Question

A single core cable of conductor diameter 2 cm and lead sheath of diameter 5.3 cm is to be used on 66 kv 3-phase.1st inter sheath of diameter 3.1 cm is introduced between the core and lead sheath find the voltage on the 1st inter sheath

Answer 1

is 2·5 cm and resistivity of insulation is 4·5 × 1014Ω-cm, find the insulation thickness.Solution.Length of cable,l=1 km = 1000 mCable insulation resistance,R=495 MΩ = 495×106ΩConductor radius,r1=2·5/2 = 1·25 cmResistivity of insulation,ρ= 4·5 ×1014 Ω-cm = 4·5 ×1012ΩmLet r2 cm be the internal sheath radius.Now,R=ρπ221lrrelogorlogerr21=221000495104 510612πρπl R=×××⋅×= 0·69or2·3 log10r2/r1= 0·69orr2/r1=Antilog 0·69/2·3 = 2orr2= 2 r1 = 2 ×1·25 = 2·5 cm∴Insulation thickness=r2−r1= 2·5 −1·25 = 1·25 cmExample 11.3. A single core cable 5 km long has an insulation resistance of 0·4 MΩ. The corediameter is 20 mm and the diameter of the cable over the insulation is 50 mm. Calculate the resistiv-ity of the insulating material.Solution.Length of cable,l=5 km = 5000 mCable insulation resistance,R=0·4 MΩ = 0·4 ×106ΩConductor radius,r1=20/2 = 10 mmInternal sheath radius,r2=50/2= 25 mm∴Insulation resistance of the cables isR=ρπ221lrrelogor0·4 ×106=ρπ250002510××loge∴ρ=13.72 ×109 ΩmTUTORIAL PROBLEMS1.A single-core cable has a conductor diameter of 2.5 cm and insulation thickness of 1.2 cm. If the specificresistance of insulation is 4·5 ×1014Ω cm, calculate the insulation resistance per kilometre length of thecable.[305·5 MΩ]2.A single core cable 3 km long has an insulation resistance of 1820 MΩ. If the conductor diameter is 1.5cm and sheath diameter is 5 cm, calculate the resistivity of the dielectric in the cable.[28.57 ×1012 Ωm]3.Determine the insulation resistance of a single-core cable of length 3 km and having conductor radius12·5 mm, insulation thickness 10 mm and specific resistance of insulation of 5 ×1012Ωm.[156 MΩ]