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Actual Fuse Current Rating

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ejeffrey:

--- Quote from: exe on June 20, 2019, 12:43:09 pm ---PS I suggest buy HRC fuses, not cheap glass fuses. The price difference is not that big for 250V fuses, but current ratings are quite different.

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I don't think this is really good advice.  Get the appropriate fuse for the situation.  HRC fuses are important in multimeters or other applications where there is a chance of a hard short to a very high current/energy source, and only when the rest of the device is also engineered to be robust to that sort of fault. In an ordinary appliance a 5x20 glass fuse is perfectly acceptable and has the advantage that you can see if it is blown visually.  HRC fuses are also usually fast acting.  THis makes them not great for protecting transformers or motors that have large inrush.  HRC fuses also have higher resistance than an equivalently sized ordinary fuse.  That is usually not important, but can be in some applications.

Keep in mind that ceramic body fuse != HRC fuse.  There are also ceramic body fuses that are not HRC.  If you don't want to worry about the glass breaking but don't actually need HRC you can get ceramic body slow or fast acting fuses.  You still lose the ability to visually inspect the fuses, but that may be worth it.

exe:

--- Quote from: ejeffrey on June 20, 2019, 04:08:18 pm ---In an ordinary appliance a 5x20 glass fuse is perfectly acceptable and has the advantage that you can see if it is blown visually.

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I think I've seen a glass fuse that was blown, but looked like new. The wire inside broke too close to the cap, so visual inspection didn't help... But that's a special case. In case of short the inside evaporate with a visible residue on the inside. Anyway, the reason I stopped using them because they tend to explode. Or is it just too cheap fuses?

Another thing is, I don't know how much breaking current I need. Is 400A enough? Can my power grid deliver more one day? I use fuses in DIY projects.

Unfortunately, situations when I accidentally shorten mains do happen. Last time I used a screw driver to show on camera how my power supply works. I touched the wires and... nothing happened. I thought that I was lucky, but no, it's just the fuse acted so fast that there was no spark or something. The fuse was ordinary, btw, 1A 250V glass fuse.

As for resistance, I never measured it, but I think you are right. But it's not a portable devices, so, I can live with that :).


--- Quote from: ejeffrey on June 20, 2019, 04:08:18 pm ---Keep in mind that ceramic body fuse != HRC fuse.  There are also ceramic body fuses that are not HRC.  If you don't want to worry about the glass breaking but don't actually need HRC you can get ceramic body slow or fast acting fuses.  You still lose the ability to visually inspect the fuses, but that may be worth it.

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Actually, I don't look at the body, I look at breaking current. An ordinary fuse has like 400amps, what I call HRC is 1.5kA. I bought these, seems "slow blow" HRC to me: https://www.tme.eu/nl/en/details/zct-1a/fuses-5x20mm-time-lag/eska/522-717/ . But I didn't do any tests, so don't know how they perform.

schmitt trigger:

--- Quote from: Siwastaja on June 20, 2019, 01:52:19 pm ---
* Time-to-break curve (this is fairly easy to understand, just look at your overcurrent of interest and read out the time it takes to blow)


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In fuse parlance this is called the I2t (I-squared-t) curve.

An example from common 5X20mm medium acting cartridge fuses:

Kindly check on the attached file, the graph on page 2 labeled: "Average Time Current Curves." Which define the boundaries of the expected time to blow as a function of the ratio between the fault and rated currents.
You can see that when this ratio approaches unity, the blow time becomes infinite.

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