Around 13:45, a hydrogen leak on a pipe occurred in a refinery. Cold cutting of a purge was underway on the equipment as part of a work stoppage. The worker, who detected the leak, stopped the air saw and gave an alert. The installation was safe. The thousand employees presented was confined. A solid buffer was set up to isolate the section concerned by the leak.An error in identifying the location of the blowdown purge caused the event. A preparatory visit had been carried out a few months previously between the subcontractor in charge of the works and the subcontractor in charge of their preparation. A photograph was used, in addition to the isometric plans, to identify the installations. But the used photograph did not represent the good purge but its symmetrical compared to a manifold. The label materializing the location of the intervention was then placed on a hydrogen pipe in service. The pre-work checked and verified, focused on energy logging, failed to detect this error. There was no point on the spot, before starting work, between the intervener and the operator.Immediately after the incident, the operator reviewed its process of materializing the cutting sites. Tagging at the cut point by the facility manager become mandatory. The operator was reflecting on the solution to be retained for the entire refinery.The inspection of the classified installations, during a visit 6 days later, reminded the operator of its regulatory obligations in terms of joint inspection, operator / subcontractor, the pre-construction intervention zone and plans of prevention.