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IEC 60287-1: Calculation of current rating and losses of electric cables

This blog post discusses the topics covered by IEC standards 60287-1. It specifically emphasizes where and why Cableizer is deviating from the standard.

Posted 2015-03-11
Categories: Theory

Part 1 of the IEC standard 60287 covers current rating equations at 100% load factor and calculation of losses.

Section 1: General

IEC 60287-1-1 contains formulas for the calculation of the conductor resistance, the dielectric losses, the sheath and screen losses, and the armour losses. Furthermore, the standard contains methods for calculating the permissible current rating of cables based on the permissible conductor temperature, conductor resistance, losses, and thermal resistivities.

Cableizer interprets IEC 60287-1-1 as follows:

  • The equations for the calculation of the permissible current rating and the conductor temperature have been generalized to consider both partial drying-out of the soil, and direct exposure to solar radiation in one single equation. The formulas have also been modified for the case where two or three single-core cables are laid in a common duct.
  • Cableizer assumes that multiple single-core cables are installed with constant spacing between them all along the route. If that is not the case, the calculation of sheath circulating current losses for single-core cables with sheaths solidly bonded at both ends and possibly at intermediate points is affected. This effect can currently not be calculated in Cableizer.
  • For installations with two single-core cables, the sheath eddy-current losses are neglected. The standard does only provide equations for three single-core cables in trefoil or flat formation.
  • Cables with a wire screen are assumed to have an equalising tape or foil screen over the wires, and thus negligible sheath eddy-current losses. The eddy-current losses in the screen are considered negligible.
  • Steel tape armour is treated as steel wire armour, and vice versa, if the standard is not explicitly making a distinction of the two cases. Also, for the calculation of the sheath eddy-current losses of multi-core cables with common sheath, both non-magnetic and magnetic armour is treated as steel tape armour. This might somewhat overestimate the sheath eddy-current losses for steel wire armour and non-magnetic armours, configurations that are not covered by the standard.
  • Cables with each core in a separate sheath (touching the sheaths of the other phases), be it of lead (SL-type), aluminium (SA-type), or copper (SC-type), are treated the same (considering their different material properties), regardless which type the standard refers to, if there is not an explicit distinction.
  • In Cableizer, the cross-sectional area of steel tape or TECK armour is calculated directly from the armour thickness, assuming a filling ratio of 1.0. If that is not the case in practice, i.e. the armour thickness is larger than the thickness of a solid cylinder with the cross-sectional area of the armour, you should add some thickness to the armour bedding and outer sheath, in order to get the correct mean diameter of armour and external diameter of cable.
  • For non-magnetic armour or reinforcement, the standard defines four different scenarios for calculating the armour resistance. In Cableizer, the values are more generally defined as a continuous function of the length of lay:
    Length of layTouching wires$R_A$ acc. IEC$R_A$ acc. Cableizer
    very longyes or no1.0 p.u.1.0 p.u.
    54° layyes or no2.0 p.u.ca. 1.7 p.u.
    very shortnoinfinitelarge
    very shortyes 2.0 p.u.2.0 p.u.
    Cableizer considers steel tape and TECK armour as touching. Steel wires are considered touching, when they cover more than 90% of the cable bedding surface. The armour resistance of touching wires is considered to be limited to 2.0 p.u., as defined in the standard for steel tape with very short lay and two or more layers of tapes in contact with each other.
  • For the calculation of the armour losses of single-core SL-type cable with steel wire armour, bonded to the sheath at both ends, the standard says that the loss in sheath and armour may be assumed to be approximately equal. In Cableizer, however, the loss is distributed according to the actual relation between between the screen/sheath resistance and armour resistance.

Section 2: Sheath eddy current loss factors for two circuits in flat formation

IEC 60287-1-2 is not yet implemented in Cableizer due to its very narrow scope. It only applies to the calculation of eddy current losses in the metallic sheaths of single-core cables arranged in a three-phase double-circuit flat formation, with the sheaths bonded at one point or cross-bonded. In addition, the calculated correction factor is negligible unless the cables are aluminium-sheathed or have an unusually large lead sheath.

Section 3: Current sharing between parallel single-core cables and calculation of circulating current losses

Cableizer does currently not explicitly support parallel single-core cables, and as a consequence, IEC 60287-1-3 is not yet implemented in Cableizer. However, you can always define multiple cable system in parallel and adjust the load current as it would be shared between the parallel cables.

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