Actual Fuse Current Rating - Page 1
Forum section: electronics / beginners

author: codingwithethanol
author_rank: Regular Contributor
timestamp: June 19, 2019, 01:40:57 pm
content: I bought some 1 amp fuses off of Ebay, and I hooked one up to see how accurate the rating was. To my surprise, it didn't blow anywhere near 1 amp. As a matter of fact, I used my PSU to step up the current 100mA at a time, and found that it blew at 1.8 amps! I did this with another and confirmed that it wasn't a fluke. Do you think they sent me 2 amp fuses by accident, or is there something i'm missing here? This could have been a drastic issue, had I actually depended on it to blow at its specified value in a circuit.

author: Cubdriver
author_rank: Unknown
timestamp: June 19, 2019, 01:54:31 pm
content: Fuses blow as a function of heat, and are designed to pass their rated current for some time without failure. As the current passing increases, the time required for the fuse to open will decrease. The time for opening will also depend upon the fuse characteristics; there are time lag or slow blow types that intentionally take longer to cope with things like starting surges, and others designed to be extra sensitive and open more quickly.This may help shed some light:https://www.allaboutcircuits.com/technical-articles/understanding-the-details-of-fuse-operation-and-implementation/-Pat

author: codingwithethanol
author_rank: Regular Contributor
timestamp: June 19, 2019, 02:25:28 pm
content: @Cubdriver thank you for the info

author: wraper
author_rank: Unknown
timestamp: June 19, 2019, 02:31:58 pm
content: Rated current is maximum current at which fuse must hold indefinitely. And current you should expect it to blow at within a few seconds generally is around 1.5-2x of rated current.

author: Nerull
author_rank: Frequent Contributor
timestamp: June 19, 2019, 06:45:21 pm
content: A 2 amp fuse blowing at 1.8A would be defective.

author: Gyro
author_rank: Super Contributor
timestamp: June 19, 2019, 08:36:52 pm
content: Examplefuse current / time curves (the actual curve depends on the fuse model and manufacturer)...http://www.specialtycontrolsystems.com/weidmuller/fuses/fuse_time_current_curves.php

author: Zero999
author_rank: Super Contributor
timestamp: June 20, 2019, 10:48:33 am
content: As a general rule of thumb, a fuse needs to pass roughly double its current rating to be guaranteed to blow.Another important rating is the breaking capacity, which is the maximum current a fuse can safely interrupt, without arcing over, blowing up or releasing flames. The breaking capacity of the fuse must be greater than the prospective short circuit current of the system, which is determined using Ohm's law: I = V/R, where V is the voltage and R is the resistance of the cable, battery/generator.

author: exe
author_rank: Unknown
timestamp: June 20, 2019, 12:43:09 pm
content: To make things even more complicated, there are "fast blow" and "slow blow" fuses... and fuses in-between.Anyway, a good vendor always provides a comprehensive datasheet with curves and deratings. Remember that you tested the fuse at room temperature, while it should also work at elevated temperature as well...So, fuses are not suitable for accurate current limiting, unfortunately.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.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 20, 2019, 01:49:07 pm
content: Quote from: codingwithethanol on June 19, 2019, 01:40:57 pmSounds like it's perfectly in typical spec. For accurate specs, you need to know the exact part number and look at the datasheet.At 1.8*nominal current, expect a few seconds of blow time, but it may vary from half a second to minutes.Had you left it running at, say 1.6A, it would haveeventuallyblown.At 1.0A, it's not supposed to blow, ever (or the rating is in hundreds/thousands of hours; this varies as well). This is how 99.9% of fuses are rated; for specifics, you just need the actual datasheet.Remember that fuses have quite some unit-to-unit variations. Really good datasheets show "thick" curve sets over the whole range; many datasheets just show the full curves for a "typical" unit, and maybe a few separate "guaranteed" numbers.For example, at 2*Inominal, a fuse may be specified to blow between 1 seconds and 3 seconds.Or, some fuse datasheet might outright forbid any continued use between 1*Inominal and 2*Inominal.Fuses are inaccurate, but extremely robust (when chosen right with enough current and voltage breaking capability, remember that DC ratings are different from AC ratings). But for really accurate overcurrent protection, you need to use active smart semiconductor devices (and then you need an additional traditional fuse to protect against a fire in case this accurate device fails; but such a fuse only protects against fire when the electronics have died already, and as such, don't need to be user-replaceable.)

author: Siwastaja
author_rank: Super Contributor
timestamp: June 20, 2019, 01:52:19 pm
content: Quote from: exe on June 20, 2019, 12:43:09 pmIt's indeed true that all these labels make things more complicated.One way around this is to always refer to the actual datasheet, and actual hard numbers/curves. Then, any generic labeling and generic wisdom is not needed.But you need to understand:* Breaking capability (current and voltage, at DC/AC depending which one you have). It's important your upstream source cannot supply more voltage and current than these limits, otherwise your fuse isn't safe and you need to pick a better suited fuse. For example, in a battery-powered system, you need to figure out the internal DC resistance of your battery, to figure out how much current may flow in a fault condition.* 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)* Unit variations to said curveOnce you have these actual numbers sorted out, it doesn't matter how they describe the fuse.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 20, 2019, 04:08:18 pm
content: Quote from: exe on June 20, 2019, 12:43:09 pmI 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.

author: exe
author_rank: Unknown
timestamp: June 21, 2019, 04:10:41 pm
content: Quote from: ejeffrey on June 20, 2019, 04:08:18 pmI 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 pmActually, 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.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 21, 2019, 05:41:40 pm
content: Quote from: Siwastaja on June 20, 2019, 01:52:19 pmIn 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.