dry and collapse when wet - test both states if possible.

Isolation tree

Branch A - Damaged module lead or connector. The most common location: a pinched, chafed, or UV-degraded module lead, or a connector with water ingress, lets the conductor leak to the frame/ground. Confirm: with the string isolated, measure Riso of each module's leads to its frame, or split the string in halves and meg each half to narrow the fault, then quarter it. A module or connector reading low to ground is the fault. Remedy: replace the damaged lead/connector or module.

Branch B - Conductor insulation abrasion against the rail or roof. PV wire run across a sharp rail edge, an unbushed conduit entry, or a roof penetration can wear through to the conductor, grounding through the metal racking. Confirm: inspect the conductor routing along the rails and penetrations of the suspect string for chafe points, and meg the wire harness section to ground. A low reading on a harness run with a visible abrasion confirms it. Remedy: repair/replace the conductor and add edge protection/bushings so it cannot re-chafe.

Branch C - Water ingress at a junction box or combiner. A flooded module junction box, a combiner with failed sealing, or a transition fitting full of water bridges conductor to ground. Confirm: the wet-weather-only pattern, plus a low Riso that improves as the box dries. Open and inspect for water and corrosion. Remedy: dry, reseal, and replace corroded components; correct the drainage/sealing that admitted water.

Branch D - Pinched conductor under a clamp or mount. A homerun or module lead crushed under a mid/end clamp or a roof mount can slowly cut through insulation. Confirm: inspect under clamps and mounts along the string; meg the affected segment. Remedy: re-route the conductor clear of the clamp, replace the damaged section.

Branch E - Inverter or GFDI false trip. Rarely, the inverter's ground-fault detection or its internal isolation monitor is faulty and trips with no real array fault. Confirm: every string megs healthy (high Riso) and the fault persists or clears erratically regardless of which string is connected. If the array insulation is provably good, the inverter's GFDI/Riso circuit is suspect. Remedy: RMA/repair the inverter - but only after a clean megohmmeter result across all strings, because a real ground fault must never be masked by blaming the detector.

Confirming diagnosis

The string-isolation result names the faulted string; the megohmmeter names the conductor and, by splitting the string in halves and quarters (the halving method), narrows the fault to a specific module, lead, or harness run. A low Riso to the equipment-grounding conductor on one polarity, localized by progressive halving, is the definitive confirmation - and a visual at that location (chafe, water, pinch) ties the electrical fault to a physical cause. The wet-versus-dry behavior distinguishes a moisture-bridged insulation breach (intermittent) from a hard short (constant). Only when every string megs high is the inverter's GFDI itself the suspect.

Remediation

Repair or replace the faulted element found by the halving method - module lead, connector, abraded conductor, or water-logged junction/combiner box - and eliminate the physical cause: add edge protection and bushings at rail and conduit transitions, reseal and re-drain boxes, and re-route conductors clear of clamps and mounts. Replace any corroded component. Re-megger the repaired string to confirm Riso is back in the megohm-plus range, reconnect strings one at a time confirming no GFDI trip, and verify the inverter runs clean through a wet morning if the fault was moisture-driven.

References

• NEC Article 690 - Solar Photovoltaic Systems, including 690.5 (ground-fault protection) and 690.41 (system grounding/ground-fault detection).
• NEC 690.12 - Rapid Shutdown of PV Systems on Buildings (conductor de-energization prior to and within array boundary).
• UL 1741 - Inverters, Converters, Controllers and Interconnection System Equipment (GFDI/isolation monitoring requirements).
• IEC 62446 / manufacturer commissioning guides (Enphase, SolarEdge) for insulation-resistance (Riso) testing and ground-fault localization procedures.