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To develop high voltage circuit breaker (HVCB), numerical simulation is commonly used. These simulations are based on simplifying assumptions. One of the most commonly admitted hypotheses is local thermodynamic equilibrium (LTE). These hypotheses ensure reasonable calculation time but under severe conditions, some differences can be observed between simulations and laboratory tests. In this study, the occurrence of thermo-chemical non equilibrium in the HVCB is studied at the breakdown time. A study on a Nitrogen filled high voltage circuit breaker chamber case is performed, including hydrodynamics, Joule effect, radiation losses, chemical kinetics of the gas and thermal non equilibrium. The influence of 4 different Nitrogen kinetic models has previously been studied. Moreover translation/rotation, vibration and electron/electronic temperatures are distinguished. The aim of this work is the determination of the interest of thermo-chemical non equilibrium on the numerical simulations to design more efficient HVCB.