The explosion occurred in a copper electroplating plant test furnace when starting the operation of copper plating. EVENT DETAILS A tinned copper pipe had been placed inside another copper pipe and lined with pellets. Inside the furnace, the pellets were intended to adhere to the outer surface of the smaller tinned copper pipe under hydrogen atmosphere. The test was the third in the series. A vacuum was created inside the furnace, and an indicator light showed that a vacuum had been successfully generated. After that, the furnace was switched to automatic operation and the employee exited the hall. The intention was to supply hydrogen to the furnace while it was on automatic operation. However, at the furnace exploded. CAUSE: The furnace was equipped with two bleeding valves, one of which was manual and the other electronically operated by a pushbutton. The pushbutton-operated bleeding valve located in the vacuum under the furnace had remained in the open position when operation was commenced. However, the vacuum suction automation system kept the afore-mentioned valve closed during the period required for reaching the vacuum and the waiting period (57 min) following full vacuum. After that, hydrogen was introduced to the furnace automatically for about 10 min. After the purging gas valve had closed, the automation system no longer prevented the bleeding valve from opening; instead, air was allowed to pass to inside the furnace because the bleeding valve button was in the depressed position. The furnace was equipped with graphite rod elements that very quickly heat up the furnace to the set temperature. When the power supply switched on, the rods became quickly glowing red and ignited the hydrogen-air mixture. The hydrogen-air mixture inside the furnace exploded, the blind flanges of the furnace were dislocated out of their joints, and a pressure wave was propagated to the testing hall where no people were present at the time. The cause of the explosion was the pushbutton-operated bleeding valve that had remained in the open position, which the signal lamp failed to indicate because it was broken. As a result, air found its way into the furnace through the said valve after the hydrogen valve had closed and the waiting period was up, i.e. when the bleeding valve was no longer controlled by programd automation.