In a plastics manufacturing plant, the cracked gas compressor of the steam cracker shut down at around 9 am following an electrical fault, causing significant flare discharges (large flame and smoke). This concerned a multi-stage centrifugal compressor whose variable speed electric motor was not ADF, was protected by a slightly pressurised casing. As the compressor’s restartup tests had failed, the motor’s protective casing was removed in order to examine it. At around 3 pm, the operator decided to progressively fallback the plant’s units, of which the steam cracker’s 4 operating furnaces (13 in total). The PEL unit was shut down, the HDT, butadiene and aro 3 units were in ‘recycle’ mode, the polypropylene unit was in minimum production and exportation in C3 cut. This progressive switching over of the units to safety mode helped to reduce the amount of cracked gas and therefore the flare discharges. At around 4 pm, activation of the site’s 2 boilers that produced the steam required for suppressing the flares, resulted in the appearance of black smoke at the 2 flares still in operation. This problem, whose cause remained unknown, was solved at around 4.30 pm. Following this incident, the plant’s electrical department tried to establish a pre-diagnosis and called upon technicians from a specialist company. The inspection authorities for classified facilities asked the operator to provide an incident report within 15 days including the chronology of the events, an analysis of the causes of the incident, an assessment of the material flows discharged stating the nature and amount of each product emitted, the human and environmental effects, the measures taken to deal with these possible effects, those taken or considered to prevent similar incidents and to mitigate the effects, the gas flow rate values at the flare based on time as well as the values corresponding to the pressure and flow rate of the suppression steam for each flare.