Which Test First Isolation vs String Voltage vs Ground Fault On No Output Decision Tree

Why this matters

When a string or array produces no output, the temptation is to start anywhere with a meter and chase the first odd reading. Sequencing the tests correctly saves time and, more importantly, keeps you safe. The right first test depends on what the inverter is telling you: a ground-fault or isolation alarm changes the order entirely, because you must not energize and load-test a system with a live ground fault. The three core measurements are insulation resistance (isolation), string open-circuit voltage, and ground-fault detection. This tree gives the order to run them in so each test rules a class of faults in or out and you never load a faulted array.

Symptom presentation

• Inverter shows zero or near-zero production with a ground-fault or isolation/insulation alarm: a fault exists between an active conductor and ground; do isolation/ground-fault work first.
• Inverter shows zero production with no fault, just no DC input or low voltage: an open circuit or dead string; string voltage testing leads.
• One string dead, others normal: localized open or connector fault on that string.
• All strings dead at once: shared cause (DC disconnect open, inverter DC stage, or a common ground fault).

Quick checks

1. Read the inverter fault log and live DC input voltage/current per MPPT. The fault code dictates the test order.
2. Confirm the DC disconnect is closed and the inverter is actually awake and seeking DC; a sleeping inverter at dawn/dusk is not a fault.
3. Verify it is daylight with usable irradiance. No sun, no Voc; do not diagnose a "dead" string at low light.
4. Visually inspect for obvious damage: scorched connectors, chewed cable, water in a combiner, a tripped DC breaker.

Isolation tree

Step 1: Is there a ground-fault or insulation/isolation alarm? Yes: do NOT proceed to load-test. Go to the ground-fault/isolation branch (Step 4) first. A live ground fault plus a second fault can become a shock or fire hazard, and the inverter's GFDI has already isolated for a reason. No ground-fault/isolation alarm goes to Step 2.

Step 2: Measure string open-circuit voltage (Voc) at the combiner or inverter input. With the string isolated from the inverter, measure Voc:

• Near expected Voc (number of modules times per-module Voc, temperature-corrected): the string is electrically intact; the problem is downstream (inverter DC stage, MPPT, or the disconnect). Go to Step 3.
• Zero or far-low Voc: an open circuit in the string. Walk the string connector by connector, or split it and measure each half, to localize the open (failed connector, broken lead, blown inline fuse, disconnected module).
• Low but non-zero, odd value: a partial string (some modules bypassed/shorted by a bypass diode or a reversed module), or a measurement taken under shade. Recheck conditions, then segment to find the bad module.

Step 3: With Voc good but no output, check the DC-to-inverter path and the inverter. Confirm the disconnect passes voltage to the inverter input, that fuses/breakers are intact, and that the inverter sees DC. If voltage is present at the inverter input but it will not start, the inverter DC stage/MPPT or a configuration limit (DC voltage window, grid not present) is the issue. Compare input voltage to the inverter's MPPT window.

Step 4 (ground-fault/isolation branch): Measure insulation resistance. De-energize, isolate the array, and perform an insulation-resistance test (per the inverter and IEC 62446 method) from each active conductor to ground. A low insulation-resistance reading confirms a fault to ground; localize by progressively disconnecting sections (combiner inputs, then individual strings, then segments of the faulted string) and re-measuring to find where resistance to ground collapses. Common causes: damaged cable insulation pinched under a clamp, water ingress at a connector or junction box, a module ground fault, or a frame/conductor contact. Only after the ground fault is cleared and insulation resistance is restored do you proceed to Voc and load testing.

Confirming diagnosis

• Open circuit confirmed when Voc is zero/low and segmenting locates the break; the half with voltage and the half without bracket the open.
• Ground fault confirmed by a low insulation-resistance reading that recovers when a specific section is disconnected; that section holds the fault.
• Inverter/DC-path fault confirmed when string Voc is good, voltage reaches the inverter input, and the inverter still will not produce within its MPPT window.
• No fault at all confirmed when low/no output traces to low irradiance, a sleeping inverter, or an open disconnect.

Remediation

• Open: replace the failed connector/lead/fuse or reseat the disconnected module; re-measure Voc to spec.
• Ground fault: repair the damaged insulation/connector/module, dry and reseal water ingress, restore insulation resistance above the threshold, then clear the inverter alarm and re-commission.
• Inverter/DC path: replace failed fuses/breakers, correct configuration, or escalate an inverter DC-stage fault under warranty.
• Always re-run insulation resistance and Voc as a commissioning verification after any repair.

References

• IEC 62446-1, Photovoltaic (PV) systems - Requirements for testing, documentation and maintenance
• NEC (NFPA 70) Article 690.41, System Grounding and Ground-Fault Protection
• NEC (NFPA 70) Article 690.11, Arc-Fault Circuit Protection (Direct Current)
• UL 1741, Inverters, Converters, Controllers and Interconnection System Equipment