500mA fuses blow at 1A, am I testing them incorrectly?

[John_doe]

Greetings, electronics experts.
I usually buy my fuses at a local store but this time I couldn't find the fuses I was looking for anywhere locally, so I ordered them from eBay.
They are 5mm x 20mm glass fuses, the description says "Type: Quick Blow Glass Tube Fuses , Rated Current: 0.5A(Amp)".
I like to test everything that I buy from eBay (if I can) and so I connected one of these fuses in the simplest circuit I could: DC power supply with adjustable current connected to the fuse in series with a low resistance high power resistor.
I start cranking up the current and nothing happens when I get over 500mA. I even tried staying at 900mA for a few minutes and still nothing, but the second I get to around 1A the fuse blows. I tried connecting a DMM in series to see if maybe the PSU current sensing is bad but the DMM shows the same current as the PSU.
I tried 4 of these fuses and they all behaved the same.
The fuses are labeled "250V 0.5AL" on the metal part.

This is the first time I ever tested a fuse's current rating so before I go complaining to the seller I just wanted to make sure that I'm not doing something wrong, or maybe I have a misconception as to how fuses work.
So... Did I get 1A fuses mislabeled as 0.5A ?

[Psi]

That's 100% normal, it takes twice its rated current to immediately blow a fuse.

A 500mA fuse will handle 500mA pretty much 24/7, though it might blow after a year or two since you are running it right on the edge of its rating.
A 500mA fuse would blow at 750mA but it might take hours.


A 500mA fuse doesn't magically know when you reach 501mA. Its all based on heat and getting the fuse wire hot enough to melt and that takes time. The fuse manufacture doesn't know what ambient temp is or how well the fuse is heatsinked to its holder, so there has to be a massive amount of added error margin in fuse design.

The general rule of thumb in the industry is that a fuse blows at twice its rating.  So your 500mA fuse blowing right away when you apply 1A is normal

[mcinque]

I experienced similar results with a fuse kit purchased (cheap) to a fair. I strongly believe that that fuses came from a bad stock. Since I experienced wrong values also in capacitors, resistors purchased from fairs, I concluded that that market isn't reliable. Since that experience, I never purchased anymore any component from a not reliable source, expecially protection components.

I don't think you're testing incorrectly, they are probably cheap chinese junk coming from the vast ebay junk market.

[mcinque]

That's 100% normal, it takes twice its rated current to immediately blow a fuse.

According to a datasheet (http://www.littelfuse.com/products/fuses/cartridge-fuses/5x20mm-fuses.aspx), a Fast acting 500mA 5x20 should blow within a second when the current raises just 1,5x it's value, but I guess the time/current curve changes between manufacturers. Is this right?

[Benta]

+1 to what Psi said.

[Minor Tom]

I am no expert either, but I support Psi since I found this link:

http://practicalphysics.org/testing-fuses.html

Citation:
Students could try to determine whether the manufacturer's rating on the fuse is 'right'. The 'rating' is the current which the fuse can carry for some time (say several hours) but not for ever. If the current rises to twice the rating, the fuse should blow in less than one second.

[Psi]

yeah, its a bit of a hard thing for the manufacture to predict and specify, due to heatsinking and ambient temp.

1.5-2x is normal.

[John_doe]

Many thanks guys, now I know!

But now I have a related question: I would like my circuit to work with ~250mA regularly and if it reaches 300mA - 400mA for short times (less than 30 minutes) it's OK but at 500mA and above I'd like it to stop as soon as possible (less than 2 seconds).

What fuse should I buy (type and current rating)?

[Minor Tom]

Well, i suppose you will use a fuse for purposes its not intended for. But if it is a once-off project for your own use, you may actually meet your goals by buying fuses and just measuring if they will meet your needs. And live with occasional "unintentional" fuse blows, unreliable function on hot days .... And of course, have a stash of your "known to work" fuses handy as well as state your decisions in your project notes, you will have forgotten about it when you have a look in 10 years ...

As your measurements suggest, start your quest with a 250 mA fast fuse ....

[amyk]

Fuses are inherently not precision devices, because they're based on physical processes which are approximate. If you want precision overcurrent shutoff, you'll need active circuitry.

[Macbeth]

Fuses are inherently not precision devices, because they're based on physical processes which are approximate. If you want precision overcurrent shutoff, you'll need active circuitry.

Don't forget to protect the active circuitry with a fuse.

[Specmaster]

That's 100% normal, it takes twice its rated current to immediately blow a fuse.

A 500mA fuse will handle 500mA pretty much 24/7, though it might blow after a year or two since you are running it right on the edge of its rating.
A 500mA fuse would blow at 750mA but it might take hours.


A 500mA fuse doesn't magically know when you reach 501mA. Its all based on heat and getting the fuse wire hot enough to melt and that takes time. The fuse manufacture doesn't know what ambient temp is or how well the fuse is heatsinked to its holder, so there has to be a massive amount of added error margin in fuse design.

The general rule of thumb in the industry is that a fuse blows at twice its rating.  So your 500mA fuse blowing right away when you apply 1A is normal

You are spot on. Because fuses are so coarse and are capable of carrying far more than their rated current, MCBs were introduced to provide closer protection. This is a technical discussion in its own right and is far to deep for this medium. Most decent equipment designers will have rated their fuses so that at full power they were already creeping up to the point of blowing, ie the circuit at full chat might be taking 750ma and by rating the fuses at 500ma means that fuse will blow that much sooner as it only has to see a 33% increase in current to blow instantly instead of 100%.

From mobile device so predictive text might have struck again [emoji83]

[John_doe]

Thanks again, everyone.
I'll do some tests with 250mA fuses and if they are insufficient, I'll change my design.

I'm really glad I asked this question, because before I did I just assumed that the current rating of fuses is the current in which they are guaranteed to break (or within a reasonable margin, something like +20/-20 %).
I don't even know why I assumed that, it's been like that for as long as I can remember (30+ years).

[Brumby]

With a lack of background knowledge, it is not uncommon for a simplified understanding to be taken as a sound basis for everyday living.  I mean, it seems to work, doesn't it?

But the fact is that fuses are fickle beasts.  While you can rely on them acting, you can't be particularly certain as to when they will act.

This characteristic is demonstrated in household power protection in Australia - well, NSW at least.  Our standard power point circuits that were protected by 15A wire fuses could have those fuses replaced by 20A circuit breakers - with absolutely no wiring rework.  The reason being that the point where the wire fuses would blow was so varied that they added a safety margin in the gauge of the wiring.  However, with circuit breakers, the breaking current was much more predictable, so they allowed them to be higher rated, using up some of that safety margin - but without impacting safety.

[IanMacdonald]

House supply MCBs also require about 2x rated current for a rapid trip. Did some test years ago with a single turn secondary transformer and a clamp meter. They are more accurate than open wire fuses, though.

The logic behind this is that the cable takes time to heat up, and so can tolerate some overcurrent provided it's not maintained for too long. If you run a fuse or MCB at over the rated current it will eventually trip, though it make take some considerable time.

In low power electronics served by a small mains transformer, fuses are not particularly good at preventing overload/overhating because  the relatively high impedance of the transformer prevents enough current flowing for a clean blow. If the fuse were rated low enough to prevent the transformer burning out, it would cause nuisance blows. This is why most wall warts and the like use a thermal trip attached to the primary winding.

[westfw]

Aren't fuses designed to mostly protect against severe overloads?  What failure mode do you envision where your 400mA-max device would start drawing 600mA, that justifies blowing a fuse?

(Hmm.  Moving parts?  I once blew a fuse in an "automotive 12V adapter" by putting a load on an air pump.   If you use lubrication or overload a motor that should be running at 200mA and it starts running at 400mA instead, that seems like a reasonable time to blow a fuse before "extreme overload.")

[bson]

For such small values it doesn't terribly matter.  Just use a fuse that won't eventually fail under normal use.  I'd pay more serious attention to a 10A fuse...

[hamster_nz]

The other thing to watch out for is that if you have appreciable resistance (say a long run of thin wire between) the source (say a car battery) and the load (say a 12V/25W LED bulb) that the wire resistance might delay or even prevent the fuse from blowing, even with a dead short across the load.

In such cases you need to make sure that you use chunky enough wire that your fuse can do its job.

[james_s]

Since the fuse is sized to protect the wire, this shouldn't matter. If the fuse doesn't blow, the wire should not get hot enough to be a problem.

[EEVblog]

As others have said, it's actually quite a complex field if you want to get into it:

http://www.littelfuse.com/~/media/electronics/application_guides/littelfuse_fuseology_application_guide.pdf.pdf

[Zero999]

That's 100% normal, it takes twice its rated current to immediately blow a fuse.

A 500mA fuse will handle 500mA pretty much 24/7, though it might blow after a year or two since you are running it right on the edge of its rating.
A 500mA fuse would blow at 750mA but it might take hours.


A 500mA fuse doesn't magically know when you reach 501mA. Its all based on heat and getting the fuse wire hot enough to melt and that takes time. The fuse manufacture doesn't know what ambient temp is or how well the fuse is heatsinked to its holder, so there has to be a massive amount of added error margin in fuse design.

The general rule of thumb in the industry is that a fuse blows at twice its rating.  So your 500mA fuse blowing right away when you apply 1A is normal

You are spot on. Because fuses are so coarse and are capable of carrying far more than their rated current, MCBs were introduced to provide closer protection. This is a technical discussion in its own right and is far to deep for this medium. Most decent equipment designers will have rated their fuses so that at full power they were already creeping up to the point of blowing, ie the circuit at full chat might be taking 750ma and by rating the fuses at 500ma means that fuse will blow that much sooner as it only has to see a 33% increase in current to blow instantly instead of 100%.

From mobile device so predictive text might have struck again [emoji83]

The problem with that is the fuse might blow, even though there's no overload present, especially at higher temperatures and after the fuse has aged somewhat. The correct approach is to use a 750mA fuse and overrate the circuit, giving plenty of safety margin, but that will incur an additional cost.

[CJay]

There are several 'electronic' fuse projects out there which with a few components will give predictable and repeatable results but it's more complexity and could fail in a way that causes damage.

[james_s]

The fuse is generally there to prevent a fire, it's sized to protect the smallest wiring in the chain, it's not going to protect a semiconductor in most cases. If a semiconductor does fail shorted that's when you need the fuse, to prevent the power cord from catching fire. Ideally you want some sort of active overload protection built into the device, and then you want a fuse there to prevent a major fault from cascading into a catastrophe.

[Zero999]

There are several 'electronic' fuse projects out there which with a few components will give predictable and repeatable results but it's more complexity and could fail in a way that causes damage.

The correct solution is to use an electronic fuse, in addition to a real fuse, which will blow in the event it fails.

The same is true with a transformer powering an LM317 circuit: never rely on the regulator's thermal/overcurrent for protection against fire. Always use a real fuse for that!

[CJay]

There are several 'electronic' fuse projects out there which with a few components will give predictable and repeatable results but it's more complexity and could fail in a way that causes damage.

The correct solution is to use an electronic fuse, in addition to a real fuse, which will blow in the event it fails.

The same is true with a transformer powering an LM317 circuit: never rely on the regulator's thermal/overcurrent for protection against fire. Always use a real fuse for that!

It did occur to me on my drive home that I should have added that but you beat me to it