A power cut in a chemical plant caused the automatic fold-back of carbon bisulphide (CS2), sulphuric acid (H2SO4) and methyl mercaptan (MeSH) production units. The power supply of the plant was managed by an independent EIG. The decompression of the facilities led to the release of 700 kg of sulphur dioxide (SO2) in less than 15 min (200 kg via the MeSH unit’s flare and 550 kg by the flares of the H2SO4 and CS2 units). Given the relative atmospheric stability, an SO2 cloud was formed over the site. The internal emergency plan was triggered, staff confined or evacuated and the neighbouring towns informed about the accident. The onsite SO2 measurements indicated values close to 0, but the measurements made by the local air quality monitoring network situated at Roches de Condrieu showed a SO2 peak of 2.5 mg/m³ 30 min following the accident. The situation was back to normal after an hour.

A new power supply substation and new protections were set up two days before the accident to create lines supplying a new production unit. Since then, the site was supplied by two 20 kV lines with a coupling switch in the platform substation to balance the load between the two links. The accident resulted from an earth fault detected by the power supply company that interrupted power supply to the chemical platform by the protective relay. The units supplied from this substation were deprived of power for 11 min. It was later on confirmed that the 20 kV power supply was protected at 0.7 A initially by an electric transformer due to the mixed supply of the chemical platform and an overhead network supplying residences while the site’s internal protection was set at 3 A to avoid sudden power cuts. The plant requested the power supply company to raise the protection level to 3 A or replace it with an alarm. In the meanwhile, the coupling switch will remain in tripped position.