Arc Fault Detection
Arc Faults
DC arc faults are a leading cause of PV system fires and one of the most difficult electrical faults to detect with conventional protection devices. Series arcs in particular can sustain high temperatures at low current levels that a standard overcurrent device will never interrupt. This section covers arc fault types, AFCI device requirements, detection methodology, and field testing approaches for both new installations and operating systems.
AC arcs self-extinguish at each current zero crossing — 100 or 120 times per second. DC current has no zero crossing, so a DC arc is self-sustaining once established. In a PV array, the source impedance is low and the voltage is persistent, meaning an arc can maintain temperatures exceeding 6000°C indefinitely until the circuit is physically broken or the arc is starved of fuel. Standard overcurrent breakers do not interrupt DC series arcs.
A series arc occurs in a break within the current-carrying conductor — typically at a degraded connector, damaged cable termination, or corroded junction box terminal. Current still flows through the arc; the fault current equals string operating current. A parallel arc occurs between two conductors at different potentials — typically positive and negative cables contacting each other. Parallel arcs produce much higher fault current and are more likely to trip overcurrent protection.
Yes, to a degree. A degraded connector or damaged cable section with compromised insulation will show reduced Riso before it develops into a full arc fault. Regular Riso testing at commissioning and at periodic O&M intervals creates a trending baseline — a downward trend in Riso on a specific string or circuit points to a developing fault site. Low Riso should always prompt physical inspection of connectors and cable runs.
Once the string is de-energised and isolated, begin with visual inspection of all accessible connectors, junction boxes, and cable runs for burn marks, melted insulation, or discolouration. Thermal imaging under re-energised conditions (safely, with inverter running) can reveal hot spots at resistance sites before they arc again. Insulation resistance testing identifies which section of the string has compromised insulation, narrowing the search area.
No. AFCIs detect and interrupt arc faults in progress — they are a safety response device, not a diagnostic or prevention tool. They do not flag developing fault conditions before arcing begins, do not provide location information, and do not substitute for periodic Riso testing or connector inspection. A system with functioning AFCIs can still develop progressive insulation degradation that Riso testing would detect and AFCI devices would not.