Calculation of the internal thermal resistance and ampacity of 3-core screened cables with fillers

All 3-core cables require fillers to fill the space between insulated cores and the belt insulation or a sheath. The equations given in IEC 287 and in the Neher-McGrath paper for the internal thermal resistance of S-core cabl es were nel eloped for paper insulated cables. For such cables, it was assu med that the insulation and filler materials have the same thermal resistiv ities. In reality, in 3-core cables a variety of materials are used as fill ers. The majority of these will have a higher thermal resistivity than the insulation. In the previous paper, a new formula was developed to compute t he value of the internal thermal resistance of belted cables taking bite ac count the thermal resistivity of the filler. In this paper, screened cables are considered and a nerv formula for the computation of the internal ther mal resistance of such cables is presented. The effect of filler resistivit y on cable ampacity is also discussed.