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Technical 12v fusible link

Discussion in 'The Hokey Ass Message Board' started by Katuna, Nov 12, 2016.

  1. Fuses do NOT blow (go open) quickly, if you hit a 20 amp fuse with 30 amps it could take it several minutes to open based on temperature and other factors. For it to blow in 3 seconds it might take 60 amps. If you have a fuse that blows as soon as you plug in a new one it is being hit with a direct dead short. Fuses heat up as the current increases and cool when load goes away, the metal fuse link inside also expands and contracts. Just like bending a piece of wire over and over will cause it to break a fuse will fail over time just from fatigue. If you look at an old failed fuse and there is a crack in the metal part that fuse probably didn't fail from overcurrent, just metal fatigue. If the blown part is melted THEN it was overcurrent. This link has a lot of engineering speak and lots of charts but it explains the role of a fuse and it is not as simple as a 10 amp fuse will blow over 10 amps, it might be a lot over 10 amps for a long time. Wire must be sized accordingly.
    Here is another article that is a little simpler to understand.
    FishFry and G-son like this.
  2. G-son
    Joined: Dec 19, 2012
    Posts: 1,130

    from Sweden

    Simple! In a perfect world no fuse would ever blow, because there would never be an overload, and as long as that is the situation a fuse is completely, absolutely useless. Everything works perfectly fine without fuses - until there is an overload. With a well built system overloads should be very rare, ideally none should happen through the lifetime of a car.

    But when there is an overload, the brown stuff can really hit the fan - on the other hand, the old cars had more metal parts, more open spaces around wiring (around everything really) and less flammable materials, so even if they had a length of wire glowing red and burning off its insulation chances are nothing more happened. Fix the short, rewire what was BBQed, and get back on the road.
  3. 2OLD2FAST likes this.
  4. Fusible links were designed to allow the harness designer to substitute an undersize wire while still offering a form of overcurrent protection during the overload. They differ from fuses and circuit breakers by their 'current slope' which is typically of a longer time duration. The problem for us is there is no documentation on just what that slope is. I have no doubt that the OEMs test to determine that slope for their particular application (and probably adjust it by link length, although that's a guess) but all we get is a generic recommendation and zero current slope info.

    All this can be avoided by simply using wire rated for the anticipated load, don't try to 'cheat' it smaller.

    As to fusible links being a 'bottleneck', that's a non-issue. They will add a miniscule amount of additional voltage drop but will have pretty much zero effect on current flow. As an example, let's say you have a #10 circuit using a #14 link for protection. The link is 6" long, #14 wire has a resistance of about 2.5 Ohms per thousand feet. That gives you a link resistance of .00125 Ohm (well below the measurement threshold of home instruments). Assume the overload is 100%, or 60 amps. A 30A fuse will blow in roughly 20 seconds at that current level, too little time for the anticipated load, so a fusible link is used to gain more time. Voltage drop across the link will be Vd = 60 x .00125 or .075 volt. Assuming a 3' length of #10, the drop there will .21 volt, total ends up being a mere .285 volt.
  5. 2OLD2FAST
    Joined: Feb 3, 2010
    Posts: 4,343

    from illinois

    Sooo , being that we're working generally on old cars (or supposed to be:rolleyes: ), someone is making a bunch of money convincing us we need all this modern electrical " stuff" !
  6. Not that so much as they're selling us stuff that needs this other stuff to correct the problem caused by the first stuff.

    On a properly designed, sized, and fused harness there should no damn reason for a fusible link, ever....
    bobss396, FishFry and 2OLD2FAST like this.
  7. Thank you, sir.

    FishFry and 2OLD2FAST like this.
  8. Blues4U
    Joined: Oct 1, 2015
    Posts: 7,322

    from So Cal

    That's what I'm thinkin...
    2OLD2FAST likes this.
  9. FishFry
    Joined: Oct 27, 2022
    Posts: 260


    Can somebody please write this in stone and hang it over every workshop door?

    Thanks, Frank
    firstinsteele and bobss396 like this.
  10. Budget36
    Joined: Nov 29, 2014
    Posts: 10,848


    If you consider the HAMB will last for awhile, Steve has a pretty good write up on wiring. Several pages of q and a, etc.
    bobss396 likes this.
  11. I'm learning a lot here, keep it coming guys :cool::cool:
  12. Without getting TOO modern and OT fuseable links were there to cover OEM (too damn many acronym's) main harness potential issues. If a car crashed and that caused a big short circuit the fuselink was there to protect the wiring harness, same thing if the harness went through a hole in the metal somewhere and rubbed through. They had to think about safety in these rare instances and plan for the "what if". Today there are many better and cleaner ways to protect the harness than a fuseable link, many different large current fuses and circuit breakers, even solid state devices. I usually use a couple of Maxi Fuses when building my own harness and sometimes run a main power disconnect for safety. Yes, I shouldn't ever need to use them but makes me feel better because "what if" does sometime happen and it doesn't cost very much to design it into the car. Kinda like a dual master cylinder!
  13. Elcohaulic
    Joined: Dec 27, 2017
    Posts: 2,213


    Don't forget the car's D.C. electricial system is run by a battery. If there was a major short the battery's lead terminals would melt in seconds and the circuit would open.

    Thats why electricians run aluminum wires in a residental service drop. We could run copper but aluminum has three big things going for it, it's light, its less expensive and it has a built in fail safe.

    I don't miss doing those 500 MCM, parallel drop 800 amp services! We used benders to bend the wire. In the winter it was even worse.
    Budget36 likes this.
  14. FishFry
    Joined: Oct 27, 2022
    Posts: 260


    After reading all this, I tried to remember when I had actually a blown fuse on a car.
    I can't remember - I had fuses going bad over the years, so I had to replace them, but never actually popped one because of a short somewhere in the system. And it's not like I'm overmaintaining my cars.

    I think all those "what ifs" are actually very rare events and fringe/edge cases, given the number of cars that never develop a fire (as long as you don't make a major mistake in your wiring).

    2OLD2FAST likes this.

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