Explosion in an electrolyzer producing chlorine The explosion occurred after the opening of an auxiliary circuit-breaker. The opening of the auxiliary circuit breaker trigger the shut down of the pumps and chlorine compressor. Two of the circuit breakers open but one remained closed providing the electrolyzer with 20 kA, while the mercury and brine pumps and the chlorine compressor were shut down. The explosion occurred when the auxiliary circuit breaker was finally closed 15mn later, following various unsuccessful attempts.The reason for the auxiliary circuit breaker to open was an overload of the electric transformer, aging of the circuit breaker and high temperature. It is not clear what caused the specific circuit breaker, which was used since 2 years and in good state, to remain open. One hypothesis is that the 50kA neighboring line lead to a magnetization of the circuit breaker.A plausible cause for explosion is the formation hydrogen by the acidic attack of the electrolyzer cells and production of sodium and potassium hypochlorite. The still running chlorine compressor extracted the hydrogen and reacted with the chlorine remaining in the pipe. This hypothesis is confirmed by the small pressure increase in the cells and the absence of explosion in the electrolyzer cells. The formation of hypochlorite and hydrogen was possible by the reaction of the salt and the water contained in the brine due to the absence of mercury to fix the sodium and the potassium.