The most dangerous fault in a direct-current system is often the one no protective device notices. A DC arc fault in series with the load — at a loose lug, a corroded connector or a cracked conductor — passes the same current the circuit normally carries, so fuses and breakers see nothing unusual. Meanwhile the arc concentrates the circuit’s power into a gap a fraction of a millimeter wide, and direct current gives it no reason to stop.
Why a series arc is invisible to overcurrent devices
An overcurrent device measures current and opens when there is too much of it. A series arc adds resistance to the circuit, so if anything the current falls slightly. The circuit keeps working; the array keeps producing, the load keeps running, and nothing indicates a problem except perhaps a small drop in performance that nobody notices. All the while, the voltage dropped across the arc times the current flowing through it is being dissipated as heat in a very small volume. Arc temperatures are high enough to melt copper and to ignite insulation, terminal blocks, enclosures and anything nearby. Photovoltaic arc faults have started roof fires exactly this way.
What makes direct current different
In an alternating-current circuit, current passes through zero twice per cycle, which gives an accidental arc regular opportunities to extinguish. Many incipient alternating-current arcs self-quench for that reason. Direct current offers no such moment. Once an arc is established across a small gap, it is sustained continuously by the source, and it will persist as long as enough voltage is available to maintain the ionized channel. Photovoltaic strings and battery banks both provide plenty. This is the same property that makes direct-current switching hard, expressed as a fault rather than as an intentional operation, and it is why photovoltaic arc-fault detection exists as a distinct product category.
Where these faults actually start
Terminations are the usual origin. An under-torqued lug relaxes over thermal cycles until contact pressure is marginal. An over-torqued one damages the conductor. A crimp made with the wrong die has high resistance from day one. Aluminum conductors terminated on connectors listed only for copper oxidize at the interface. Photovoltaic connectors from different manufacturers, mated together because they physically fit, make poor long-term contact and are a well-documented source of array faults. Rodent damage, conduit abrasion and repeated flexing all produce partially broken conductors. Each of these begins as a resistance that warms, and warmth accelerates oxidation, which raises resistance further. The progression is slow enough that a system can run for years with a joint quietly getting worse, and fast enough at the end that the transition from warm to arcing happens between one inspection and the next.
Finding them before they find you
Heat is the symptom you can detect. An infrared thermometer or a thermal camera surveying terminations while the system is under substantial load will show a warm connection long before it becomes an arc, and a connection warmer than its neighbors is always worth investigating. A voltage drop measurement across a connection under load is even more direct: any meaningful voltage across what should be a solid joint indicates resistance. Do this at commissioning, so you have a baseline, and then annually. Look, listen and smell as well; a crackling sound, a smell of hot plastic, or discolored insulation near a terminal all deserve immediate attention rather than a note for later.
Detection equipment and what it covers
Arc-fault detection for photovoltaic systems is built into much modern equipment and is required in many installations. These devices listen electrically for the characteristic high-frequency noise an arc produces and shut down the string when they detect it. They are genuinely useful and they have limits: they cover the circuits they monitor, they can nuisance-trip on electrical noise from other equipment, and they do nothing for the battery side or for alternating-current wiring. Battery systems rely instead on correct terminations, per-string fusing and monitoring. No detection device substitutes for connections made and verified properly in the first place.
What to check before you act
- Torque every termination to specification with a torque wrench.
- Never mate photovoltaic connectors from different manufacturers.
- Survey terminations with a thermal camera under load annually.
- Investigate any warm, discolored or noisy connection immediately.
Common questions
Will my fuse protect me from an arcing terminal?
No. A series arc passes normal current, so overcurrent devices see nothing. Only arc-fault detection or physical inspection finds it.
Can I mix photovoltaic connectors that fit together?
No. Connectors from different makers are not listed as an intermateable pair, and the resulting contact degrades over time into exactly this failure.
The practical takeaway
Assume your overcurrent protection will not save you from a bad connection, because it cannot. Torque terminations properly, use matched connectors, and survey the system thermally under load on a schedule. Where photovoltaic arc-fault detection is available, use it, and treat it as a backstop rather than as the primary defense.
References and further reading
- NFPA: National Electrical Code background on arc-fault and photovoltaic requirements
- OSHA: wiring design and protection requirements for conductors and terminations
- NREL: photovoltaic system research including reliability and failure modes
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
