Arc Fault

NEC 690.11 Requirements

• DC PV source and output circuits operating ≥ 80 V.
• AFCI required on circuits “on or penetrating a building” (NEC 2017+).
• Detection within seconds; disconnect within minutes (per UL 1699B).
• Integrated AFCI in modern inverters typically satisfies the requirement.

Common Causes

1. Loose MC4 connectors — most common.
2. Damaged wire insulation (sun/water degradation).
3. Corroded terminations.
4. Damaged bypass diodes — see bypass diode activation for how this failure mode develops into a hot spot.
5. Animal damage (rodent gnawing).

Prevention

• Use manufacturer-listed MC4 crimp tools.
• Inspect connections during commissioning.
• Maintain conductor strain relief.
• Use UL-listed combiner boxes and other listed components.
• Periodic IR thermography surveys identify hot connections.

How AFCI Detection Works

Modern DC arc-fault circuit interrupters continuously sample the current waveform on each PV string for the broadband, high-frequency signature a real arc produces — a pattern distinct from the normal switching noise of an inverter or the ripple from cloud-driven current changes. When that signature crosses a validated threshold under UL 1699B, the AFCI logic confirms it isn’t a nuisance trip and opens only the affected string, rather than shutting down the whole array. That selective disconnection is why a single loose MC4 connector or a chafed conductor can be isolated without derating the rest of the plant. Because integrated AFCI now ships inside most string inverters, the practical design task for an EPC is less about specifying a separate device and more about confirming the inverter’s AFCI is listed for the operating voltage and correctly annotated in the solar SLD software or electrical drawing package, so the protection boundary is visible to permit reviewers.

Key Takeaways

• Arc fault = unintended electric arc between PV conductors; fire risk.
• NEC 690.11 requires AFCI protection on DC circuits ≥ 80 V.
• Modern string inverters include integrated AFCI per UL 1699B.
• Most common cause: improper MC4 connector termination.
• Prevent via proper crimping, periodic IR thermography.

Related Reading

Arc-fault protection rarely gets specified in isolation — it shows up on the same drawing set as the DC and AC electrical layout our Electrical CEIG Drawings service produces, where AFCI, ground-fault, and rapid-shutdown devices all have to be called out for the permit reviewer. If a string keeps tripping after AFCI has ruled out an obvious wiring fault, the root cause is often on the inverter side rather than the DC circuit itself; QbitsEnergy’s inverter troubleshooting guide is a useful next step for isolating that class of fault. Teams assembling the full DC protection scheme — AFCI, combiner sizing, and grounding together — will also find the Ground Fault entry useful, since the two failure modes are detected separately but are frequently confused during commissioning walk-throughs.