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The context or reason for this question is the following:

Reminder: the question is about the fuse, not about the TRIAC. The context may help to understand the question.

A 230V output is controlled by a DPAK2 TRIAC that is protected with a fuse of type F6.3A/250V 5x20mm glass type. The TRIAC can take 8A.

A test was made with a load (heater) of 1000W (about 4 Amps). The circuit worked fine for several hours. Then the heater was switched to 2000W (about 8 Amps). The fuse did not blow, but the TRIAC did. The reason for that is that at 8A, the TRIAC has to dissipate too much heat and there is no specific cooling. Possible 6A is also too much, but that is not the question.

Question

Thermal fuses are rated for speed with T, M or F, and are rated for a given current. This is in fact misleading. The fuse does not break above the given current - il requires a lot of time for a glass fuse rated at 4 Amp to blow at 4.5 Amp. In fact the rating T, M and F are "specified" for currents that are 10 (ten) times the nominal current.

According to my analysis of the F6.3A glass fuse, at 8A the fuse will blow after about 200s. Which leaves enough time for anything else to overheat and blow itself.

So this made we realise that glass fuses, ceramic fuses and other fuses exists possibly because they have "better" ratings with regards to overcurrent protection.
Are there fuses that would allow a 4Amp current to flow indefinitely, but blow pretty fast (say about one second) for a current slightly exceeding 4 Amps, for example 4.5 Amps.

A comparison between fuse technologies that are commonly available for PCBs would be nice but the principal information I am looking for is the existance (or not) of fuses that do what most people would expect them to do: blow in a short time as soon as the current exceeds the current rating.

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    \$\begingroup\$ It is my understanding that the rating of a fuse is the maximum amount of current it can carry indefinitely without blowing. Hence a 4A rated fuse would safely carry 4A forever and will blow at some threshold greater than 4A depending on (I^2)*t and the specific curve for said fuse. I do not expect a 4A fuse to ever blow at 4A or 4.000001A for that matter. They're really intended to disconnect in the event of heavy current flow (i.e. short circuit), not offer overload protection. I think that focusing on just the fuse is not the right angle to your problem. \$\endgroup\$ – Adam Lawrence Jun 17 at 17:25
  • \$\begingroup\$ I am not just focussing on the fuse, but it seems an interesting angle. Circuit breakers for the home seem are better current limiters (they use magnetics), so there may be solutions in between. \$\endgroup\$ – le_top Jun 17 at 17:41
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    \$\begingroup\$ For product safety matters, circuit breakers aren't allowed - CSA insists on fuses for motor circuits even if they have inherent protection. Products tested to EN60950 are tested on a branch circuit with a 20A breaker even if the fuse in the product is much smaller. As for not focusing on the fuse, your original question was pretty blunt: "Reminder: the question is about the fuse, not about the TRIAC." - glad you're considering other angles. \$\endgroup\$ – Adam Lawrence Jun 17 at 19:31
  • \$\begingroup\$ Yes, I mentionned explicitally that the question is about the fuse because I am interested in learning about fuses that I do not know about. Other approaches are about heatsinks, TRIACS with lower drop or TRIACS in parallel (not recommended), as well as switching to a relay. But using a TRIAC keeps the cost low if we can live with this limitation. * I mentionned circuit breakers to highlight that there is a technology with a more precice current limit. \$\endgroup\$ – le_top Jun 17 at 20:22
  • \$\begingroup\$ The question is (still) about the fuse options, but for readers of this question, you might be interested to know that there are a few overtemperature protected triacs out there. Which means that I am now "waiting" for the D2PAK version. \$\endgroup\$ – le_top Jun 19 at 19:23
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I am looking for is the existance (or not) of fuses that do what most people would expect them to do: blow in a short time as soon as the current exceeds the current rating.

Fuses don't protect a kettle from damage should it suddenly develop the urge to take more current. They protect infrastructure i.e. the wiring of your home and, the wiring of your home can take excessive currents for a pretty long time before getting too hot and starting a fire. The fuse stops the house burning down and isn't capable of protecting your kettle, hi-fi or TV should it develop a fault that causes it to take more current.

If your kettle, hi-fi or TV takes more current (than it should do) then it's because it's about to fail.

Fuses don't protect electrical items, they protect infra-structure (including the "usually-more-costly-to-repair" product infrastructure).

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  • \$\begingroup\$ I am talking about the 5x20mm fuses inside a device, not the fuses in your distribution box. Possibly the fuse inside the device is mandatory first to protect the infrastructure, but its also nice if it can protect from misuse. Apparently glass tube fuses break slightly faster than ceramic fuses for overcurrents. \$\endgroup\$ – le_top Jun 17 at 18:16
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    \$\begingroup\$ A fuse inside a product protects the product from failing in a way that catches something else on fire. It might even protect it to the extent that is repairable. Protecting it the the extent that all you need to do is replace the fuse is next to impossible. \$\endgroup\$ – Charles Cowie Jun 17 at 18:22
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    \$\begingroup\$ @le_top as Charles points out (correctly) the fuse is not to protect a faulty semiconductor but to stop the appliance catching fire. After all it won't use internal wires the same thickness as those in your wall so, it needs a fuse to stop those burning (or the PCB burning). \$\endgroup\$ – Andy aka Jun 17 at 18:31
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    \$\begingroup\$ I disagree that fuses are not for protecting devices. For example, many DMMs have fuses, for ohms mode and sometimes the high-current input, to protect the meter from overcurrent. Simply replace the blown fuse and you're back in business. (I was going to say more here but decided to turn it into an answer instead.) \$\endgroup\$ – Glenn Willen Jun 17 at 20:41
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    \$\begingroup\$ @le_top I can't think of one situation that I've ever come across where a fuse has saved a chip or transistor. Seriously, that is not what they are intended for. If there is a port on a piece of equipment that can be abused by a customer then there is only so much that can be done - if people short things out and there isn't adequate current limiting (not a fuse but a proper circuit) then failures will occur. Same with over-voltage if there is a chance of a situation occurring then protection circuits can be designed for this. \$\endgroup\$ – Andy aka Jun 18 at 9:40
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I think the big problem here is the narrow margin that you're asking for. If you want to reliably allow X amps, and rapidly reject Y amps, you need Y to be as large as possible compared with X. Fast action with a narrow margin is harder to engineer than slow action with a wide margin, and requires more precision. That all means it's going to cost more.

In searching, I found some "ultra-fast" fuses (from Littelfuse) marketed specifically for protecting semiconductor devices in power equipment. (Listed as "FF", "Very Fast-Acting".) These "specialty fuses" are a whopping $15 each (for a single-use consumable! I found a whitepaper noting that it might be cheaper to replace the protected component than the fuse.) Nevertheless, the 4A fuse is only rated to open after 1 second at 5 amps, marginally worse than what you asked for.

So I think (although I am not a fuse expert!) my bottom-line answer to your question is no, there is probably not a fuse that meets your needs (at a realistic price point.) You have set yourself an excessively difficult problem, I think. Instead, rethink your approach to work within the limitations of the components that are available.

One option, which I expect is the best one in practice: widen your margin. I see you already discussed this in a comment, a bit, so forgive me for repeating: that means lowering your current draw, or oversizing your triac, or adding a heatsink, or doing something to give you more headroom between the desired current and the upper limit.

Another option, which is tantalizing but I regret that I don't know enough to know how feasible it is, would a solid-state approach. Something to look into could be a "crowbar circuit", https://en.wikipedia.org/wiki/Crowbar_(circuit). This is a circuit which shorts across the power rails when a fault is detected, removing the current through the load and causing the fuse to blow immediately (thus the crowbar circuit itself does not have time to overheat.) They are normally used for overvoltage protection, and perhaps someone more experienced can set me straight, but it seems like an approach that could be workable here (suitably adapted). HOWEVER, it sounds like your circuit may be running directly from the mains voltage. In that case, I'm not sure whether such an approach is practical or advisable (or permitted, by whatever regulatory or certification authorities you may be subject to.)

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  • \$\begingroup\$ When I say "1 second" this is an order of magnitude, maybe I should write a few seconds. The standard fuses blow after 30minutes for twice the nominal current which is enormous. \$\endgroup\$ – le_top Jun 17 at 22:22
  • \$\begingroup\$ The crowbar approach is an interesting idea, but yes, this circuit is running on the mains voltage. There is actually one optically coupled TRIAC controlled from a microcontroller which triggers the high current triac. \$\endgroup\$ – le_top Jun 17 at 22:25
  • \$\begingroup\$ I am not very familiar with repair costs, but the BOM is about 8€, and closer to 13€ with the PCBA process included. \$\endgroup\$ – le_top Jun 17 at 22:29
  • \$\begingroup\$ An active approach to avoid that the fuse blows is definitively more complex as the output is on the mains, and the control is on 3V3. A PCB mounted miniature circuit breaker (not exactly a fuse) represents a cost, but could be a solution in certain cases. \$\endgroup\$ – le_top Jun 17 at 22:33
  • \$\begingroup\$ Here is an example of a listing for a circuit breaker for your specs: mouser.com/ProductDetail/Phoenix-Contact/… You will note that (1) it costs about $15 each, so it will more than double your BOM, and (2) it is significantly slower than the fuse I mentioned above; the trip curve in the datasheet says it will take 1 second to trip at 5x to 8x the rated current. If you allow 30 seconds, you're at 1.5-2x rated current, which is still 6-8 amps for your 4A nominal current rating (so not low enough, and probably also not fast enough.) \$\endgroup\$ – Glenn Willen Jun 17 at 22:56
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Thermal Overcurrent Circuit Breaker

A Thermal Overcurrent Circuit Breaker is faster than the classic glass fuse for overcurrents and breaks in less than 10 seconds for double the nominal current compared to more than 30 minutes with the glass fuse. Their disadvantage is their cost. My understanding of the datasheet suggest that the circuit breaker might not break below about 1.8 times the nominal current.

From the datasheet for the ETA 1410-L2/G1:

From the datasheet https://www.e-t-a.com/fileadmin/user_upload/Ordnerstruktur/pdf-Data/Products/Schutzschalter_Sicherungsautomaten/Thermisch/2_eng/D_1410-L2_G1_ENG.pdf

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