A study of ignition time for materials exposed to DC arcing in PV systems

This study examines the factors that influence the time to first ignition and burn through for materials, found in PV power systems, when exposed to DC arcing. Materials of interest include PV wire insulation, connectors, metal conduits, and insulation. The most important factors for the time to ignition are the power density absorbed and the material threshold. The most important factors for burn through are power density, heat released by polymers, thickness and flame retardant chemistry. Ignition occurs when the arc power and time of exposure are sufficient to produce localized heating that exceeds material thresholds. Data shows that arcs as low as 200 watts, at a radius of 10 mm, will ignite most plastics in 4 seconds. A radiation model is presented to calculate the absorbed power density. Times to the first ignition, re-flash, burn rate and flame retardants are factors in the prediction. The ratio of arc power density to the peak heat release rate of polymers is used in the burn through time prediction. A burn through time vs. arc watt correlation has been established. Burn through time is sensitive to the power density absorbed by the exposed material and includes geometry factors and material thresholds. Radiated power density is driven by arc power density. Arc power is determined by arc current, and arc gap. Ignition time & burn through estimates can be used to establish an AFCI trip curve to reduce the risk of fires due to arcing.