The ability of an arc suppression blanket to self-extinguish depends on three major factors: selection of materials, external contamination, and arc energy levels. Each of these factors is independent of the others. This means that each can reduce the self-extinguishing ability of the blanket by itself.
Selection of Materials
Two decades ago, initial testing was performed on individual fabrics and the combination of fabrics. The results supported the use of blankets that combined inner ballistic layers with outer protective layers. So materials need to reflect the ability of the combination of fabrics, thread, finishing, and storage of the finished blanket.
It should be noted that flame-resistant and arc-resistant designations are not the same. Arc testing has shown that an article made for firefighters does not have the same self-extinguishing properties as an arc blanket.
FR garments are tested with flame temperatures in the 1500°F or higher while arc rated products are tested with an arc plasma of 5,000°F to 10,000°F. The arc plasma additionally contributes high levels of ultra-violet radiation which impacts the degradation of fabrics.
Thus, making an arc suppression blankets from regular FR fabrics would probably not pass ASTM F2676 standards.
External Contamination
Also, arc suppression blankets fail self-extinguishing standards if external contamination is present. Low levels of oils or flammable solids such as sawdust, or coal dust prolong blanket after flame.
Arc Energy Levels
And the final discovery is especially significant for blankets protecting against energy levels of 30,000 amps or more. After-flame time increased. As an example, a blanket that passes the 40,000 amp (40KA) test according to ASTM F2676, testing at 5 cycles show the blanket does not blow through and self-extinguishes within a few seconds.
So, 5 cycles is 5/60 of one second in a 60 cycle per second AC current. As the arc exposure time is extended past 15 cycles, self-extinguishing time expands to a point where the blanket may not self-extinguish within standards.
By Victor L. Petrovic PhD
Look for the next article, Arc Suppression Protection: Blanket Inspection & Cleaning, to come out in December.