Fire crews had requested the Environment Agency to attend the fire at a building on Collaton Lane in Tavistock, to help control a diesel spillage.
Western Power has also been at the site and disconnected the electricity to help make it safe.
At 0700, a cordon has been put around the area and fire crews were monitoring the incident from a safe distance because of the volatile nature of acetylene cylinders.
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Drier cloudier conditions to follow