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Recently I powered on an electrical device that was (unbeknownst to me) quite defective. It tripped the circuit breaker almost immediately (after shooting off some sparks). There was a small (600W) transformer between the mains supply and defective device, which has its own internal fuse to guard against overcurrent events.

What struck me as odd was that the fuse was fine, despite there having been enough current drawn to trip the main breaker.

In essence, I've got things rated like:

  • Main Breaker - 30A @ 240V
  • Transformer Fuse - 10A @ 120V

Which seems to say that the fuse should always blow before the breaker trips, as it takes significantly less current to blow the fuse.

How is it that the breaker can be tripped while the fuse remains intact?

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    \$\begingroup\$ But it might take more heat. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 21 '18 at 2:53
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    \$\begingroup\$ Have a look at the datasheet of both, you will get graphs about the possible ranges of current and times it needs to blow. Draw in your supposed situation of current draw and you will see that the timing is quite different between both. \$\endgroup\$ – PlasmaHH Mar 21 '18 at 10:22
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Fuses generally come in two types:

  • Slow blow: These will sustain their maximum current, and possibly current not grossly over that, for some time. Generally on the order of a second or more.
  • Fast blow: These simply blow faster than slow blow.

Fuses aren't exactly precision instruments. It is literally a piece of wire that needs to heat and vaporize. Because of this variation, fuses are rated somewhere that might be fairly far off their actual blow current. To make things more interesting, there is a curve that fuses have which shows that as the peak current increases, the time to blow decreases. It makes sense if you think of the fact that you are actually heating up a piece of metal beyond the red-hot point (briefly). The more current (energy) you dump into it, the faster you'll heat up the fusing wire.

Breakers on the other hand are more precise. Your standard AC house breaker operates by having the mains wire wrapped around a ferrous bar, turning it into a small electromagnet. When the resulting magnetic field increases to a certain point, it actually pulls on a spring-loaded mechanism inside the breaker which separates the contacts. The amount of current required to induce a magnetic field strong enough to release the latch on the contacts can be much more precisely controlled than the fusing current of a fuse.

In short, your breaker tripped first because the fuse hadn't yet heated up enough to blow. Breakers can respond sooner to overcurrents since they don't rely on something heating up, though if the overcurrent is grossly high enough I imagine that the fuse might actually overtake the breaker in speed (though it may physically detonate, if it isn't an HRC fuse).

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    \$\begingroup\$ There are specialty fuses made with flat platinum wire with holes in them, embedded in sand, that blow fast on surges, but not motor start currents. These fuses are expensive. \$\endgroup\$ – Sparky256 Mar 21 '18 at 3:40
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Circuit breakers are commonly both thermal and magnetic.

They instantly trip magnetically for big overloads.

They trip slowly (thermally) for overloads that are just a little past the rating.

Thus a total short is instantly disconnected, but starting your skilsaw is tolerated

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Amusing the fuse was on the output side of the transformer, 240V primary 120V secondary, this also would have spared the fuse but primarily the trip time of the breaker is designed to save equipment from being further damage due to short circuit conditions.There are such things as quick blow fuses which have the fuse element connected to a spring to allow a quicker interruption to the over-current event.

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    \$\begingroup\$ the spring pulls the copper wire apart as soon as the wire gets hot enough to become a bit gooey \$\endgroup\$ – Henry Crun Mar 21 '18 at 7:03
  • \$\begingroup\$ "is designed to save equipment from being further damage" I think it's more correct to say they are designed to prevent cabling (or other components) overheating and catching fire: by the time the equipment has failed into an over-current situation, preventing further damage isn't the priority. \$\endgroup\$ – TripeHound Mar 21 '18 at 10:40

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