Fuses vs breakers?

[saumon]

Just wondering: is there's a practical reason why fuses are still in use these days on boats instead of resettable breakers, other than their low cost? On mine, there's one breaker from the battery going to the main bus, but from there it's all fuses...

They've get rid of them a long time ago in house construction...

 

[foodfisher]

Re: Fuses vs breakers?

Easily replaced. Can't run to the store for a bad breaker when on the water

 

[saumon]

Re: Fuses vs breakers?

Thanks, I thought they very seldom "go bad"...

 

[gm280]

Re: Fuses vs breakers?

Just wondering: is there's a practical reason why fuses are still in use these days on boats instead of resettable breakers, other than their low cost? On mine, there's one breaker from the battery going to the main bus, but from there it's all fuses...

They've get rid of them a long time ago in house construction...

There are pros and cons to each type circuit interrupter. Fuses are cheap and easily replaceable IF they are located in a convenient place to be easily accessible. But they do create some problems of their own too. I've seen fuses (the knife type) installed for so long that they get corrosion on their blades (connections) and become intermittent. But again if you remove them and insert then back a few times it cleans the connection off to make good contact again. Another drawback is you have to carry a few spares for every type and size you have in your fuse panel. Never replace a 5 amp fuse with a 10 amp fuse. That is asking for a problem or even a fire. So while fuses do a great job at very little cost, you have to keep them clean and also have spares for every type and size.

It would seem that because you have to keep spare fuses for that time they do blow, that circuit breakers would be the greatest thing going. Well not so fast. Circuit breakers do share similar problems with fuses. That being they too get corrosion on their metal connections but they also have the problem both on the outside and inside as well. So circuit breakers can be so finicky for that reason. And another thing most folks don?t know about circuit breakers is that fact that a 5 amp circuit breaker doesn?t trip at 5 amps. And depending on type and age, that same 5 amp breaker would not break until it is under near twice that rating. No circuit breaker will trip at the exact amperage they are listed at. Most will take a longer time to trip as well. They are not an instance circuit interrupter as fuses are. BUT, circuit breakers allow you to simply reset them...until they get finicky as well and refuse to reset.

So there you have it. Fuses or circuit breakers are a great thing to install in every circuit, especially on a boat. But you have to understand that they too need maintenance just like everything else on any boat to operate properly. JMHO!

 

[saumon]

Re: Fuses vs breakers?

Thanks gm280 for the great explanation!

 

[Silvertip]

Re: Fuses vs breakers?

Fuses are still used in cars because they are small and reliable. Used to be a car had 8 or 10 fuses. Even back then, if circuit breakers were used that would have required a fairly large panel. Today there are usually at least two fuse panels with a dozen or more in one panel and 20 or so in the underhood panel. A breaker panel for a car wold be the size of most trunk lids so size is a factor. Breakers also suffer from de-rating so even if they did trip the first time at exactly 5 amps in the above example, the next time it may trip at 4.5, then 4 then 3.5 until they cannot maintain the "normal" load any longer. But if the breaker trips repeatedly, it is either too small or there is a circuit issue.

 

[Grandad]

Re: Fuses vs breakers?

Also breakers will trip at different current values based on the ambient temperature. At cold temperatures, they take longer to trip. An investigation into a near disaster airline incident a few years ago, identified that the location chosen for a breaker panel was so cold when flying at altitude that the breaker that could have prevented a smouldering fire in the cockpit would not trip soon enough to save the wiring. Fuseholders may corrode, but so do breaker mounts. Additionally, a breaker can corrode internally, but a typical fuse won't. Anyone can change a fuse, but changing a damaged breaker needs a more knowledgeable tech. - Grandad

 

[saumon]

Re: Fuses vs breakers?

Oh man, you guys are right!

Had a chance to put the boat on the water today for the first time to check if everything's working before the fishing opener next week. Lauch at a nearby ramp 2 miles from home; engine start on the first crank but, oops, nothing work at the helm; no GPS, no SONAR, no VHF. I checked the fuses in the bus bar but they're all OK.

Bring the boat back home and start poking around with a voltmeter. Turn out that the main 30 amp breaker in the line that feed the bus bar is getting 12.56V at the input but only around 4V at the output. I tried resetting it a few times and cleaning the mounts, to no avail; it's bad, probably corroded inside. Replaced it and everything's fine!

 

[Don S]

Re: Fuses vs breakers?

Lauch at a nearby ramp 2 miles from home; engine start on the first crank but, oops, nothing work at the helm; no GPS, no SONAR, no VHF. I checked the fuses in the bus bar but they're all OK.

Why didn't you simply check your boat over at home to make sure it starts and all systems work before you stick it in the water and block the ramp to find that nothing works after setting all winter?

 

[saumon]

Re: Fuses vs breakers?

Everything (all electronics) is working two weeks ago in the garage but I disconnected the batteries to add a trolling motor 50amp breaker to the deep cycle battery, maybe that's when it went bad? (And, being on a weekday at a dock-less ramp, I was alone!)

 

[Grandad]

Re: Fuses vs breakers?

Here's a link to an interesting discussion by professionals on circuit breakers and their use in aircraft. - Grandad
The Deliberately Weak Link in the Electrical Chain

 

[saumon]

Re: Fuses vs breakers?

Great article; what I'll remember is that fuses protect devices while breakers are meant to protect wires.

 

[Silvertip]

Re: Fuses vs breakers?

Fuses cannot protect a device. If the fuse blows it is the device that caused it so it is already toast.

 

[Don S]

Re: Fuses vs breakers?

Great article; what I'll remember is that fuses protect devices while breakers are meant to protect wires.

No they both do the same thing.

Since many of the devices can be run off of larger fuses protecting the wires, they have smaller fuses near the unit. (Notice the fuse for your VHF. GPS, depth sounder etc.) What those smaller fuses do is keeps the smallers wiring in the unit from becoming flames because the main wiring fuse or breaker is too big. for the unit.

It's not uncommon for a 15A fuse to have a stereo, vhf, gps or whatever hooked to it, but each item has it's own fuse either built in or in the supplied harness for the unit.

 

[bruceb58]

Re: Fuses vs breakers?

The advantage of a fuse over a circuit breaker is size/cost/reliability. Fuses are cheaper, smaller and more reliable.

 

[glenn property of pam]

Re: Fuses vs breakers?

another advantage of a fuse is that it acts as a choking device for additional protection. Old glass fuses use to be marked for example 6/10 the fuse was rated at 6 amps and instant blows at 10. The fuse being inline turned in to a heater when the ampage trying to pass the wire was unable to squeeze through the wire so it acted as a choke restricting the ampage. On a breaker the ampage is not restricted so you will get a spike at max amps the battery can put out for a short period of time (in electrical terms a millisecond reaction is a long time and the diffrence between a item being repairable or unrepairable)

Of course silvertips has a good point in this throw away society we dont fix things anymore so it doesnt matter is one component fryed or if the whole thing fryed as long as it didnt catch fire its all good

Edit: as pointed out in private pm and a little research it turns out that english fuse ratings were diffrent from american fuse rating so 6/10 makes no sense on the american fuse chart. i believe that modern fuses are now the same. uk use to rate on instant and usa use to rate on short time which makes me wonder what fuses i put in the mgb when i restored it.

also pointed out was the word choke is not used in the same context as a current-limiting device

 

[Auger01]

Re: Fuses vs breakers?

Another issue with circuit breakers in thier ability to open the circuit in a dead short situation. By that I mean that if enough current flows though a breaker, the contacts can be welded shut, therby preventing it from doing its job. That is usually not a problem in the marine environment, but if you have a large battery bank, it is something to consider.

 

[bruceb58]

Re: Fuses vs breakers?

The fuse being inline turned in to a heater when the ampage trying to pass the wire was unable to squeeze through the wire so it acted as a choke restricting the ampage.

OMG...:facepalm: the BS is flying! The inductance of a fuse is probably in the nano henrys if you can actually even measure it.

 

[glenn property of pam]

Re: Fuses vs breakers?

OMG...:facepalm: the BS is flying! The inductance of a fuse is probably in the nano henrys if you can actually even measure it.

so a fuse is not a current-limiting device in a short circuit condition

 

[bruceb58]

Re: Fuses vs breakers?

so a fuse is not a current-limiting device in a short circuit condition

Its not an inductor.