EMERGENCY CABLE RATING IN AIR

For cables installed in air, the conductor temperature changes rapidly with changes in load current, and hence, daily load cycles do not allow cyclic rating to exceed the steady-state value. However, short term emergency ratings may significantly exceed steady-state values without exceeding permissible temperature limits. The paper identifies issues with the IEC 60853-2 (Calculation of the cyclic and emergency current rating of cables. Part 2: Cyclic rating of cables greater than 18/30 (36) kV and emergency ratings for cables of all voltages). This standard gives manual methods for calculating cyclic rating factors for cables whose internal thermal capacitance cannot be neglected; in general this applies to cables for voltages greater than 18/30 (36) kV. It also gives a method for calculating the emergency rating for cables of any voltage. However, the emergency rating calculations are not applicable where load applied exceeds 2.5 times the steady-state current value, if this condition is not met, the formulas become irrelevant. This paper will present possible solutions for calculation for emergency cable rating in air which are outside the scope of IEC 60853-2.