In the IEEE study, if the closest upstream circuit breaker did not operate and the next higher upstream OCPD cleared the arcing fault, in approximately what fraction of cases would the worker lack adequate arc-rated PPE?

• A. 1/4
• B. 2/3
• C. 1/2
• D. 3/4

Answer: (B)

The main idea is how arc flash energy depends on how long the arc is left unisolated. Incident energy at the worker increases with arc duration, so when the closest upstream protective device fails to operate and only a higher upstream device clears the fault, the arc stays energized longer. That extra time lets the energy build up, and in the IEEE study this situation led to the worker exceeding the rating of their arc-rated PPE in the majority of cases. In other words, the PPE would be inadequate more often than not when the fault is cleared by a higher-up device after the nearest upstream circuit breaker didn’t trip. This highlights the importance of assuming worst-case arc duration and selecting PPE that covers the higher potential incident energy.