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ekstra   ara
 Post subject: Arc flash in cable box of low voltage motors
PostPosted: Sun Sep 02, 2012 2:27 pm 
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Joined: Sun Sep 02, 2012 12:17 pm
Posts: 1
Recently we have had two serious incidents with an arcflash in the cable box of a low voltage motor.
Our High Voltage motors have standard a pressure relief in the wall of the cable box

One motor was supplied by 400 volt (delta connected) AC 50 Hz, 200 kW, DOL, fuse was 630 Amps . A winding failure (earth fault with short circuit) has made an overpressure inside the stator-housing. This overpressure could reach into the cable box and the cover of the cable box was fragmented by the overpressure. This incident has happened during starting by the operator near the motor. The load was a FAN. The feeding transformer was 1250 kVA, 10 kV/0,4kV

The second motor was supplied by 690 volt (star connected) AC 50 Hz, 200 kW, DOL, fuse was 315 Amps.
During normal operation the bearing was blocked by lack of greasing. The MCC has tried to disconnect the motor but the contactor contacts were sticked by the high current. Inside the cable box an arc flash caused an overpressure and the cover of the cable box was fragmented by the overpressure and was landed 15 meters from the motor.
The load was a FAN. The feeding transformer was 3150 kVA, 10 kV/0.69 kV.

We were very astonished that this could happen. Who has experience with similar incidents and how can we take measures to prevent such a near miss.


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