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I have a 240VAC, 200A switch. It has very tight space requirements, and has to be sealed - so no external air flow. I need to minimize heat as much as possible. I have redesigned bus bars, spacing, etc. One of the things I did to save space was change from a two-pole circuit breaker to two fuses. What surprised me is that most of the heat being generated is coming from the fuse itself. In fact, when I measured the resistance, it was 10x what I expected. All the other changes I made to reduce heat only resulted in small incremental changes, not enough to get in the range I need.

I am working on making heat sinks connected to the fuse - but I am unsure that will be enough. If I were to get a lower resistance fuse somehow, it would make the biggest impact.

I have searched, and my results are unrelated to what I'm searching for. I can't seem to find any studies or papers on the resistance of fuses. Manufacturer spec sheets do not have resistance as part of their spec. I am contacting manufacturers for this information and additional suggestions - but that is a long process.

Are there any studies / guides, etc. on the impedance of fuses? Have there been comparisons of different brands or classes?

I am using a class T fuse. I need fast acting, 100 kA interrupting, current limiting. Based on the circuit requirements, I could use class J, CC, R, or T. However, I have no idea if any of these classes may have different thermal performance or resistance. Nothing in their specifications necessarily points to how much resistance they may have. I do know that the higher the voltage rating, the more resistance they tend to have, so I am sticking to 300 V rated fuses vs. 600 V rated. Is there anything else I can use as an indication of how much resistance a fuse may have (short of getting a sample of every single one and testing with a milliohm meter)?

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  • \$\begingroup\$ how does the fuse connect with the circuit? At 200 A, it probably bolts down? The contacts will generate signnifcant heat due to the contact resistance. \$\endgroup\$
    – Neil_UK
    Commented May 23, 2022 at 19:24
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    \$\begingroup\$ @Neil_UK yes, it is bolted on to bus bars for the connection. I was expecting that to be the biggest source of heat. It certainly has been significant, but I've done lots of things to maximize contact area and reduce the resistance at the contact points and it's only made a slight difference. When I take the fuses out for testing (replacing with straight bus bars - don't worry, test supply is protected) it makes a much more significant impact. \$\endgroup\$
    – Trashman
    Commented May 23, 2022 at 19:36
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    \$\begingroup\$ @user_1818839 yes, this is USA. 240V, split bus, two "hot" 120V circuits and one neutral - so I need two fuses. We do have loads that use both "hot" legs (180 degrees out of phase) so it is 240 VAC for those loads. \$\endgroup\$
    – Trashman
    Commented May 23, 2022 at 19:38
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    \$\begingroup\$ This document from one manufacturer says resistance data is available on request: littelfuse.com/~/media/electronics_technical/application_notes/… \$\endgroup\$
    – DamienWontContributeToAITheft
    Commented May 23, 2022 at 19:51
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    \$\begingroup\$ This may be inherent in the nature of a fuse. They work by melting the metal conductor after all. And any conductor that can handle 200 A is bound to be hefty, so a large amount of heat would be required to melt it. Is it possible that some type of mini circuit breaker would work better? \$\endgroup\$
    – user57037
    Commented May 23, 2022 at 19:56

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Fuse losses are often specified rather than resistance for larger fuses.

For example this 200A fuse (with your required 100kA break capacity) has a 32W loss at rated RMS current, so the resistance is around 800\$\mu\Omega\$.

That is indeed a fair bit of power.

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    \$\begingroup\$ Interesting. Weird how spec sheets for other lines of fuses, including from the same manufacturer that I'm looking at, don't include that watt loss figure. Also, the 200A fuse, if you look closely, is only 20kA. 100 kA starts at 250A, which has 50W loss at rated RMS. Also comes out to 800 microOhms - so in theory would also be 32W at 200A. \$\endgroup\$
    – Trashman
    Commented May 24, 2022 at 22:15
  • \$\begingroup\$ Yes, interesting how the power loss goes up with the RMS rating generally. BTW, you might want to check fuses used in aircraft, they might have better characteristics. I recall some 200A-ish fuses but not sure if they meet your break requirements. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented May 24, 2022 at 22:51

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