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Is there a reason that both a fuse and a circuit breaker would operate? From my understanding, the fuse should blow first and kill any current flowing through it leaving the breaker intact. So why would a fuse blow, and then an upstream breaker trip and lock out?

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    \$\begingroup\$ If the current was high enough, both could possibly go together. Fuses are thermal, but breakers can be magnetic and thermal. The thermal breaker would be slower than the thermal fuse, which is why the breaker has a magnetic trip. The current squared would be proportional to the magnetic field. \$\endgroup\$ – StainlessSteelRat May 17 at 18:04
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I would also think it unusual, most breakers have higher current ratings than that of products, but it really comes down to the details of the situation.

It depends on the fuse. There are slow blow fuses that take a few msec to blow, which could have been carrying a decent amount of current before the breaker shut off.

If the fuse rating was near the rating of the breaker, then the explanation is simple, the current rose and they both blew.

Most breakers have two mechanisms for tripping, one for a high fast magnetic current. And the other is slower and from heating an element in the breaker with current. Most fuses have an element that melts from ohmic heating and current. It depends on how much current for how long.

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I've seen this effect in my microwave oven when the bulk plastic cap failed with reduced capacitance and leakage. 15A fuse and 15A breaker. (AFAIK)

It failed on surge startup and was repeatable. Its an unusual race condition that may be activated by the reactive load with arcing until fuse gap extinguishes the arc.

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