Jerome K. Hastings

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.

Electrical Arcing and Material Ignition Levels

Experimental data and analysis show the relationship between arcing watts, the time of the arc exposure and the degree to which common polymers experience damage. Arcing test apparatus for 125V DC and 48 volt DC are shown. Arc gap control allows 125-volts to mimic arcs in lower voltage systems. Arcing current waveforms depict the chaotic behavior of arcs and differences due to anode / cathode materials. Levels of degradation are defined and used for “Constant Damage Contours” for Vinyl, Carpeting and Sound insulation. The polymers were exposed to conditions of constant arcing watts for increasing times. Connecting the points of “first flame” establishes a “contour of constant damage”. The data indicates that a “3 second flame free” limit is between 50 and 100 watts. 4000 watts of arcing takes 1/4 to ½ second to cause burning. The influence of circuit resistance on the maximum arc power transfer is presented.

Application Feasibility of Detecting Glowing Contacts Using Acoustic Sensing Technology

Glowing contacts may be precursors to arc faults and could lead to fire hazards even before arc faults occur. As of today there is no cost effective technology available to detect overheated or glowing contacts. Experiments have been conducted in order to investigate both the acoustic characteristics associated with glowing contacts and the feasibility of their detection under various conditions by employing acoustic sensing technology. Experimental results show that the propagation or attenuation of the acoustic signal through electrical circuits is significantly influenced by size and length of electric wires and types of electric connections. The results also further demonstrate that the acoustic signal can propagate effectively through large cable conductors and busbars, hence enable glowing contacts detection for industrial applications. However, the attenuation of acoustic signal through electrical circuit could be a potential limitation for residential applications.