If I would like to use a piece of copper conductor as a 50A fuse, what size of conductor should I use? What is your advice for dimension?

It doesn't have to be 50A very accurately, the parameters I need the conductor to meet are:

  • it needs to conduct continuously 30A without getting red or overheating
  • it needs to conduct 50A for only 10 seconds without melting.
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    \$\begingroup\$ I'd be pretty worried about what will happen when the fuse blows. Exploding metal is a real danger at those currents. \$\endgroup\$ – ratchet freak Sep 26 '17 at 13:09
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    \$\begingroup\$ And what are you going to do with the molten copper? Have you researched the melting point of copper? Can you find any examples of copper being a suitable fuse material? \$\endgroup\$ – Transistor Sep 26 '17 at 13:09
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    \$\begingroup\$ I would advise to not do it and use a proper fuse instead. 150A fuse blowing is no fun \$\endgroup\$ – PlasmaHH Sep 26 '17 at 13:11
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    \$\begingroup\$ This is called an xy question. You have asserted that in order to do x you must do y, and want advice on how to do y. You should, rather, ask advice on doing x. You need a 150 amp fuse, so you've decided to make a copper fuse yourself. Don't. As PlasmaHH has advised, get a real fuse. At the least, the manufacturer has done all the pesky testing which you hope to avoid, and a real fuse will work the first time. \$\endgroup\$ – WhatRoughBeast Sep 26 '17 at 13:22
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    \$\begingroup\$ Big Clive on Youtube recently made a video about fuses: youtube.com/watch?v=kx35WN3uLis Pay special attention to the parts where he explains what can happen if you do not use a proper fuse. It is still your choice if you want the copper strip but if it burns down your lab/workplace, don't say we didn't warn you. Also, what voltage will the fuse have to break, if it arcs over you can be in deep trouble. \$\endgroup\$ – Bimpelrekkie Sep 26 '17 at 14:12

free advice

If you have no idea how to do thermal analysis on melting or exploding conductors and insulators and ohmic losses with adjacent thermal heating issues PTC effects on conductors and NTC effects on battery ESRs, then find an expert to define the whole project and not ask how does this look? ok?

Start with a complete environmental spec and performance curves.

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  • \$\begingroup\$ That is not applicable for my system and cells so that I have to use my own copper fuse like the first post of the question. \$\endgroup\$ – Ugur Baki Sep 26 '17 at 14:10
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    \$\begingroup\$ The "littelfuse MEGA" for 150 A is hilarious ;-) \$\endgroup\$ – Bimpelrekkie Sep 26 '17 at 14:10

Designing fuses is harder than it sounds. Lets start with a very rough approximation, the fusing current for 15 gauge copper wire is normally given as 140A. The cross section of that wire is 1.7mm^2, so that is about the cross section you need. If you are using 0.1mm thick foil, then you'll need it to be 17mm wide. Since that is wider than the 10mm ends, you'll probably want to go up to (e.g.) 0.4mm thick foil, and 4mm wide.

But that could fuse anywhere over quite a wide current range depending on:

  • heat flow from the fuse to the air, which depends on:
    • air temperature (only if it's hundreds of degrees - but it could be next to the high current wires)
    • orientation of the fuse, flat, vertical, on end...
    • humidity and air pressure
    • air flow, enhanced by a fan, or restricted by a case
  • heat flow to the bus bars, which depends on:
    • the size of the tabs at the ends
    • how the tabs are secured to the bus bar
    • the material and size of the bus bar
    • the temperature of the bus bar with high currents flowing through it
  • The copper alloy actually used, resistance and melting point
  • The details of the overload, such as how long it's been at 70A heating up.
  • Any damage to the (soft) copper fuse during installation.

In addition to fusing current you should also consider:

  • Arc extinguishing. The copper may establish an arc as it melts, which may continue to conduct until extinguished by a voltage reduction, strong air blast etc.
  • Where the molten copper goes. It will be expelled with some force. You may need a tough case around the fuse to stop it destroying something or killing someone.
  • Whether running for long periods at 70A will heat the copper enough to soften it, and let it bend or deform, increasing resistance.

You won't be able to calculate all these. So if you are set on making your own fuses, you need to commit to a series of tests. Start by getting a dozen fuses with cross sections between about 1.2mm^2 and 2.2mm^2, and set them up in a mock-up of your equipment with the right temperature, airflow etc. See where they fuse, pick the best size, and get more made. Test these in all the variations of conditions you expect to see in the field. Exactly how much testing you do will depend on what level of protection you need.

Given that 150A bolt down automotive fuses are not very expensive this kind of testing is probably only worth it if you plan on making lots of these.

  • \$\begingroup\$ Unfortunately,it is not applicable for my system and cell level fusing so I have to use my own fuse \$\endgroup\$ – Ugur Baki Sep 26 '17 at 14:27
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    \$\begingroup\$ And why is that? \$\endgroup\$ – Dampmaskin Sep 26 '17 at 14:30
  • \$\begingroup\$ Because, I am using fuse for generic of system. But I would like more safety so that I need cell level protection. My cells short circuit current is 150A . \$\endgroup\$ – Ugur Baki Sep 27 '17 at 5:24
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    \$\begingroup\$ "Safety" and "Roll Your Own Fuse" are mutually exclusive terms. \$\endgroup\$ – R Drast Sep 27 '17 at 12:54
  • \$\begingroup\$ DIY fuses of the type you're discussing are not uncommon on battery packs in the 5-10A range. The whole pack is protected by a rated, tested, proper fuse, and subsections of the pack are protected by fuseable sections of bus bar. Generally you only see this where thousands of battery packs are made and sold. Because you need the volume to make testing the fuseable sections worthwhile. If you are making enough battery packs to make it worthwhile, then you need to do some tests. Or subcontract someone to do them. \$\endgroup\$ – Jack B Sep 27 '17 at 13:28

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