At a steam plant, a boiler had been operating in steady state at minimum flow rate, with both gas-powered and fuel oil-powered burners running simultaneously. The mode setting was automated, yet due to a lack of reliability in the measurements of combustion air, both the gas discharge controls and blowing regime were shifted into manual operating mode in order to make adjustments to better meet demand. An electronic component malfunction on the fuel regulation system triggered the regulation valve to fully open, extinguishing combustion and generating a large quantity of unburned fuel, which blocked the flame detectors, thereby causing a general fuel oil and gas supply outage. Since the fan was still running, the unburned fuel reached the upper flammability limit and exploded.