Arc Fault Protection in PV: Why DC Arcs Persist
A DC arc has no zero crossing to extinguish it. Series against parallel faults, and why repeated trips are an instruction to inspect rather than reset.
Arc fault detection exists in PV systems for a reason that follows from physics rather than from regulation. Direct current behaves differently from alternating current when an arc forms, and the difference is the whole subject.
Why DC Is Different
An alternating current reverses direction at mains frequency, passing through zero on every reversal. An arc sustained by AC is extinguished at each of those crossings and has to be re-established. Often it is not, and the arc self-extinguishes.
Direct current does not cross zero. Once an arc is struck in a DC circuit, nothing in the waveform interrupts it. It persists for as long as current flows and the gap conditions allow, at temperatures high enough to ignite surrounding materials.
A PV array is a current source running through conductors on a roof, often for decades. That combination is why dedicated detection was introduced.
Where Faults Come From
Almost all of them come from connections.
| Origin | Mechanism | When it appears |
|---|---|---|
| Connector not fully mated | Higher resistance at the joint, heating | Can appear early, or after thermal cycling |
| Mismatched connector brands | Designed to mate with their own kind; cross-mating fits but seals and contacts differ | Months to years later |
| Corrosion at a terminal | Moisture ingress raises resistance | Years, often after a seal fails |
| Loose terminal screw | Thermal cycling backs it off | Years |
| Chafed insulation | Conductor rubbing a sharp edge or a clamp | Any time; depends on installation detail |
| Rodent damage | Insulation chewed | Any time on accessible arrays |
Connector cross-mating deserves particular mention because it is common and it looks fine. Connectors from different manufacturers can physically mate while differing in contact geometry and sealing. The joint works initially, resistance rises over time, and it becomes exactly the degraded connection that sustains a series arc.
Series and Parallel
A series arc forms in a break along the current path, such as a failing connector. The array’s own current crosses the gap. Detection is difficult because the current does not increase; if anything it falls slightly. Conventional overcurrent protection does not see it at all, which is why arc fault detection is a separate function.
A parallel arc forms between two conductors of different potential, or between a conductor and earth. Current rises, so other protection may respond, though not necessarily fast enough or at all depending on the fault current available.
Detection circuits look for the electrical noise signature an arc produces, distinguishing it from the normal switching noise of the system. That discrimination is the hard part of the design, and it is why nuisance trips occur and why some real faults are harder to catch than others.
When the Inverter Reports a Fault
Treat it as an instruction to inspect.
A first trip may be a genuine fault or may be interference. A recurring trip is not noise; something in the array is degrading, and the detection is doing what it was fitted to do.
Resetting repeatedly returns the system to service with the fault still present. Because the hazard is fire rather than equipment damage, this is not a fault worth living with.
A qualified installer should inspect connectors, terminations and conductor routing. Thermal imaging under load identifies hot joints, and visual inspection of connectors identifies cross-mated pairs and failed seals. Anyone without the training to work on an energised array should not open anything: the modules cannot be switched off, the conductors are a lethal hazard in daylight, and this is electrician work.
Reducing the Risk at Installation
Use connectors of one type, mated as their manufacturer intends, with the correct tool.
Route conductors so nothing bears on an edge, and support them so movement does not chafe insulation over years of thermal cycling and wind.
Torque terminations to specification and re-check them at commissioning.
Keep conductors out of standing water and protected where rodents have access.
None of this is exotic. It is ordinary care, and it is the difference between an array that ages quietly and one that develops faults.
What People Get Wrong About This
The first mistake is resetting a recurring arc fault. The detection is reporting a degrading connection.
The second is cross-mating connectors from different manufacturers. It fits, it passes commissioning, and it fails years later.
A third is assuming overcurrent protection covers arc faults. A series arc does not raise current, so it does not.
A fourth is treating detection as complete protection. It reduces risk; installation quality determines how often it has anything to detect.
And the last is inspecting only the inverter end. The fault is usually in the array, at a connector, and finding it means going through the string.
Related
Rapid shutdown addresses the same underlying property of PV arrays, that they cannot be switched off. System grounding covers the earth-fault side of the same protection question.
Common questions
Why are DC arcs more dangerous than AC arcs?
An alternating current passes through zero twice per cycle, and an arc extinguishes at those moments unless conditions re-establish it. Direct current has no zero crossing, so once an arc is struck nothing interrupts it naturally. It can sustain itself at high temperature for as long as current flows.
What causes an arc fault in a PV array?
A poor connection that has degraded, most often. Connectors that were not fully seated, mismatched connector brands mated together, corrosion at a terminal, damaged insulation, or a conductor chafed against a sharp edge. Thermal cycling over years loosens joints that were adequate at installation.
What is the difference between a series and a parallel arc fault?
A series arc occurs in a break along the current path, such as a failing connector, and the array's own current sustains it. A parallel arc occurs between two conductors or between a conductor and earth. Series faults are harder to detect because the current does not rise, which is why dedicated detection exists.
My inverter keeps reporting an arc fault. Can I just reset it?
Repeated trips are an instruction to inspect, not to reset. Detection exists because a sustained DC arc is a fire hazard, and a recurring fault means something in the array is degrading. Have a qualified installer inspect the connections and the conductors before the system is returned to service.
Does arc fault detection eliminate the risk?
It reduces it substantially and it is not absolute. Detection relies on recognising the electrical signature of an arc, and signatures vary with the fault and the array. Good installation practice, correct connectors properly mated, and periodic inspection remain the primary defences.
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