Circuits operation control and overloads protection: power cables equivalent ampacities

Abstract

The evaluation of the ampacity of electric power cables and the overload protection are currently based on criteria, to which the operation of the circuits will have to satisfy each time like the first time, without self-memory of the way of life, without discounts or debts correspondent to the actual profile load, certainly different from a steady-state line. The number and the duration of the emergency overload events are taken statistically into account (IEC rules) or are fixed in advance for an allowable average time per year over the life of the cable (ICEA approach). To enhance these criteria, this paper suggests a method to define in the low-voltage range an equivalent prospective power cables ampacity, based on the global allowable cables life loss and considering the performance in the time. Since still monitoring and memorizing the actual life of power cables are not possible, the paper proposes a simple and quick evaluation method for the duty cycles characterized or approximated by load phases of continuous currents. The aim of this paper is to try to help the feeling of performance and behavior of insulated power cables. In reference to the prospective actual load profile of each circuit or grouping of circuits, this feeling is very useful for the designer to presuppose how much the power cable size will be allowable and for the maintenance operator to presume during the life of the circuits the real conditions of the power cables.