REQUIREMENTS TO BE MET BY PROTECTION AND SWITCHING DEVICES FROM THE ARCING PROTECTION POINT OF VIEW

摘要

Investigations of low-voltage networks and switchgear show that nearly all short-circuits are arcing faults. From the specifics of arcing faults, special requirements are resulting to the protection devices. These needs are of partially significant higher extention as usual in the normal short-circuit protection. A high degree in equipment and personnel protection requires very short switching-off times. Fuses are principly useful for this. At fault points with highest short-circuit currents, there is the need of a very fast and - under circumstances - current-limiting interuption (switching-off times in the range of 5 ms). In case of faults in the sub-distributions or at the coupling points of great loads, which are mostly characterised by smaller fault currents, there is often the problem of the "fault detection" because there are no great differences between fault currents and maximum operational ones. The function of fuses is generally connected with the real fault currents being used for selecting and checking selectivity. Short switch-off times are only reached if there are high ratios between the real fault currents and the rated fuse currents. Guaranteeing a short-circuit current limitation requires highest ratios. In case of arcing faults the currents are significantly decreased in comparison with the calculation values for I"_(k(3)). From this, the following general requirements to the use of fuses can be summarised from the arcing fault protection point of view: The selection and checking has to be based on the real minimum fault currents I_(karc) (considering k_B). Otherwise significant increases of the interuption times have to be expected (factors for t_k of 5...20). Fuses should only be used if it is guaranteed by an according high ratio I_(karc)/I_(rSi) that short-circuit times t_k are permissible. If a short-circuit current limitation effect by the fuse is necessary, the integral current heat values ∫ i~2dt resp. P~2*t have to be checked. Fuses being not able, arcing fault protection has to be ensured by additionally use of special secondary protection means. More exact calculations of the fuse switch-off behaviour in case of arcing faults are not possible yet because there is a deficiency in suitable models (for describing the fuse characteristic or the time behaviour of the fuse arc voltage). Such calculations, especially when considering the interactions between network, fault arcs and fuse switching arcs would increase the safety refering to the evaluation of the fuse switching-off, and enable to draw further concrete conclusions for the application of fuses.