Electrical discharges caused by opening contacts in an ignitable atmosphere – Part II: Spectroscopic investigation and estimation of temperatures

Abstract Electrical discharges are a potential source of ignition in explosive atmospheres. Spectroscopic investigations of the radiation emitted during such discharges give a better understanding of the processes that take place during ignition. The discharges investigated here are comparable to discharges generated in the spark test apparatus defined in IEC 60079-11 (2011). In these investigations, such discharges are generated with a maximum voltage of 30 V and constant currents from 50 mA to 100 mA between opening electrical contacts (cadmium cathode, tungsten wire anode). Here, discharges with currents higher than 40 mA may normally lead to an ignition in the hydrogen/air mixture (21% vol. H2). This article describes the measurement setup necessary in order to investigate these discharges between slowly separating electrodes, which are approximately 200 μm long and typically last 2–4 ms. The method for evaluating the measurement data is described, and the measurement uncertainty is considered. The characteristic spectrum of these discharges is presented and discussed together with the temporal curves of this spectrum. The excitation temperatures determined with the Boltzmann plot from this spectrum are shown and compared with temperatures calculated from the specific electrical conductance and geometry. Based on these excitation temperatures, the characteristics of discharges with and without ignition are shown. An outlook on the further steps necessary for the characterization of these discharges and further open questions are mentioned.