Fuses and Circuit Breakers, Removable and Non-?
February 22, 2021 9:07 PM   Subscribe

Could someone describe to me in simple language the difference between fuses and circuit breakers? If it matters, when I think of fuses and circuit breakers, I think primarily of automotive fuses and circuit breakers (due to an early summer job).
posted by metabaroque to Home & Garden (11 answers total) 2 users marked this as a favorite
Best answer: Both fuses and circuit breakers are electrical devices that protect an electrical circuit from an overcurrent condition that could cause dangerous overheating of the wire conductors. Both devices are connected in series with the load so that all of the current in the circuit passes through the fuse or circuit breaker. The device opens the circuit to interrupt the current in an overload condition.

A fuse has a small wire or metal strip inside like the filament of a light bulb. The filament is carefully designed to be of such a size that when too much current flow through it, it gets hot enough to fuse (melt) which causes the filament to come apart and stop the flow of current. A fuse is a sacrificial or one-time device. When the filament breaks, you have to remove the burned out fuse and replace it with a new one, just like a burned out light bulb.

A circuit breaker is a switch in which current flows between two contacts in the switch. The switch is similar to the light switch on your wall. A circuit breaker has a spring and devices inside that when they sense too much current, triggers the spring to open the switch, sort of like a mousetrap. A circuit breaker can also be switched manually just like a wall switch.

The major difference between a fuse and a circuit breaker is that a fuse can only trigger once and then has to be replaced like a light bulb. A circuit breaker can be switched back on either automatically or manually like a light switch after the circuit fault is fixed.
posted by JackFlash at 9:40 PM on February 22, 2021 [15 favorites]

Fuse: a piece of metal engineered to melt, or fuse, into two pieces at a specific energy level. Once triggered, must be replaced.
Circuit breaker: a piece of metal (or two pieces of two metals) engineered to bend, or flex, into a new position at a specific energy level. After triggering, it can be mechanically restored to the functioning position.
Both are safety devices designed to break an overloaded circuit, powered by the overload itself.
One is a little glass doodad, one is an engineered component.
Colloquially, it's still common for people to refer to a modern circuit breaker panel as a 'fuse box', its early 20th century equivalent.
on preview, what jackflash said
posted by bartleby at 9:43 PM on February 22, 2021 [4 favorites]

At a simple level, both fuses and circuit breakers are trying to do the same thing, but they work a little differently. The purpose of both fuses and circuit breakers is to disconnect an electrical circuit when too much current is flowing through it (overcurrent). If a circuit is carrying more current than it's designed for, it could cause damage to electrical devices and/or a fire. The purpose of fuses and circuit breakers is a safety device that performs the action "if there's too much current, disconnect the circuit."

A fuse is designed to break when there's too much current. It's essentially a wire, usually wrapped in glass or plastic, that's designed to break if too much current goes through it. If that happens, the wire melts and the fuse needs to be replaced. The idea is that it's better for the fuse, which is carefully designed to break if there's a specific amount of current, to break than it is for the overcurrent to cause damage or start a fire.

A circuit breaker, on the other hand, is a type of switch just like a light switch. Normally, the switch is turned on. If too much current is flowing through the circuit, the switch turns off. Unlike a fuse, nothing physically breaks when this happens, so the switch can be turned back on again after the problem is solved.

Some circuit breakers can do other fancier things like switch themselves off if they detect other types of potentially dangerous problems with the circuit besides overcurrent, but you don't need to care about that if you don't want to.

Both fuses and circuit breakers will have a label on them showing how much current they consider to be "too much." An electrician needs to make sure to use the right rating for a particular circuit, and anyone who replaces a fuse needs to make sure to use the same rating as the broken one.

Analogy time. Imagine we have a bathtub we want to fill. We turn on the tap and, because of a problem with the plumbing, 5,000 gallons of water/minute comes pouring out, resulting in massive damage. We don't want that to happen again, so we install a fuse into the tap. The fuse says it's designed so that if the flow is more than, say, 15 gallons of water/minute, it breaks and doesn't let any more water come out at all, preventing a flood. If we want to try to turn the water back on again after we fix the plumbing, we need to replace the fuse with a new one. Or we can put in a circuit breaker instead. The circuit breaker is turned on and lets the water flow through, but if more than 15 gallons of water/minute starts to flow, the switch turns off, also preventing a flood. If we want to try to turn the water back on again, we need to turn the switch back on.
posted by zachlipton at 10:03 PM on February 22, 2021 [3 favorites]

Those were some of the best replies to any question I've seen so far, and I'm reluctant to try to add to them.
However, Jackflash's statement on what the fuse protects is worth emphasizing.
A circuit breaker doesn't care what's plugged into the outlet. If your toaster shorts out and suddenly conducts a hundred times as much electricity as it should, the wires in your walls can easily get hot enough to light fires. This is bad anywhere, but especially so inside a wall where you probably won't spot a fire until it's too late. The breaker protects the wires, and your toaster doesn't matter at all.
For this reason a fuse or breaker is sized to the load the wiring can take. If you run 20 amps of appliances on a circuit with a 15 amp breaker, it'll turn off the current.
That's an irritation (I live in a house that was wired in 1912) but it's wonderful, because once or twice a week a circuit breaker doesn't let my house burn down.
It's tempting to replace a fuse with a washer or a circuit breaker with a bigger one, but you're literally playing with fire.
posted by AugustusCrunch at 11:19 PM on February 22, 2021 [2 favorites]

People have done a great job of explaining how fuses and breakers work, but if you're not sure what you're dealing with, here are some visual aids: If a building has circuit breakers they will be in a breaker box, which some people will sometimes refer to as a "fuse box." But a fuse box doesn't have switches, it has fuses. The most common kind of fuse/fuse box I know is the kind that screws in, like little lightbulb bases with no lightbulb attached, but there are other kinds and I'm sure it varies regionally and with the age of the building.
posted by mskyle at 3:59 AM on February 23, 2021

Something that may be throwing you off, but may also be useful:
In home wiring, fuses are almost universally of the Edison type - they screw into what looks like a light bulb socket. Most breakers use a different mounting/distribution arrangement.

HOWEVER. After replacing fuses over and over at the old house I lived in (turns out some wiring in a wall was shorting, but it was blowing the fuse too fast for me to even do diagnostics), I discovered a product that is reset-able - a circuit breaker - but is made to screw into fuse (Edison screw-type) sockets. These little buggers saved me a lot of pain and suffering - no more stocking up on fuses, only to blow the last one when you're setting up for a party and have to run to the hardware store that closes in five minutes.

Reviewing your question, I notice that you're coming from an automotive setting. In cars, the fuses (or in older cars, fusible links) burn out, and must be replaced. Same as houses. Circuit breakers in cars, though, sometimes have an extra twist - they're self-resetting. That may be where the confusion is coming from.

(interesting side note: blinkers in older cars were controlled by a special electromechanical device that was *made* to break, like a circuit breaker, at a low amperage, then reset. Over and over. That's why old cars made the clicky-clicky noise when the blinkers or flashers were on - a little wire passing power to the blinker bulbs, breaking the circuit, then resetting and passing power again. Nowadays, it's computer controlled, but the skeuomorphism of the clicky noise remains...)
posted by notsnot at 5:21 AM on February 23, 2021 [1 favorite]

FWIW, there are circuit breakers for automotive use, but they are USUALLY seen on larger vehicles... trucks or buses, not regular cars. Think of them as reset-able fuses.

These for example, will fit in most places that call for a regular ATC blade-style fuses, but are resetable.
posted by kschang at 6:33 AM on February 23, 2021

Second interesting sidenote: as @notsnot have mentioned, the older blinker boxes (4-way switches?) make click-click noises. One way you know one of the bulbs "broke" was it stopped blinking. That's because the circuit is no longer complete, so it is no longer triggering the click-click circuits inside. Some of them will let you put in different wattage bulbs to control the blink rate.
posted by kschang at 6:37 AM on February 23, 2021

Many automotive turn-signal circuits were wired to use the resistance of the bulb as part of the delay, and a broken bulb would make the relay click faster as an audible indication you had a problem (since you can't typically see the turn signal bulb from inside the car). Some cars still do this even if the bulb is now controlled by solid-state means.

Something else to note about circuit breakers is that they can also do more than just trip for an overcurrent situation. There are Ground Fault Circuit Interrupter (GFCI) breakers that will trip when leakage current between hot and ground is detected. They're similar to the ones you use in a bathroom or kitchen but they can service an entire circuit. There are also Arc Fault Circuit Interrupters (AFCI) that can detect abnormal arcing between lines or a broken conductor inside an appliance.
posted by JoeZydeco at 6:47 AM on February 23, 2021 [1 favorite]

Best answer: You may be wondering why anyone would still use fuses instead of circuit breakers in building power applications if circuit breakers have so many advantages over fuses.

1: The installed price can be lower at higher amperages.
2: Fuses are 100% reliable and always fail open. Circuit breakers are mechanical devices and for a variety of reasons can fail to operate.
3: Fuses aren't user resettable so it prevents people from repeatedly closing a shorted circuit.
4: Fuses are capable of interrupting larger fault currents [a serious short can put 10s of 1000s of amps on the circuit before clearing, breakers can themselves be damaged by this current and fail to open]
5: This is getting out in the weeds but as an electrician I often have to coordinate fusing where several fuses/circuit breakers protect a load and I want those devices to trip in a particular order so that a toaster shorting out doesn't blow the fuse on the pole/transformer. There are way more options to accomplish this with fuses than circuit breakers.
posted by Mitheral at 2:33 PM on February 23, 2021 [1 favorite]

Response by poster: Although I've highlighted two in particular, my thanks to everyone who responded. Additional thanks to Mitheral who answered an additional question that my mind posed late at night but I had not yet posted in the thread.
posted by metabaroque at 6:30 PM on February 23, 2021

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