In a plant producing felt textiles for the automobile industry, a fire broke out at the level of the transformer room, which contains a 630 kVA transformation unit. The origin of the fire is correlated with the direct or indirect action of lightning on the site or in the vicinity during the day of July 5, 2006. The fire was fanned by the combustion of oil residues contained in the regulatory retention tank (i.e. a volume of less than 1 litre). Control of the dielectric quantity (without PCB) contained in the transformer compartment, undertaken by the subcontractor, indicates that this quantity matches the filling level. An examination of the transformer does not reveal any blisters present in the case of a direct lightning strike; furthermore, the absence of a leak on this transformer offers proof that the integrity of this unit has been preserved. The presence of smoke on the wall and traces in back of the transformer demonstrate that fire had occurred in the tank. This onset of fire could thus be explained by the creation of an electric arc at the level of the tank and the ignition of oil deposits (i.e. a potentially fouled tank). The plant’s lightning study underscores that the transformer must be protected by lightning rods, which are indeed very present on the site’s supply post, as per the study’s set of recommendations. Their state of repair shows no underlying destruction. This study however must be updated every 5 years, and the last study dates back to July 7, 1998. The operator is thus requested to update this study. On the technical side, the lightning study recommends introducing other measurements on the following installations: interconnect the incinerator chimney and structures, remove the former dust cleaning installation, interconnect the dust recovery system with structural masses and the external lampposts with the electrical ground network of the building, equip the protective control and power circuits with a lightning arrester as well as the automatic sprinkler circuits, the suction and the hot air duct, the cooling electrical cabinet, the telephone lines and the secondary power transformer. This study also recommends installing temperature probes on both the cooling fan and charcoal filter box. The operator was required to submit to the Hazardous Installations Inspectorate within one month the verification report demonstrating that these protective measures have indeed been completed. Since it was scheduled that this site would soon be relocated to a neighbouring locality, the Hazardous Installations Inspectorate reminded the operator that all site operations still needed to comply with the general conditions set forth in the environmental code.