NFPA 70E informational Note to Table 130.7(C)(15)(a) states that molded case circuit breakers have a typical fault clearing time of 3 cycles with an instantaneous integral trip. True or False?

NFPA 70E informational Note to Table 130.7(C)(15)(a) states that molded case circuit breakers have a typical fault clearing time of 3 cycles with an instantaneous integral trip. True or False?

• A. True
• B. False
• C. Not specified
• D. Unknown

Answer: (B)

Fault clearing time is how long it takes for a protective device to interrupt a fault after it is detected. When a molded case circuit breaker has an instantaneous trip, it is designed to respond immediately once the fault current exceeds its instantaneous pickup, using a magnetic mechanism. That means the clearing time is dominated by very fast electromagnetic action and the breaker’s mechanical reset time, typically a small fraction of a second, often well under one cycle at standard power frequencies. Describing such a situation as a typical fault clearing time of three cycles would imply a much longer delay than what the instantaneous trip is intended to provide. Therefore, the statement is not accurate. The note reflects that instantaneous-trip MCCBs interrupt faults quickly to reduce arc energy, but it does not set a 3-cycle standard.

Fault clearing time is how long it takes for a protective device to interrupt a fault after it is detected. When a molded case circuit breaker has an instantaneous trip, it is designed to respond immediately once the fault current exceeds its instantaneous pickup, using a magnetic mechanism. That means the clearing time is dominated by very fast electromagnetic action and the breaker’s mechanical reset time, typically a small fraction of a second, often well under one cycle at standard power frequencies. Describing such a situation as a typical fault clearing time of three cycles would imply a much longer delay than what the instantaneous trip is intended to provide. Therefore, the statement is not accurate. The note reflects that instantaneous-trip MCCBs interrupt faults quickly to reduce arc energy, but it does not set a 3-cycle standard.