The power cord of the furnace recorder was intentionally disconnected during furnace check by maintenance operator while he opened the door of the furnace control box.
The investigation found that the power cord of the recorder is too short and risks to be disconnected if the furnace door is opened too wide (the recorder is attached to the control box door while the power cord connects to the power socket inside the control box).
After the power cord had been detached, the recorder continued operating using the power supplied from its UPS (which lasts for 0.5 – 1 hour). Thus, the maintenance operator couldn’t detect the problem right after he had finished checking the furnace.
After the recorder shut down, nobody could detect the problem because there is no requirement to check the furnace recorder which located at the backside of the furnace. The operator only checked the temperature from 4the display of the temperature controller.