The NFPA 70E informational Note provides typical fault clearing times for which devices?

• A. Only molded case circuit breakers
• B. Only current-limiting fuses
• C. Both molded case circuit breakers and current-limiting fuses
• D. Neither

Answer: (C)

Understanding arc-flash energy calculations relies on how long a fault is allowed to persist before it’s cleared. The NFPA 70E informational Note gives typical fault-clearing times for the protective devices you’re most likely to encounter in electrical equipment: molded-case circuit breakers and current-limiting fuses. These two device types have published, representative clearing times under short-circuit conditions, which you can use to estimate incident energy when you don’t have exact timing data. Since both molded-case breakers and current-limiting fuses are common interrupting devices in many installations, the note provides typical times for both to help with hazard analysis. It wouldn’t be accurate to limit the note to just one device because these two types cover the majority of fault-clearing scenarios engineers assess for arc energy.