A lightning strike caused a voltage drop on the power supply of a chlorine-alkali plant using the mercury process. The emergency electrical system was activated, though restoring the external supply voltage in less than 200 ms rendered the entire system inconsistent; some units were operating normally, but all units connected to the emergency devices (including the mercury pump) remained idle. An explosive mix of 500 kg of chlorine and an unknown amount of hydrogen accumulated inside the low-pressure chlorine treatment circuit and, upon exploding, injured two employees and destroyed the whole circuit (damage: 237 000 euros).
This accident demonstrates the need to surpass a simple binary approach (i.e. on/off) in order to incorporate short-duration voltage drops (electrical disturbances) into the safety reports. An installation’s emergency power supply must be designed by integrating a SIL 2 safety integrity level (according to the Standards IEC 61508 / 61511). The site operator extended personnel training modules to include the facility’s operating details, in a step to better cope with unidentified defects.