Burning Smell Near Combiner No Fault Shown Which Check First Decision Tree

Why this matters

A burning smell at a combiner box with no fault on the inverter is the most dangerous "everything looks fine" in PV. The inverter's arc-fault and ground-fault detection catch specific electrical signatures, but a slowly heating, high-resistance connection can cook insulation and char a lug long before it produces a detectable series arc or a ground path. By the time the homeowner smells it, you may be minutes from a parallel arc or a fire. The smell is the alarm the electronics did not raise.

The first check is not a meter, it is heat, because a hot connection is the overwhelming cause and a thermal scan finds it without opening a live box you do not yet understand. This tree orders the checks from safest to most invasive.

Symptom presentation

Get the picture before the panel.

• Acrid, plastic/insulation burning smell localized to the combiner, no inverter fault: the classic high-resistance-connection signature.
• Visible discoloration, melting, or char around a lug, fuse holder, terminal, or connector: confirms a hot joint, do not energize further.
• Smell strongest at high production (midday): heating scales with current squared, so a marginal joint smells worst under load.
• Smell with an inverter arc-fault (AFCI) or ground-fault flag: the electronics did catch it, follow that fault, but the same thermal cause may be present.

Quick checks

Lowest-risk data first.

• Thermal-scan the combiner exterior and, where safe and visible, the terminations with an IR camera or non-contact thermometer during production. A hot spot on one lug, fuse, or terminal against cooler siblings names the joint.
• Compare string currents in monitoring: a string carrying current through a degrading joint may read slightly low or erratic.
• Inspect connectors and conduit entries for water ingress staining, the combine of moisture and current accelerates corrosion and heating.
• Read the inverter log for any isolation, AFCI, or ground-fault events near the time the smell appeared, even transient ones.
• Note the weather history: a box that took water and then dried can leave a tracking path that heats.

Isolation tree

Order checks from external/thermal to internal/electrical, all on a de-energized box for anything internal.

Step one, thermal scan first, energized but unopened. The single most common cause is a loose, corroded, or under-torqued termination heating under load. A clear hot spot localizes the fault without opening the box. If one lug/fuse/terminal runs markedly hotter than its peers, that is your joint.

Step two, de-energize, then visual and tactile inspection. After rapid shutdown, AC disconnect, and verified-dead with a rated meter, open the combiner. Look for char, melt, discoloration, green/white corrosion, and the smell's source. A blackened lug or a melted fuse holder confirms a high-resistance joint. A scorched landing on an over-torqued or under-torqued terminal confirms a mechanical-connection failure.

Step three, branch on what you find:

• Charred/loose termination: the fault is the joint. Connection was loose, over- or under-torqued, oxidized, or dissimilar-metal corroded.
• Melted/discolored fuse holder or breaker: the overcurrent device or its contact is the heat source, often a marginal fuse clip or an undersized/aged device.
• Water ingress with tracking marks: moisture created a leakage/tracking path that heated and carbonized.
• Connector (MC4) burn at the box entry: a mismatched or improperly crimped connector, the leading cause of inter-brand connector failures.

Step four, if no thermal or visual cause is found but the smell is real. Check for an intermittent series arc that the AFCI catches only transiently, measure insulation resistance of each string conductor to ground (a low IR reveals a degrading insulation/tracking path), and inspect upstream and downstream of the combiner since the smell can migrate. Do not return to service on a real burning smell with no found cause.

Confirming diagnosis

Lock the cause to evidence.

• High-resistance termination confirmed: thermal hot spot plus char/discoloration plus a loose or corroded lug; torque check finds it out of spec.
• Fuse/breaker fault confirmed: heat and melt centered on the device or its clip, device tests bad or out of rating.
• Moisture/tracking confirmed: water staining, carbon tracking, and a low insulation-resistance reading on the affected conductor.
• Connector failure confirmed: burn localized to an MC4 or junction connector, often a cross-brand or bad crimp.

Remediation

Replace, do not just retighten, a charred connection: cut back to clean conductor, install a new lug or connector with the correct crimp and contact material, and torque to the manufacturer's spec. Replace any heat-damaged fuse, holder, or breaker with the correct rating. Dry, seal, and correct the entry path for water ingress, and replace conductors with degraded insulation rather than re-terminating burned ends. Re-scan thermally under full production after the repair to confirm the hot spot is gone, verify insulation resistance is restored, and document torque values. Never return a combiner with a real burning smell to service until a specific cause is found and corrected.

References

• NEC Article 690: Solar Photovoltaic (PV) Systems (combiner, overcurrent protection, 690.12 rapid shutdown, AFCI).
• UL 1741 / UL 3741: Inverter and PV hazard-control system requirements (arc-fault and rapid-shutdown context).
• IEC 62446-1: Grid-connected PV systems, commissioning and inspection (insulation resistance and connection verification).
• NEC Article 705: Interconnected Electric Power Production Sources (system disconnection and labeling).