Insulation
Thermal resistance of insulation T1 (IEC 60287-2-1)
- where
- thermal resistance of the insulation per core (K·m/W)
- thermal resistivity of the insulation (K·m/W)
- insulation thickness between conductor and sheath, including semiconducting screens (mm)
- conductor diameter (mm)
Explain this equation
Gives the thermal resistance of the insulation between the conductor and the metallic screen of a single-core cable, the first thermal term in an IEC 60287 rating.
ρ_T is the thermal resistivity of the insulation material, t₁ the total thickness from conductor to sheath including both semiconducting screens, and d_c the conductor diameter.
Heat flows radially through a cylindrical shell, so the same log form as for electrical insulation resistance applies: ln of outer over inner diameter, here written as 1 + 2t₁/d_c. Dividing by 2π spreads it round the circumference.
An 11 kV XLPE core with 3.4 mm over a 20 mm conductor gives about 0.16 K·m/W; a 132 kV core with 18 mm over 30 mm about 0.44 K·m/W. At 30 W/m of conductor loss that is roughly 5 K and 13 K of temperature rise across the insulation.
For three-core cables a geometric factor is needed instead; this simple form is for single-core cables only.
Thermal resistance between conductor and metallic sheath/screen of a single-core cable, the T1 term of the IEC 60287 rating equation.
Enter your values to see the result.
Good to know
IEC 60287-2-1: for single-core cables t1 includes the semiconducting screens (they are treated as insulation thermally), and dc is the conductor diameter. Thermal resistivities from IEC 60287-2-1: XLPE and PE 3.5 K·m/W, EPR 3.5 K·m/W (up to 3 kV) / 5.0 K·m/W (above 3 kV), PVC 5.0 (up to 3 kV) / 6.0 (above), impregnated paper 6.0. For conductors with a longitudinal water-blocking or other layers, follow the standard.
Common mistake
Using only the main insulation thickness and leaving out the two semiconducting screens, which understates T1.
Limits
Single-core cables only. Three-core cables need the geometric factor G from IEC 60287-2-1. Steady state; no moisture migration.
Sources
- IEC 60287-2-1 Electric cables – Calculation of the current rating – Part 2-1: Thermal resistance (part of the IEC 60287 series)
- IEC 60287-1-1:2023 Electric cables – Calculation of the current rating – Part 1-1: Current rating equations (100 % load factor) and calculation of losses
- Moore, G.F. (ed.) Electric Cables Handbook, 3rd edn, Blackwell Science / Wiley
For competent high-voltage engineers. Results are calculations from the published methods shown and depend entirely on the data you enter. They are not a substitute for a system study, manufacturer data, the applicable standards and Engineering Recommendations, or your organisation’s safety rules. Never use a result to justify working on or near live HV equipment.