NOVELTY - The electrolysis installation for decomposition of water into hydrogen and oxygen at high pressure, comprises an electrolysis stack (5) constituted of electrolytic cells each comprising an anode and a cathode, a water recirculating system supplying water to anodes and cathodes, a unit for supplying water to a recirculating circuit (11, 13), and an extinguishing unit to inactivate first and the second separators at the time of the cessation of the installation. The recirculating system comprises a pump (15), and first separator (7) to separate hydrogen gas from water.    USE - The electrolysis installation is useful for decomposition of water into hydrogen and oxygen at high pressure.    ADVANTAGE - The electrolysis installation is cost-effective and highly efficient and has a simplified structure for decomposition of water into hydrogen and oxygen at high pressure with less consumption of energy, and avoids deterioration of the quality of hydrogen mixed with nitrogen during inertization process.    DETAILED DESCRIPTION - The electrolysis installation for decomposition of water into hydrogen and oxygen at high pressure, comprises an electrolysis stack (5) constituted of electrolytic cells each comprising an anode and a cathode, a water recirculating system supplying water to anodes and cathodes, a unit for supplying water to a recirculating circuit (11, 13), and an extinguishing unit to inactivate first and the second separators at the time of the cessation of the installation. The recirculating system comprises a pump (15), the first separator (7) to separate hydrogen gas from water produced in the stack, and the second separator (9) to separate oxygen from water produced in the stack. The extinguishing unit comprises first and second depressurizing valves that are designed to simultaneously depressurize two separators while maintaining constant level of water inside the two separators. The extinguishing unit is arranged: to automatically control the water supplying unit; to entirely fill both separators with water using a hydraulic supply unit when hydrogen pressure in first separator is descended down below two bars; and to stop the electricity supply to the electrolysis stack before opening the depressurizing valves. The first depressurizing valve releases hydrogen in the atmosphere. The water supplying unit comprises a deionized water tank associated with a pipe to make the contact of the tank with the recirculating circuit in upstream of the recirculating pump entrance. The pipe is equipped with a valve, which is opened when the quantity of water in the recirculating system comes down below the predefined level, and with a non-reverse stop device. The recirculating valve is designed to reduce the flow of water coming from the two separators to increase the proportion of water coming from water supplying unit. An INDEPENDENT CLAIM is included for a process for inertization of an electrolysis installation.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of an electrolysis installation.    Electrolysis stack (5)    First separator (7)    Second separator (9)    Recirculating circuit (11, 13)    Pump. (15)