IEC 60287-2: Calculation of thermal resistances of electric cables

This blog post discusses the topics covered by IEC standards 60287-2. It specifically emphasizes where and why Cableizer is deviating from the standard.

Posted 2015-03-18
Categories: Theory

Part 2 of the IEC standard 60287 covers methods for calculating the internal thermal resistance of cables, as well as the external thermal resistance for cables laid in free air, in ducts, and buried.

Section 1: Calculation of thermal resistances

IEC 60287-2-1 contains formulas for the calculation of the thermal resistance between one conductor and sheath, the thermal resistance between sheath and armour, the thermal resistance of the outer sheath, and the thermal resistance to ambient. The latter is derived both for cables in free air, as well as for buried cables, either isolated, in groups (touching or not touching), or in ducts or pipes.

Cableizer interprets the standard IEC 60287-2-1 as follows:

The calculation of internal thermal resistances of multi-core cables is discussed in a separate blog post.
The outer half of the corrugation depth is added to the insulation thickness between sheath and armour, thus increasing the thermal resistance between sheath and armour, as opposed to the standard where it increases the thermal resistance of the outer sheath. As a consequence, for cables with corrugated sheaths, the thermal resistivity of the armour bedding is a required input and it is used as the property of the outer half of the corrugation filling.
The difference between the surface temperature of a cable or duct in air and the ambient temperature is solved by iteration, instead of explicitly calculating it based on $K_A$, which has proven unreliable for certain installations, e.g. multiple cables in a common duct.
For the calculation of the thermal resistance to ambient of a single cable or duct, it is assumed that only cables in metallic ducts are metallic sheathed. All other cables are considered non-metallic sheathed.

Section 2: A method for calculating reduction factors for groups of cables in free air, protected from solar radiation.

This section of the standard covers methods for calculating group reduction factors of cables in free air. Cableizer deviates from the standard IEC 60287-2-2 as follows:

The thermal analysis of groups of cables in free air exposed to solar radiation is possible, even though it is not directly addressed in the IEC standard.
In the standard, a reduction factor $F_g$ for the hottest cable in the group is calculated, which is then applied to determine the current-carrying capacity of the hottest cable. In Cableizer, however, the reduction factor $F_g$ is not calculated and a ratio of thermal resistance to ambient is instead directly applied to the thermal resistance to ambient of cables in air. In such a way, all calculations are done with the correct current of the hottest cable, as opposed to the standard which only reduces the current after the calculations are done. In our understanding, this is a more correct way, even though no big differences are expected due to the different approaches.
When more than one reduction factor (vertical spacing, horizontal spacing, close to wall) for groups of cables in free air applies, then that specific installation is not supported by the standard. Cableizer still outputs a result considering only the largest reduction factor, together with an error message. Similarly, Cableizer does also calculate installations with cables having external diameters that exceed the defined range of the standard, together with a warning.

Section 3: Cables installed in ventilated tunnels

This section of the standard deals with the rating for cables installed in ventilated tunnels. In this situation, the air flowing in the tunnel results in longitudinal temperature gradients along the cables. This is different from the first two sections of the standard, where only heat transfer in a plane perpendicular to the cables is considered.

This content was updated! IEC 60287-2-3 is now implemented in Cableizer and you can simulate cables installed in ventilated tunnels.

IEC 60287-2: Calculation of thermal resistances of electric cables

Section 1: Calculation of thermal resistances

Section 2: A method for calculating reduction factors for groups of cables in free air, protected from solar radiation.

Section 3: Cables installed in ventilated tunnels

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