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Circuit breakers and fuses have trip curves which describe how quickly they interrupt various currents. How does the manufacturer of a molded-case circuit breaker choose one value of current to represent the product? In other words, how would you determine the trip rating of this molded-case breaker given its trip curve?

example trip curve

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2 Answers 2

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Did more research.

Table 7.1.2.2.1 of UL 489 lists the maximum automatic tripping time at 200% of the rated current.

For example, breakers rated 101-150 Amps must trip within 8 minutes (480 seconds) at 200% of their rating. Since at 480 seconds this breaker has interrupted about 300 Amps, this breaker is probably rated 150 or 125 Amps.

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  • \$\begingroup\$ The rated current of this breaker is what it can sustain 'indefinitely', a little over 150A from the graph, the UL specification indicates the maximum current after some 'generous startup' time. The highest 20kA (1-30kA) value is for a local short and the 2kA (1-3kA) for a short at the end of a correctly rated cable. \$\endgroup\$
    – KalleMP
    Commented Dec 17, 2015 at 8:15
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The lower range (0.02 seconds) is a magnetic effect. It is about 1 cycle of a 50/60 Hz AC. So once the current is enough to cause that (here about 3 kA), it is a pretty well-defined time.

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  • \$\begingroup\$ Does that tell me the trip rating? (Also, I think you meant 3 kA as the beginning of the instantaneous region.) \$\endgroup\$
    – njivy
    Commented Sep 29, 2011 at 23:44

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