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Thursday, December 15, 2022
Question:
Good morning, Charlie, I installed a 2000A service, and per Section 110.21(B)(4) the service equipment must be labeled with clearing time of the overcurrent device. How do I determine the clearing time of a circuit breaker? Why is this information required? Thanks, Jake Philbin.
A
Answer:
Hello Jake, thanks for your question. (1) The clearing time of a circuit breaker, or any over current device (OCPD), can be obtained from the manufacturer in the form of a
Time Current Curve or
Trip Curve graph. Most OCPD manufacturer’s will also make these graphs available on their website by simply searching the part number. This graph is an X/Y axis graph, representing the Time/Current relationship in the form of a Time/Current curve. The graph will show the Time on the Y-axis, and Current on the X-axis. To use this graph, you will also need to know the available fault current at the circuit breaker location. If this is at the service disconnect location, the available fault current can be obtained from the Utility. Typically, the higher the fault current the fast an OCPD will operate. (2) By posting the clearing time of an OCPD, on the equipment, you are providing a valuable piece of information to the next technician that may have to service the equipment while energized. If there has
not been an Incident Energy Study on the system, the technician can use the 2021 Edition of NFPA 70E applying the “PPE Category Method” found Section 130.7(C)(15). This method provides a means for the technician to select the appropriate Arc-Flash PPE using Table 130.7(C)(15)(a). The clearing time is a crucial piece of information needed in order to use this Table.