[Weekly Accident Case Sharing] Accident Case 2: Cable sealing issues.

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This post was last edited by tiandiyi*an on 2017-8-14 08:08. I. Fault phenomenon: Water flowed back into the explosion-proof flexible conduit of a cable in a certain company, reaching the primary fan 6A; this caused the electric actuator of the outlet damper to activate, resulting in the shutdown of the primary fan. At 22:41 on July 11, 2015, the operators of Unit #6 detected a trip of fan 6A, which caused the tripping of coal mills 6A/6C as well; as a result, the unit’s load dropped to 500 MW. After the fan tripped, the operators determined that the cause of the trip was the activation of the protection mechanism due to the fan being in operation with all the outlet dampers closed. Further inspection revealed water accumulation inside the electric actuator of outlet damper 6A of the primary fan; the water was removed and the control circuit board inside the actuator was dried, but the device still could not operate properly. Eventually, after the maintenance crew replaced the control circuit board, the actuator functioned normally. At 15:10 on July 12, outlet damper 6A of the primary fan was brought back online. II. Cause analysis: (1) Water entered the electric actuator of the outlet damper of primary fan 6A, resulting in the tripping of primary fan 6A. (2) After the rainy season begins, increased rainfall causes water to enter the cable trays. This water then flows through the cable trays, along the cable of the electric actuator at the 6A outlet flap of the primary fan, and into the interior of the actuator, resulting in a short circuit in the control circuit board of the actuator. III. Identified issues: (1) Improper use of explosion-proof flexible hoses. The explosion-proof flexible pipe was not connected using explosion-proof flanges, resulting in poor sealing of the electric actuator. (2) No measures have been taken to direct rainwater away from cable trays and cable conduits. IV. Preventive measures: (1) Perform drainage by drilling holes in the cable trays and cable conduits throughout the plant, or use tees for this purpose. (2) Inspect the instrument seals throughout the plant; add flange connectors where explosion-proof flexible pipes are lacking, and add sealing gaskets where they are missing, to ensure proper sealing of the instruments.