When the Cable Insulation Burns: Experimental and 2D Modeling Study of Cables with Flame-Retardant Sheaths and Flammable Insulation

The integration of flame-retardant (FR) materials into electrical cables mitigates fire hazards; however, the widespread industrial practice of pairing flame-retarded sheaths with non-flame-retarded insulations offers protection only below a critical external heat flux. Beyond this threshold, the sheath’s suppression capacity is exceeded, allowing the insulation to pyrolyze and significantly elevate the fire risk. While non-FR insulations are widely utilized for their dielectric strength, flexibility, durability, and cost-effectiveness, their contribution to fire growth remains a concern and requires further elucidation. Therefore, this study investigates the fire behavior of two industrial cables: MKEM-HF, comprising solely an FR sheath, and XMK-HF, consisting of an FR sheath over a highly flammable insulation, under a heat flux exceeding the sheath capacity to protect the inner insulation layer. Thermogravimetric (TGA) and Differential Scanning Calorimetry (DSC) analyses were conducted to characterize thermal degradation of the cable layers and develop pyrolysis models. Cone calorimetry at 50 kW · m⁻² assessed fire behavior under gasification and flaming, for both single- and multi-cable configurations. MKEM-HF displayed a descending MLR-HRR profile over time, whereas XMK-HF exhibited a continuously rising trend with a dominant final peak, driven by the decomposition of the insulation. A classical 1D slab model of the cone calorimetry was used for inverse estimation of thermal properties and heats of combustion. A geometrical modification on the 1D model allowed the simulation of early insulation exposure, offering a possible approach for modeling the sheath’s mechanical rupture. A more advanced 2D model, accounting for cylindrical geometry and improved boundary coupling, demonstrated superior performance in transient heat transfer and peak prediction compared to the 1D model. The study highlights the critical influence of non-flame-retarded insulation on cable fire behavior and demonstrates the necessity of advanced modeling approaches beyond simple slab approximations to capture these effects.