At a site on the urban fringe specialised in packaging industrial gases, 4 kg of chlorine (Cl2) escaped during a transfer between bottles. The “source” bottle, which had been heated to 50°C by an electric blanket, contained 8.2 kg of Cl2 and was fitted with a fuse set at 75°C. The untimely tripping of the fuse caused this leak. The “destination” bottle was cooled by means of circulating a refrigerant. A toxic gas alarm was triggered; a technician isolated the two bottles and, in recognising a continuing leak, left the workshop. The other employees placed the unit in safety mode and evacuated the premises. Confined to an enclosure designed for this purpose yet whose seal had been breached, the leaky bottle was dispatched to another isolated spot onsite. The remaining gas was drained onto a sodium carbonate bath. Both the internal and external emergency plans were activated within 2 hours as a precaution; a technician had to be hospitalised for a few hours for testing and the site’s 27 employees were evacuated. At a neighbouring site, only the Cl2 smell was noticeable. To increase workshop productivity, a 3-kW heating mantle had been installed instead of the existing 1-kW mantle; it had been used for the first time and the poorly-positioned temperature probe had not made good contact with the bottle. Excessive uncontrolled heating of this mantle caused a temperature rise in the tap that actually melted the fuse. The hood design was incapable of suctioning such a massive leak as its ventilation system had not been intended to neutralise discharged gas. The confined enclosure was verified and retested; the operator was requested to provide both a Cl2 extraction servo-controlled to detection and a gas treatment process. Moreover, the operator: updated the site’s safety report (with new scenarios), suspended use of the new mantle, recalled change management procedures, revised packaging steps, and modified both the workshop ventilation and processing. The corresponding capital investments were valued at 8 million francs.