The importance of the metal reinforcement of low voltage cables in the process of identifying defects

Abstract

Regarding the identification of defect in the electrical energy cables, in exploatation, usual or high-performance methods are used, based in particular on the phenomenon of reflection and refraction of the impulse transmitted in the cable, phenomenon generated by the impedance change at the defect location. In order to identify the defect, an important role is played by the existence of a current path on the cable route between the ends of the electrical cable and the place where the defect occurred. In the case of low voltage cables, there are situations where, following the sleeve process the continuity of the mechanical protection armature of the steel strip or the copper screen has not been restored. To measure the distance to the defect location, location equipment uses low and high voltage pulses which moves to the defect site with a certain speed and which is repeated after a selected time interval. These impulses are transmitted to the location of the defect at one end of the cable, through the circuit generated by the short circuit between the faulted conductor and the metal armature, which is usually found at ground potential. At the same time, there is situations where the defect is difficult to identify precisely because of the continuity of the reinforcement or screen and their connection to ground potential. This paper describes the stages of fault identification in a low-voltage power cable and presents a case study that highlights the importance of the metal reinforcement in the case of the high-voltage impulse method. Ways to detect defects using the arc reflected method (ARM) using the shock wave generator as well as the inductive method are also briefly presented.