Fusible links, ABYC, and the NEC

[thinwater]

After another thread about a guy that wanted to use rigging wire as a MacGyver fuse, and another guy brought up that 14 AWG would work as a 100 amp fuse, I got to thinking about the fusible links we often find on alternators, starters, and cigarette lighters. Basically, a length of ordinary wire, but very importantly, with special insulation intended to contain the molten metal and allow it to fail safely, without a fire.


They are cheaper than high amp fuses, compact, and very corrosion resistant if properly installed. The down side seems to be possible fire risk of their own. But I could find very little discussion about applications (other than starters and alternators); acceptability, prohibitions, or sizing (generally 4 sizes smaller than the rated wire, low voltage systems only, generally direct-to-battery connections, and ratings over 50 amps). Nothing in a text search of ABYC E-11.


Discuss. Maybe something interesting will come of it.

[Jammer]

The automotive industry uses "fusible links" mainly as a hedge against shorts caused by collisions. Typically it is a short (6 inch) section of wire two or three sizes smaller, that is, 4-6 numerical gauges smaller, so a 10 gauge wire would be protected by a 14 or 16 gauge fusible link, and yes, they are in a fire-resistant sleeve.



Again, the idea is to prevent a fire in the event of a short, where overcurrent protection on fanout circuits is provided by fuses or breakers.


These fusible links don't provide any sort of nuanced overcurrent protection. A fuse will blow at 200% in 5 seconds where a fusible link will carry 200% more or less indefinitely. Either will blow quickly at, for example, 1000 amps. The goal with a fusible link is that in the event of a short, it will blow before the insulation on the balance of the circuit is hot enough to burn.


They are a cost saving measure, because for a few dollars more, there are various fuses now available, including the older maxi-fuse and newer J case fuses.

[smac999]

why? uses fuses or breakers.

[YANDINA]

Originally Posted by smac999

why? uses fuses or breakers.

For applications where routine electrical interruptions are expected the convenience of a fuse or breaker is justified.
For circuits that are only expected to be interrupted once under extreme circumstances the ability to restore is not needed and the cost is only a few percent of a fuse or circuit breaker.
We use fusible links in some of our Trollbridges and Combiners to avoid melting cables and the risk of fire in the event of a catastrophic failure.

[ItDepends]

Originally Posted by YANDINA

For applications where routine electrical interruptions are expected the convenience of a fuse or breaker is justified.
For circuits that are only expected to be interrupted once under extreme circumstances the ability to restore is not needed and the cost is only a few percent of a fuse or circuit breaker.
We use fusible links in some of our Trollbridges and Combiners to avoid melting cables and the risk of fire in the event of a catastrophic failure.

I don't know anybody who uses a fuse for "routine electrical interruptions," I am not at all sure what you mean there. On my boat blowing fuses are not considered "routine."

I can install a mega fuse complete in a proper ignition protected fuse holder for $40... not exactly a bank breaking cost.

There are many places I use fuses that would only ever blow in the event of a "catastrophic failure." Having the arc and molten metal of a blowing fuse reliably contained is well worth the cost. To me, anyway.

[thinwater]

Originally Posted by Jammer

The automotive industry uses "fusible links" mainly as a hedge against shorts caused by collisions....

Makes sense. You can put them in places (right on the starter, for example) where a fuse of adequate size would be hard to fit and exposed to terrible weathering. Placing the fuse up on the fender requires a good length of wire, which will get shorted in a crash, and adds expense.


Boats don't generally get wadded up into crumpled masses of short-causing steel, so no good applications.

[Sun and Moon]

I'd rather pay a few dollars more and put in a breaker that can be reset. The last thing I want is to be in the middle of a crisis and have to hunt down and install a new section of fusible link.

Jammer is probably right about the use in autos to protect during an accident. Not really an issue on a sailboat traveling 7 knots. Maybe on a power boat.

[Lake-Effect]

Originally Posted by thinwater

...I got to thinking about the fusible links we often find on alternators, starters, and cigarette lighters. Basically, a length of ordinary wire, but very importantly, with special insulation intended to contain the molten metal and allow it to fail safely, without a fire.

They are cheaper than high amp fuses, compact, and very corrosion resistant if properly installed. The down side seems to be possible fire risk of their own. But I could find very little discussion about applications (other than starters and alternators); acceptability, prohibitions, or sizing (generally 4 sizes smaller than the rated wire, low voltage systems only, generally direct-to-battery connections, and ratings over 50 amps). Nothing in a text search of ABYC E-11.

Discuss. Maybe something interesting will come of it.

I think the reasons that fusible links aren't so desirable on boats are:

• most boats are more flammable than vehicles and heavy equipment
• fusible links would be harder to replace than fuses, unless there's a nice block or terminal to connect them to, and at that point they're no cheaper than fuses.
• on a boat there's fewer chances for the sort of catastrophic wire injury that would cause a full-current short. Just the starter wiring, if ABYC is complied with.
• High current fuses can be more expensive than fusible links, but still inexpensive in boat terms, and fuses are faster to respond and less risky.
• DC panels and other load wiring are already protected by fuses or breakers (ABYC)

So I can't really think of where fusible links would improve safety on the average boat.

[ItDepends]

Originally Posted by Sun and Moon

I'd rather pay a few dollars more and put in a breaker that can be reset. The last thing I want is to be in the middle of a crisis and have to hunt down and install a new section of fusible link.

This logic is a bit flawed. I am not at all a fan of fusable links, so consider this a comment on breakers vs. proper fuses.

Remember, we are talking about high amperage circuits, >50 amps or so. In these sizes breakers start to get rather expensive, rather more than a "few dollars."

"In the middle of a crisis" the real time consuming part of the problem is FIXING the cause of of the blown fuse, not so much finding a new fuse. On my boat, all the spare fuses are in ONE place, so "hunting one down" isn't really an issue. In fact, in most cases, there is a spare fuse taped to the wire right next to the fuse holder.

High amp breakers are great for those things that might need to be shut off for one reason or another, and I have a few for things like that. Breakers also have use in protecting a motor from stall current, in lieu of internal thermal protection. Neither of those cases are really suitable for a fuse. For simply protecting DC wiring high amp rated breakers seem very much an extravagance. (AC systems are a bit of a different story, for a lot of reasons...)

On a boat with properly sized wires--and properly sized fuses--a blown fuse should be an incredibly rare event. The "quick reset" seems a benefit that will just never be realized. I have simply never had to replace a high amp rated fuse in 25 years of living on boats.

On the other hand, I have had to replace at least 3 breakers that aged out and began to trip well below their rated value. So my experience, breakers are more of a PITA than fuses...

[Jammer]

Originally Posted by ItDepends

On a boat with properly sized wires--and properly sized fuses--a blown fuse should be an incredibly rare event. The "quick reset" seems a benefit that will just never be realized. I have simply never had to replace a high amp rated fuse in 25 years of living on boats.

On the other hand, I have had to replace at least 3 breakers that aged out and began to trip well below their rated value. So my experience, breakers are more of a PITA than fuses...

After too many years of working on electrical and electronic systems, I have a few comments.


The great thing about fuses is that they work reliably. Most especially, they will blow when they are supposed to blow. Over time with a history of near-overloads they can blow for little reason but even so that is less a problem than it is with breakers. I have never, ever had a fuse fail to blow when it was supposed to, and I have never, ever had a fuseholder conduct electricity after the fuse was removed.


The great thing about breakers is that, like comparing lighters to matches, you can use them more than once. You will not be faced with (for example) the choice of running the windlass manually vs. applying seizing wire where the fuse used to be because you ran out of fuses. Historically many houses burned down from the use of pennies as a bypass for household fuses. With breakers you just push the button, and the circuit reconnects with overcurrent protection in place, and this enhances safety.


The reason this is great is that overcurrent events can occur for a wide variety of reasons. I still have a nice cheap Chinese copy of a Dometic fridge that has really good surge protection in it. So good that the fuse on the boat panel for the fridge blows when there's a thunderstorm. I went through 3 fuses a year on that until I sold the boat I had it on. This sort of thing is not unusual especially if your circuits are tightly fused (fuses rated close to the actual load). You can also blow fuses from a motor stall or undervoltage to an a/c refrigeration load or somebody plugging in the induction burner and the teakettle to the same circuit. It happens, and unless you carry a bunch of spares in all sizes the temptation will come to bypass the fuse or use a larger fuse.


The problem with breakers is that they fail. They get "weak" and trip early. And the damn things fail closed. I have personally witnessed a Square D QO series breaker -- the best available household breaker -- fail closed on a residential branch circuit. You could move the handle to the "off" position and it would click and it would still conduct. I replaced it without taking it apart to try to figure out what happened but I was a very confused electrician that day because I had all the branch breakers in the service panel shut off and there were still lights on in the house. You can see the potential hazard. This sort of thing is not as rare as we might like and occurs far more often than never, which is how often it happens with fuses. For another example google the "stab-lok breaker recall."



Anyway the smart way to approach this is to think about what problem you're trying to solve, what accident you're trying to prevent. As a rule fuses in cars and trucks should never blow, except maybe the one for the cigarette lighter if someone plugs in a 400 watt inverter, or the fuse for charging the trailer battery. The fuses are there mainly to improve collision safety and to deal with the very rare component failure or damaged wiring event. With modern wiring and modern wiring technique in cars that just doesn't happen. You'll see self-resetting breakers on cars for things like the wiper motor (which can stall due to ice) or the trailer wiring (which can stall due to poor wiring on the trailer, which is common), but not much else.



As a rule, I like fuses, I trust fuses, and I use them on retrofit projects. It's cheaper to carry a whole box of spares than it is to use breakers instead, and safer.