Characteristics of Power Cable Sheathing Materials with Thermal ageing

Sheathing materials of the power cables play an important role in the protection of the cable against mechanical damage and fire accidents. The sheathing materials are selected based on the application and the installation environment of the cables. Generally used sheathing materials for power cables are Poly Vinyl Chloride (PVC), Flame Retardant Poly Vinyl Chloride (FR PVC), Flame Retardant Low Smoke Poly Vinyl Chloride (FRLS PVC) and Low Smoke & Zero Halogen (LSZH) materials. PVC possesses high mechanical strength, and electrical strength. However, the smoke and toxic gas release during fire is more for PVC. To improve the flame retardancy and smoke properties, additives and fillers are blended to PVC to make it as FR PVC and FRLS PVC. The optimal selection of additives is done in such a way that the materials retain its basic mechanical and electrical properties along with improved fire characteristics. LSZH materials emit low smoke and no halogens. It is having high amount of mineral fillers to enhance the fire and smoke properties. Lubricants, stabilizers, plasticizers, antioxidants, and colorants are also added to retain the flexibility of the cables, avoid the deterioration of the materials and to achieve the desired colors. In the present study, the various sheathing materials of new power cables are analyzed for its mechanical and fire properties with ageing. The sheathing materials are aged at its field operating temperatures for duration of up to 672 hours and the properties are measured periodically. Ageing of the special sheathing materials have not resulted into significant changes in the fire properties except the loss of chlorine in PVC & FRLS PVC. However, the ageing of the sheathing materials has resulted into loss of certain mechanical properties. Hence it is suggested to evaluate the properties of sheathing materials with accelerated ageing at the compounding stage itself for its optimal compounding blend and to ensure the integrity of the compounds throughout its lifetime.