Читать книгу Arc Flash Hazard Analysis and Mitigation - J. C. Das - Страница 20

Of necessity and for the continuity of processes, maintenance of electrical equipment in energized state has to be allowed for. If all maintenance work could be carried out in deenergized state, short circuits cannot occur and therefore there is no risk of arc flash hazard. For the continuous process plants, where the shutdown of a process can result in colossal amount of loss, downtime and restarting; it becomes necessary to maintain the equipment in the energized state. Prior to the institution of arc flash standards, this has been carried out for many years, jeopardizing worker safety, and there are documented cases of injuries including fatal burns.

The time/motion studies show that human reaction time to sense, judge, and run away from a hazardous situation varies from person-to-person. A typical time is of the order of 0.4 second. This means that 24 cycles is the shortest time in which a person can view a condition and begin to move or act. In all other conditions, it is not possible to see a hazardous situation and move away from it. As will be further demonstrated, this reaction time is too large for a worker to move away and shelter himself from an arc flash hazard situation.