Aviation Investigation Report A98H0003
1.16.8 Electrical Ignition Tests of MPET-Covered Insulation Blankets
Electrical discharges in the form of arcs or sparks can produce localized temperatures in excess of 5 500°C (9 932°F). A sustained short-circuit event will cause a conventional CB to trip and de-energize the faulted circuit. However, a CB may not trip when an intermittent short circuit exists.
Tests were conducted to characterize the ignition properties and determine whether MPET-covered insulation blankets would ignite when exposed to electrical sparks produced by ground shorts from wires carrying 115 V AC and 28 V DC current. It was observed that MPET-covered insulation blankets would ignite and propagate a flame when exposed to an electrical arc or spark. However, ignition was sporadic in that it sometimes occurred with the first strike of the arc and other times it was not achieved after numerous attempts. The arcs were struck by hand and typically resulted in the tripping of the CB. It appears that electrical arcs were sufficiently rapid in onset and localized to overcome the propensity of a cover material constructed with thin-film material, such as MPET, to shrink away from a heat source.
In one such test, an MPET-covered insulation blanket, similar to those in the occurrence aircraft, was placed between the vertical frames in a section of aircraft fuselage. The blanket was exposed to an intermittent electrical short between an exposed 115 V wire and the grounded fuselage. The MPET-covered insulation blanket ignited, causing a flame to propagate vertically and horizontally across the face and rear surface of the blanket.
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