Definition
An arc fault in a solar PV DC circuit is an unintended electric arc between conductors caused by loose connections, damaged insulation, or component failure. NEC 690.11 requires Arc-Fault Circuit Interrupter (AFCI) protection on DC circuits operating above 80 V.
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
- Loose MC4 connectors — most common.
- Damaged wire insulation (sun/water degradation).
- Corroded terminations.
- Damaged bypass diodes (module hot-spot).
- Animal damage (rodent gnawing).
Prevention
- Use manufacturer-listed MC4 crimp tools.
- Inspect connections during commissioning.
- Maintain conductor strain relief.
- Use UL-listed combiners and components.
- Periodic IR thermography surveys identify hot connections.
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.
Frequently Asked Questions
6 commonly searched questions about Arc Fault.
What is an arc fault?
An unintended electric arc between conductors in a PV circuit. Causes: loose MC4 connector, degraded wire, corroded terminal, damaged module bypass diode. Generates heat and can ignite surrounding materials.
What does AFCI detect?
High-frequency current signatures characteristic of arc faults. Modern inverters with integrated AFCI continuously monitor for these signatures and disconnect the affected string.
Is AFCI required?
Yes per NEC 690.11 for DC circuits operating ≥80 V on or penetrating buildings. Most modern inverters include integrated DC AFCI.
Difference between arc fault and ground fault?
Arc fault: arc between conductors (not necessarily to ground). Ground fault: unintended current path to ground/earth. Both detected separately; both required by NEC.
Common arc fault causes?
Loose or improperly crimped MC4 connectors (most common), damaged wire insulation, water-corroded terminations, module hot-spot escalation.
What happens when AFCI trips?
Inverter disconnects the affected string and signals an alert. System remains offline until manual reset after the cause is identified and corrected.
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