In order to replace cyclones at the exist of a chip drier at a plant manufacturing wood panels, an employee of a subcontracting company began cutting an external element of the drier with a side grinder while it was still in operation. This resulted in a dust explosion and fire; a crack had appeared on the cyclone’s access door that had allowed sparks to pass. The firefighters sprayed water on the installation to cool it down. Production was interrupted the time needed to replace a vent destroyed by the explosion and clean and check the installations. The operator modified his work permit to require that the drier be shut down before work involving hot spots.