CSLB C-10 Electrical ContractorSafety and TestingHard
An electrical contractor is performing an arc flash risk assessment for a 480V motor control center. According to NFPA 70E, what is the primary factor that determines the severity of an arc flash incident and dictates the required Personal Protective Equipment (PPE)?
- AThe duration of the arc and the working distance.
- BThe incident energy at the working distance.
- CThe system voltage and available fault current.
- DThe type of overcurrent protective device (OCPD) and its clearing time.
Show answer & explanationAnswer & explanation
Correct answer: B. The incident energy at the working distance.
While voltage, fault current, OCPD clearing time, and working distance all contribute, the incident energy at the working distance is the ultimate calculated value that quantifies the thermal hazard of an arc flash and directly determines the required PPE category according to NFPA 70E.
Why the other options are wrong
- A. Arc duration and working distance are critical factors in the incident energy calculation, but incident energy itself is the direct determinant.
- C. System voltage and available fault current are inputs to the incident energy calculation, but not the direct determinant of severity or PPE.
- D. The type and clearing time of the OCPD significantly influence arc duration, which affects incident energy, but are not the final determinant.
Incident Energy
The amount of thermal energy impressed on a surface, a certain distance from an electric arc, typically expressed in calories per square centimeter (cal/cm²).
- Primary metric for arc flash hazard assessment.
- Determines the required Arc Flash PPE category.
- Influenced by voltage, fault current, arc duration, and working distance.
Memory trick: Incident Energy: The Heat of the Hazard.