Fusible Links vs Maxi Fuse

[SquareRoot]

Fusible links are not fuses . . . they are current limiters intended to protect relatively fat wires that carry power to distant distribution points, usually fuse blocks where the power splits out to various appliances and sundry tasks around the vehicle. Fusible links are simply pieces of ordinary copper wire sheathed in an insulation that stands up to molten copper within while producing a minimum of low toxicity smoke. These insulations were developed for the electric locomotive industry where humans shared close proximity with at-risk wires carrying a LOT of current. The fusible link is sized at four wire-gauges smaller than the protected feeder. I.e. a 10AWG feeder should get a 14AWG fusible link at the BATTERY end of the feeder. In some cases where the feeder supplies a single load, no additional fusing is necessary for protection of the wires. But it may well be that a fuse-block at the downstream end of the feeder will carry fuses appropriate to the load of each circuit.

Fusible links are VERY slow acting compared with fueses as you might guess from their construction. They are NOT interchangeable. Most of the explanations on the 'net are simplistic and miss the point for incorporation of SLOW fuses in a power distribution system. Worked in aircraft for the last 50 years and while we don't use fusible link wire, we DO use very slow, robust current limiters that kinda look like a fuse . . . but they're not.

You will note that fusible links are 'rated' by AWG wire size, not AMPs like a fuse.

THANK YOU for posting this. I've been too lazy to say what you did but you're exactly right. Everybody wants to go with a fuse but they don't know why other than it's easier for them. Fuses and fusible links serve different purposes. If the OEMs thought fuses would suffice, they would have used them.

Rolls of different size fusible link is available at Napa.

Another point...you should NEVER fuse or limit the charging cable coming off the alternator. Like an uneducated *******, I put a 150amp maxifuse on mine and quickly burned it up. Don't do it. Just make sure the cable can support the amp rating of the alternator.

FWIW, I'm not an electrical engineer but I play one at work.

 

[Matt69olds]

I went junkyarding over the weekend, I noticed many full sized Chevy trucks have a 175 amp fuse mounted on the passenger side firewall area, just below the cowl and right above where the pass valve cover would be.

Unfortunately, the trucks had been picked over and cut up too much to determine what the fuse was protecting. The cable on it was comparable to a small car battery cable (probably 4 gauge?) so it’s obviously intended to carry a good amount of current.

I don’t think the OEM uses fusible links anymore.

 

[C10MixMaster]

THANK YOU for posting this. I've been too lazy to say what you did but you're exactly right. Everybody wants to go with a fuse but they don't know why other than it's easier for them. Fuses and fusible links serve different purposes. If the OEMs thought fuses would suffice, they would have used them.

Rolls of different size fusible link is available at Napa.

Another point...you should NEVER fuse or limit the charging cable coming off the alternator. Like an uneducated *******, I put a 150amp maxifuse on mine and quickly burned it up. Don't do it. Just make sure the cable can support the amp rating of the alternator.

FWIW, I'm not an electrical engineer but I play one at work.

OK when I was working on European cars I never saw a fusible link and I cant think of any modern American car that uses them, they all use fuses now to the best of my knowledge.

The OE configuration on my 77 going by wiring diagrams, there are 2 fusible links between the Alt charge wire and the battery, one at the junction block and one at the starter..

 

[AuroraGirl]

I rewired mine using fusible links but in retrospect I wish I would have done maxi fuses. I've never been a fan of fusible links..

fun fact, if you score parts at a junkyard, look for oldsmobiles and buicks from around 2000. My 1999 park avenue has a maxifuse block that contains 6 or so slots and wires underneath and has a cover that says "FUSES" so you could easily make it professional and add that type of protection to any desired circuits. how many do you possibly need on a square? 2-3? Wouldnt be too overkill, its like 6 inches long (haha funny) and has a stud for positive power that feeds all pins

 

[Nuckollsr]

Another point...you should NEVER fuse or limit the charging cable coming off the alternator. Like an uneducated *******, I put a 150amp maxifuse on mine and quickly burned it up. Don't do it. Just make sure the cable can support the amp rating of the alternator,

In airplanes, the alternator output (b-lead) is always protected. Alternator diode arrays are exceedingly reliable but risk for shorting is not zero. Hence a current limiter (or beefy breaker) is always situated closest to the battery end of that lead. 150A Maxifuse? Didn't know those were available so large but in any case, what ever protection you DO use in that path, it needs to be 'rated' for potentially catastrophic, battery sourced fault currents (hundreds of amps). This is where current limiters or fusible links are called for. My 87 R1500 has a 90A alternator wired with a 6AWG b-lead wire. I've got a 10AWG fusible link on the starter-contactor end of that lead. Note that an 80A ANL limiter (see https://tinyurl.com/4m6j4uet ) will carry 150 or so amps almost continuously but opens in a timely manner with hard fault levels. Real 'fuses' can be used but need to have a lot higher overhead 2-3x alternator current rating. Circuit breakers are often used on smaller aircraft and they are much slower than fuses but still need some head-room. I'd use 100A breaker on a 60-70 amp alternator.

It should be noted here that fuses melt at roughly calibrated current levels which are strongly influenced by ambient temperature. Further, some devices have inrush currents 2 to 5 times the running current. Motors and incandescent lights come to mind. It's recommended practice to operate a fuse at no more than 70 percent of rated value . . . less in high ambient temperatures. If powering a high inrush device, then de-rating to 50% or lower is called for. If a fuse powering your off-road flame throwers is loaded to the nominal 70% of rating, inrush currents over many turn-on events can thermally 'hammer' the fusible element and cause its trip current to drift DOWNWARD. I investigated an aircraft accident wherein a really nice airplane and 4 folks made an unplanned arrival with the earth because the system integrator failed to account for this characteristic common to fast fuses. Fortunately, nobody died but there were serious injuries and gross disassembly of the airplane. Check through the engineering literature on fuses downloadable from Bussman and/or Littlefuse websites.

 

[Raider L]

@dsteelejr,

That's what I did when I reworked my starter wiring when I was rebuilding my truck. I looked at how GM had routed the wire and they went from the harness at the firewall and went over the top of the engine and then back again under the engine and then down to the starter. I thought why did they take the long way around in doing that? So I took a cutter, figured out where I was going to put the block and just cut all the wires. I measured how long of wire I had cut out, and I had cut out five feet of excess wire.

Then from the block, which I ran a battery wire from the battery to the block and fixed all the power wires to a brass bar with screws of different sizes for the different size wires and attached each wire to it. The bar is fixed to a Phenolic block that is screwed to the firewall by reaching around through one of those holes in the valance , those openings under the windshield? The wiring to the stater I guided straight down the firewall where it's at a point straight with the starter, I fixed a insulated clamp down on the firewall there so I could direct the starter wires directly to the starter curving under the header tubes. I wrapped insulation along the starter wires where they went under the exhaust so it would not get hot. and then I turned the starter wires right to the starter. Now it real easuy to take the starter wires off when I need to without worry about them.

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If you look close you'll see a "S" and a "R". You connect a remote starter clip to the "S" and the other lead to the bolt where all the battery wires are. Turn your key to the "on" position and you can remotely start your truck.

 

[Raider L]

I make up all my own terminal ends which are crimped and soldered. In the photo, above the power block I made, is the original link in the ignition wiring. I left it. I may not need it but It's not hurting anything so I left it alone. Why not? The other side, to the left of the link is a fuse, new type I made up for the heater fan motor. I think it's a 10 amp.

 

[dsteelejr]

@Raider L

That looks like some nice work there. I have a remote start solenoid on the firewall. Comes in very handy at times. In the picture here, which was taken before I decided to redo the wiring, I was using the stud on the solenoid as a primary power junction, but now I’ve added a junction block bus. I’m using aftermarket exhaust manifolds so I still have that tube that runs behind the engine to keep the wires away from the manifolds, but it sounds like you had to get a little creative there and, to me, that’s part of the fun it. I appreciate you sharing what you’ve done. It starts caffeinating the hamster in my head and giving me new ideas of how to best make it all work.

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[Raider L]

@dsteelejr,

Thank you. And you are right it is the fun to see what your mind can come up with to make something you need work. I couldn't believe how GM had put all that wiring along the engine like they did when the starter is right down there! All they had to have done was drop down from where the starter wiring comes out of the harness wiring and drop down to the starter...right there. All I had to do was take a close look at what was done, and then change it to how I think it should be. Get that hampster in gear! And have fun doing it!

Just think of what you'd have to remove if all that starter wiring was on the engine if you wanted to take the engine out.

Good looking engine!

 

[Raider L]

@dsteelejr,

Man O Man!! Go to the "This weeks popular threads" and go to "Relays, Relays, Relays" and the guys have just what you are looking for!!