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While there are exceptions the majority of cars use fuses instead of circuit breakers to protect electronics and I'm wondering why circuit breakers are not more widely used. Is it practicality, cost, regulations or a combination of those factors?

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    $$$. And weather/conditions. Hard to find a breaker that holds up in automotive conditions, or trips the same at -20F vs 160F from the car baking in the desert sun all day. Sep 21 at 4:05
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    I don't know if I agree with the breakers holding up in weather conditions, light airplanes have a DC electrical system and use breakers instead of fuses. The breakers have to hold up in a very wide variety of conditions and they've always worked for me.
    – GdD
    Sep 21 at 7:42
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    Completely true @ChrisH, I'm just saying reliability isn't an insurmountable issue. I'm not drawing a parallel between planes and cars, I need to be able to reset my avionics in flight as it could impact safety, if my radio goes out in my car it's an annoyance.
    – GdD
    Sep 21 at 8:56
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    @GdD indeed, and even total electric failure on a fast road (not a fuse - mechanic didn't do up the main earth strap properly) is safer than in the air. One thing I wondered is how many breakers you might have in a fairly simple light aircraft
    – Chris H
    Sep 21 at 9:03
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    @Harper-ReinstateMonica "Hard to find a breaker that [...] trips the same at -20F vs 160F..." - To be fair, fuses also don't trip the same at those temperatures; they fuse by melting a wire, and that depends on ambient temperature as well as current. This document has some interesting information including graphs.
    – marcelm
    Sep 21 at 9:16

6 Answers 6

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(1) Fuses are cheap. Really cheap. You can get a variety pack of 100 for $8 or so. Circuit breakers for 12V systems are available, but are not cheap (more like $10 each). My car has 40 or 50 circuits, so that is a large chunk of change.

(2) Fuses are small and can be put into tight spaces, much smaller than you can get your fingers into. Look at your main fuse box and consider just how large it would get with breakers, even the push-button ones.

(3) Breakers are great for circuits you want to turn off, or quickly turn back on. But neither of those really comes into play on a car - 12V won't kill you, unlike your house wiring.

(4) Mostly your car just runs. In 45 years of driving I've never replaced a fuse.

(5) Circuit breakers have mechanical parts that need to move when needed, and not move when not needed. Cars experience shock and vibration through their lifetime, making the simple fuse a more reliable way to protect a circuit.

(6) from @Criggie, particularly in the engine bay there is oil, dust, water, etc. Fuses don't notice, many breakers will not like that. You can get marine grade breakers that are sealed, but that adds cost.

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    5b. add Moisture and Oil contamination too - fuses don't have moving parts and the contacts can be fairly well sealed.
    – Criggie
    Sep 21 at 0:02
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    Strange that you've never had to change a fuse. I probably average one every 2-3 years, but I do tend to run quite old vehicles. Most recently something must have shorted the 12V/lighter sockets as I was only using one of them for a USB charger which still works. I have had them go n other circuits though. Stuck electric windows or their switches can blow fuses due to the stall current in the motor, for example
    – Chris H
    Sep 21 at 8:49
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    @ChrisH - perhaps luck of the draw. On average I keep a vehicle about 15 years. I do always carry a set of replacements, so perhaps that deters the ones in use from failing...
    – Jon Custer
    Sep 21 at 12:45
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    @ChrisH: Adding to anecdata, across the six cars I've owned in my lifetime (including the cars driven primarily by my wife), I've never replaced a single fuse. Five of those cars (two '90 Geo Prizms, an '00 Maxima, and an '06 and '10 Prius) were still in use by myself or a close family member beyond 10 years of use (and the '14 Leaf is eight years old now). I believe I remember one car (an Oldsmobile?) of my grandfather's needed a fuse replacement once back in the '80s, aside from that, I've never personally known of a car that needed one. Never carried replacements either. knocks on wood Sep 22 at 1:50
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    4. Lucky? Last year the water in my screen-wash bottle froze because I'd forgotten to reinforce the mixture for cold weather, and trying to pump ice took out the screen-washer fuse. Much preferable to taking out the screen-washer.
    – nigel222
    Sep 22 at 15:43
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I don't have a definitive answer, but believe it is a combination of factors and boils down to the following:

  • Cost: While both do the job of protecting circuits, a fuse runs you cents per unit, while breakers can cost you dollars per unit. When you consider in a modern vehicle, there are probably 30 to 50 circuits which need protection, that creates a large cost overall to try and run breakers.
  • Practicality: Size is a large consideration here. Fuses have a very small footprint when you compare them to breakers. Engineers trying to create fuse panels in vehicles have very limited space in which to put a fuse panel. If you were to create a breaker panel, you'd have to allocate a lot more space to this endeavor.

Another factor (not sure how or if it fits in with the above) to consider is, as long as the owner of a vehicle doesn't start putzing around and changing things in the vehicle, the circuit stays in tact and the fuse is never truly needed. Engineers design enough overhead into a circuit the systems of the vehicle will most likely never overload one, and therefore the fuse will never need to be replaced. The fuse is there on the off chance something happens which was not accounted for and can stop a chain reaction of events which might otherwise have serious consequences for the vehicle and all aboard. The fuse can do this at a much lower cost than the breaker, so this fits into both a cost and practicality reasoning.

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    @JonCuster - Sorry if I stepped on your toes on this one ... the internet is slow here at work and it took me a LONG time to get this answer in. I've been working on it for some time now, lol. Sep 20 at 13:25
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    Not a problem at all!
    – Jon Custer
    Sep 20 at 13:27
  • Based on other comments, there are some reasonable scenarios where fuses blow (moving things getting jammed) so maybe the point about fuses never being needed is a bit optimistic.
    – user253751
    Sep 22 at 19:23
  • @user253751 - I'm not sure if you were saying that to me or just to say it. If it was to me (which I can assume because it was under my answer), I'm not quite sure why. If you read my entire answer, I didn't say it would never be needed, but rather, it would never be needed under normal circumstances. Usually the things not under normal circumstances are when the car is in an accident or the owner starts putting things in which weren't accounted for in the original design & overloads a circuit. I stated "As long as the circuit stays intact" meaning, it remains unchanged from original design. Sep 22 at 20:39
  • ... surely a washer tank that freezes in winter is part of the original design
    – user253751
    Sep 22 at 20:43
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In addition to the other good answers:

You'll see self-resetting circuit breakers in heavy-truck applications, which is a far more abusive environment, and prone to intermittent faults (esp. in trailer circuits).

From my personal experience, most blown fuses are from trailer wiring problems, and the rest are from people plugging things into the "power ports" ("cigarette lighters" for us geezers). Very rare to see other fuses blow besides a short circuit from a collision.

So as the others have answered, in a car the cost/size/complexity outweigh any advantages.

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With one notable exception, automotive fuses almost never blow in the absence of a catastrophic failure such as a crash which causes wiring to be severed. One of the primary purposes of the fuses is to prevent the wiring from overheating and starting a fire if that happens. In such a case, having to replace fuses would be the least of one's worries.

The notable exception is with the "cigarette lighter" socket, whose current delivery capacity varies between vehicles. If one plugs in a 120V inverter which is designed to be able to convert more current than a car's socket is designed for, and plugs a high-draw AC device into it, that will blow the fuse, but there's no nice way for a user to know in advance how much of a load could be placed on the inverter without blowing the fuse. A circuit breaker there would IMHO be an improvement over the fuse.

Additionally, many cars have (or at least used to have) a self-resetting breaker driving the windshield wiper motor. This was intended not only to protect the wiring, but also to protect the motor in case the wipers are jammed up (e.g. with ice). When the motor is stalled, it will draw much more current, and generate much more heat, than when it is running; the breaker would be chosen so that it would pass the motor's stall current for a short period of time, but eventually trip if the wipers are left on while stalled.

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    Of course, someone who designs things that plug into a 12V socket should know what they are fused for (mine are 20A fuses), and the device should not exceed that. But then there is Amazon... There is a reason I have an aux battery system and some pretty hefty fuses on it...
    – Jon Custer
    Sep 22 at 13:14
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    The owners manual ought to say what the current limit is. If it doesn't state it explicitly, it does implicitly: go to the diagram of the fusebox and look for the amperage of the cigar-lighter fuse!
    – nigel222
    Sep 22 at 15:47
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    @JonCuster: Some cars' utility sockets are fused for 20A, some for 15A, and some for 10A. Further, many inverters are designed so they can accept power from either a lighter socket or pair of alligator/crocodile clips, lugs, or other such means. In retrospect, it would have been useful to either have a standard for adding pegs, notches, etc. to utility sockets to indicate how much current they could supply, or better yet have a set of standards for adapters and connectors so that if one plugged a suitable adapter into a car's socket, it would only work with devices that were suitable, or...
    – supercat
    Sep 22 at 15:48
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    ...were capable of adjusting their current draw. For example, an adapter suitable for use in a 15A socket could have a connector that would accept a two-pin 5A, 10A, or 15A plug, or a three-pin auto-sensing plug, but would not accept a two-pin 20A plug. On the other hand, I've also long thought that the connector found on PCs and such (I forget the name) should have been designed so that cables with 120V mains plugs would have a notch in one spot, those with 220V mains plugs would have a notch in the other, devices that needed one voltage or the other would have a projection...
    – supercat
    Sep 22 at 15:52
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    ...in their socket to only accept the right cable, those with a voltage switch would put it in the socket so that a plug could only be inserted if the switch was set correctly, and auto-sensing devices could accept both plugs interchangeably. But of course that didn't happen either.
    – supercat
    Sep 22 at 15:53
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Fuses and circuit breakers are two kinds of overcurrent protection; both protect from two kinds of events:

  • Short circuit, in which a positive conductor touches the common/ground. This causes instant, nearly infinite current.
  • Overload, in which too many devices each operate normally but simultaneously. This causes current that is too high for safety, though much less than a short circuit.

Short circuits happen only if something breaks, which is rare. Replacing a fuse is a small fraction of the cost of diagnosis and repair of an electrical problem. The small convenience of breakers can’t justify their extra cost.

Overload protection generally isn’t a concern in vehicles, because you rarely connect new electrical devices, especially the high-current devices like the starter and resistance heaters. Again, fuses are cheap and replacement is rare. Overload is more of a concern in buildings: nothing stops you from plugging in a hair dryer, toaster oven, and coffee maker into the same circuit, and this happens frequently enough for the convenience of breakers to justify the cost.

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I'll add one factor which the other already good answers didn't mention in detail: weight.

Modern cars have several dozens of fuses, each one weighing a couple of grams. Imagine replacing them with even the smallest automatic breaker you find, for example this:

enter image description here

and look at the specs:

enter image description here

It weighs ten times more! Not mentioning it's much bigger and the endurance against the environmental conditions is nowhere suitable for a car. And improving its tolerance against an adverse environment will probably add still more weight.

Bottom line: you'll end up adding kilograms of weight for no more than a very small convenience (resetting a breaker instead to change a blown fuse, event which in normal car operation is quite rare). In exchange you will have a heavier and more expensive car.

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