The severity of the arc flash hazard during an arc flash event is directly dependent on the upstream OCPD, the 'protecting OCPD,' that is protecting the downstream equipment.

• A. Directly
• B. Indirectly
• C. Not at all
• D. Unknown

Answer: (A)

The key idea is that the amount of energy in an arc flash (and thus the hazard severity) is governed by how long the fault current stays unisolated. The upstream protecting OCPD— the device that protects downstream equipment—determines that clearing time. If the upstream OCPD trips quickly, the arc is extinguished sooner, reducing the incident energy (remember that incident energy roughly follows I^2 t). If it trips slowly or isn’t properly set, the arc lasts longer and the energy is higher, increasing the hazard. The protective device’s trip characteristics and settings directly control the duration of the arc, so the arc flash severity is directly dependent on the upstream protecting OCPD.