How Does A Circuit Breaker Break?

Even if you aren’t an electronics person, you probably have a working knowledge of circuit breakers. When the lights go out, you find the circuit breaker and flip it back to the on position. Most people understand, too, that the breaker will trip if you overload the circuit with too many things plugged in or with an accidental short circuit. But how does this common device actually work? Keep in mind, circuit breakers need to be super reliable and have been around long enough that you can imagine they are pretty low tech. [Learn Engineering] has a very clear video about what goes on inside a circuit breaker that’s worth the eight minutes to watch. You can see the video below.

The handle is a mechanical engineering marvel, using two springs and a special design so that even a tiny force will cause it to snap to the off position. That takes care of a human tripping it. However, you have two other cases where you want to turn it off: overload and short circuit.

The short circuit case is easy to detect with an electromagnet that will have a strong magnetic field when so much current flows. However, the overload case isn’t that abrupt. For that, the breaker uses a bimetalic coil that expands as it heats.

There is one other detail. When the breaker opens at high currents, an arc forms. There’s a special component inside to help dissipate the arc. There’s a lot going on in that little box.

Of course, nowadays there are fancier breakers that detect arcing as well as shorts and overloads. We’ve also seen at least one other breaker teardown in the past.

32 thoughts on “How Does A Circuit Breaker Break?

  1. “First of all, there is no minimum length for monetizing videos. But for optimal revenue sakes, make your video is at least 8 minutes, because you can place extra mid-role ads in the video.”

    There’s your answer.

    1. Thermal trip elements are not precision instruments. There is variability between units, and they are sensitive to ambient temperature. If you can’t find a standard nameplate rating that is close enough, a circuit breaker is not what you want.

      If you are looking for a precise trip current value, look elsewhere. Or look at breakers with digital trip mechanisms.

      I’m curious why you would want to have a custom trip rating for a breaker.

        1. Fair enough. I think you’ll be disappointed though. If you’re looking to tinker, look at bimetallic overload relays. Very similar type of trip mechanism, but the characteristics are designed to protect motors from overheating. Typically you have a selection of heaters, which have various resistances to produce heat in the relay based on the load current, and the bimetallic trip has some adjustment in the amount of deflection required to trip it.

          The bimetallic type are very traditional motor protection, but they’re becoming less common. Their electronic replacements have more advanced features as mentioned in some other replies.

          1. I should also clarify: overload relays do not open like a breaker. They open control contacts, which will typically be wired in the motor controller to cause a contractor to drop out and de-energize the motor.

    2. You might want to have a look at “motor starters” or “circuit breakers for motors”.
      They usually have a tiny wheel where you can adjust the tripping-curve to your specific motor.

      Just checked, you’ll find them from Siemens and ABB but I’m sure, there will be other brands, too.

      1. These are not breakers but thermal protections, in short they integrate the current to check thermal condition of the motor, so they can pass *very* big surge as long as they are short.
        They also do phase protection for most.

        1. That’s right, those are not breakers (MCBs) per se.
          But for most types I know there are magnetic switches inside that trigger when a short circuit situation happens as well as a bimetall switch that protects from thermal overload.

          So you can use them instead of an MCB but you usually won’t because motor protectors are much more expensive than MCBs.

          If [abjq] wants to adjust the thermal trip curve these might be useful, depending on [abjq]’s application.
          Maybe have a look at the 3RV1 or 3RV2 series from siemens. I’m not connected to this company but I used them in several industrial applications instead of MCBs.

          1. Those (3RV1 etc) are technically manual motor starters or motor starter protectors. It’s basically a breaker that has its thermal trip curve tailored for motor protection. Some are adjustable thermal trip, and they do have an instantaneous magnetic trip for short circuit protection.

  2. The arrangement of the plates to dampen the arcing makes me think that orientation matters, and that this type of breaker should always be installed vertically. Yet, I’ve seen mini-breakers of this type installed on its side too, and that strikes me as odd, to say the least.

    1. I guess for most users here orientation doesn’t matter.
      It does though when using circuit breakers and other switches for high current (speaking of ratet current of 32A and more). “High currents” may be more than 500A when working with mains voltage an in the event of a short circuit.

      You’ll find details on orientation in the datasheets and also safety clearances as with higher currents the arcs are not staying inside the device.

  3. As an American we only sang about mending a fuse. The Beatles When I’m 64. I’ve never seen one till now. We used Edison socket fuses. A penny was the cheapest way to burn your house down.

    1. I came to ask about that. Are those for real? Fuse holders where the fuse is just a wire? I’ve only ever used fuses where the wire was enclosed in some sort of casing and the whole thing gets replaced such as Edison socket fuses for houses, glass tubes for devices and plastic chips for cars.

      I know about the penny thing but I would think with fuses where one just replaces a wire people would be even more tempted to just insert any random wire. Scary!

  4. Big Clive has an excellent video coming up about a circuit breaker which…doesn’t. There’s no coil, no bimetallic strip, just a wire connecting the input and output live terminals. Under no circumstances will it actually break the circuit, not unless you physically break the device. Your house might get burned down, but, hey, the contractor saved a few pennies!

  5. If I didn’t have decades of experience knowing that circuit breakers work well and I just watched this video then given the choice I’d probably go with fuses. After all, greater complexity means more points of failure. Fuses are so simple and circuit breakers are full of moving pieces.

    1. fuses are more likely to expierence unscheduled-rapid-self-dissassembly
      this usually results in tiny bits of shattered material at VERY high velocity.

      because fuses are often made of brittle materials (like glass and such) they may penetrate your skin. this could cause you to die from massive blood-loss; even with small size cuts.

      a fuse is simply a weak-point, your actually hoping for a wire-fire, one of the type where it goes out on it’s own before the thick cabling can heat up, but this isnt always the case.

      circuit-breakers are reliable, most of the issues are from “sticking”, which can be avoided by switching them off (with little to no load) and back on once in a while. because the HOT buss-bar is not mechanically attached to the unit, any rapid-self-dissassembly helps to seperate the two current-carrying connections (with great speed); the load is a wire bent-back by the forces and the source stays bolted to the back (as the shattered plastic gets ejected with great-force).

      in a fuse, the two ARC-CONTACT-POINTS STAY PERFECTLY-STILL and do not fly apart, meaning the arc is more likely to continue and burn something.

      purposely switching off a breaker while under heavy-load is a huge personal-safety risk requiring a safety suit and other PPE. or at least stand BESIDE and FACE AWAY while doing the actual switch-off/on, i use my non dominant hand so if its vapourised, i can still sign the doctor’s bill.

      these are all on youtube.

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