Degradation characteristics of cable insulation under short-term extreme high temperatures in fires

• Built a platform simulating cable insulation temperature changes during fires. • Revealing differences between fire-overheated and thermally aged cable insulation. • Analyzed cable insulation properties changes under various treatment temperatures. • Proposed method assesses cable damage level using key performance indicators. Short-term exposure to extreme temperatures during fires can affect cable insulation performance. A dynamic heat transfer model for cable fires was first established to obtain the temperature rise characteristics of the cable insulation layer during fire exposure. Specimens were conditioned using a temperature-controlled, oxygen-free platform. The microstructural, mechanical, and electrical properties of the treated samples were then characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), tensile testing, and AC withstand voltage testing. Results reveal distinct behavior compared to conventional thermal aging: unlike the significant crystallinity decrease observed around 160°C in long-term aging, short-term exposure at 160°C resulted in crystallinity higher than the original sample due to the brief duration, with no performance degradation. Crystallinity decreased below original levels after 200°C treatment, with a more pronounced decline above 300°C, accompanied by noticeable microstructural changes. Breakdown strength and tensile strength followed similar trends, also differing markedly from conventional aging. While conventional aging samples exhibited significant breakdown strength degradation between 120°C and 160°C, the short-term heated samples showed a slight improvement at 160°C compared to the original. Breakdown strength decreased after 200°C treatment, and further reductions of approximately 5% and 10% occurred after 300°C and 400°C treatments, respectively. This indicates that short-term exposure up to 160°C has minimal impact on XLPE properties, while significant degradation initiates above 200°C and accelerates beyond 300°C. The degradation induced by conventional thermal aging under prolonged exposure to elevated temperatures involves fundamentally different mechanisms compared to those triggered by short-term extreme overheating in fire scenarios.