Evaluation of the Prospective Joule Integral to Assess the Limit Short Circuit Capability of Cables and Busways

At the occurrence of three-phase or single-phase faults, abnormal levels of thermal energy are developed during the time protective devices take to clear them. By conservatively assuming an adiabatic process, all the thermal let-through energy, also referred to as Joule integral, is accumulated within the components involved in short circuits; therefore, the temperature of their conductive materials elevates. The thermal energy is proportional to the square of the short circuit current. Evaluating the prospective Joule Integral is, therefore, crucial in order to assess the short circuit capability of cables and busways to withstand the thermal stress without failing or triggering fires in neighboring materials. In this paper, in the general case of resistive-inductive circuits, methods to both evaluate the Joule integral and to perform the assessment will be provided. The differences for power frequencies of 50 and 60 Hz are also shown.