The arc flash hazard severity is directly dependent on the upstream OCPD, the protecting OCPD that protects downstream equipment.

• A. True
• B. False
• C. Not known
• D. Sometimes

Answer: (A)

The main idea is that arc flash energy at a downstream point is determined by how long the fault current can flow before protection opens the circuit. The upstream OCPD—the device that would trip to protect the downstream equipment—sets that clearing time. If this upstream device operates quickly, the fault is cleared fast and the energy released is small; if it trips slowly or doesn’t coordinate properly, the fault persists longer and the incident energy rises, increasing the hazard. That’s why protective-device time-current characteristics and proper coordination are central to predicting and reducing arc flash energy. The current magnitude is set by the source and system impedance, but the severity at the downstream location is largely governed by how quickly the upstream protecting OCPD clears the fault.