Recently , we have three phase Non-Segregated bus fault and it melted the bus. I am little bit confused about what is the meaning of Arc Flash fault. When we do the Arc Flash analysis then we consider the Cubical door is opened and someone is going to change the state of the protective device. But if the door is closed and nobody is changing the state of the protection device and fault happens.Can we say this is Arc Flash event? If yes then we have to wear Arc Flash PPE as while you are close by Electrical Equipment as you do not know when Arc Flash event may happen. Can anyone describe about Arc Flash event?

Please look at the definition of an arc flash in Art 100 of 70E - a dangerous condition associated with the possible release of energy caused by an electric arc. An Arc Flash does not necessarily depend upon human interaction or position of the electrical equipment (enclosed, guarded, or wide open-that is "exposed" . This could be why the 70E-2015 definition of normal operations as not requiring Arc Rated PPE may be misleading. Arc flashes can happen with just equipment insulation failures, and without human intervention/interaction. You must approach energized electrical equipment with the understanding that a risk assessment (as defined by 70E) is realistically required before entering into the shock and arc flash boundaries.

Arcing faults can happen at any time, and arcing faults are different from bolted faults. Just because bus is not segregated makes no difference. Segregated bus just theoretically limits it to line-neutral faults but I've seen line-line still and since especially the tubular kind is almost impossible to inspect, its worse. The difference is that doing certain activities around electrical gear significantly increases the likelihood of a fault.

In the case of busducts, they are notorious for joint failures because there are so many joints, bus can failures during stabbing operations, and faults due to environmentalcontamination.