« Conductor protection against short circuit current: available I2·t evaluation

The admissible let-through energy I2t allows consideration of the cable protection from overheating due to excessive short circuit current flowing in its conductors. For short circuit current duration of t<0.1 s, which is a range own for instantaneous tripping of protective devices in electric power systems below 600 V AC, the asymmetry of the current is of importance in the evaluation of the through energy. The IEC Standard suggests that the value of I2t should be quoted by the manufacturer of the adopted protective devices. In the case of very short duration less than half-cycle, for which it is necessary to consider the protective device I2t limiting behavior. The aim of this paper is to free the designer from the necessary knowledge of protective device behavior or from the need of prior manufacturers choice. The paper suggests evaluation of the prospective Joule integral, always referred to as RMS short circuit current by means a factor m, which considers the worst case (maximum value). Expressions and illustrations are given for m factor evaluation, which is variable with fault circuit conditions and prospective short circuit duration