Electronic – What *fuses* an electric fuse

fusesprotection

I build a quite lot of projects to learn electrical engineering, and since I was first introduced to what's called a fuse around 2 years ago and I always put one in my high voltage 220V mains for protection, but when I worked with precious LiPo batteries, I wanted to add a fuse for protection.

The problem is that what's written in my pack of fuses is "220V 250mA" here comes the question, what make a the fuse fuse or pop-open circuit? Is it the current rating or the voltage rating?

As I am a student who loves experiments, I went and did an experiment, where I used a 7.4V 1AH LiPo battery which was rated at 20C and hooked a 1 ohm load, theoriticaly I should be able to draw 7.4 A but due to resistance of wiring I was able to draw around 4 A (used high watt resistor). When I hooked up the fuse (220V 250mA) in series with the load, I was expecting it to pop open, but it took around 1~2 seconds till it popped up, although the rating was at 250mA 0.25A which is way less than the 4 A drawn by my dummy load.

I wanted to know if is this normal for an electric fuse, or my fuse is very bad manufactured? As a side note my fuse works perfectly fine in 220V circuits

I done a bit of research if there is anything called low voltage fuse, found some webpages,but I couldn't understand anything

One of the pages I saw is: here

My fuse type is the normal glass fuse.
enter image description here

this is not my image, source : here

In short what pops a fuse (sorry but is pop the right word to use in such thing?)
Is it the over specified voltage or over specified current or it's the power through it? And in the other side what kind of fuses used in low voltage application like fuses in digital multimeter? Is it the same fuse I have?

Thanks in advance.

Best Answer

Fuses blow due to heat, and heat is due to current via \$I^2R\$. Therefore, what blows a fuse is the current.

However, a fuse needs to blow with a large enough gap that the voltage behind current cannot overcome the gap. A fuse with a 1mm gap isn't going to be able to interrupt a current with 10,000V behind it (it might make a massive, long-lived arc that eventually extinguishes at best, and at worst the 10,000V will cause sparks across the gap or melt and weld the gap.) That's what determines the voltage rating of the fuse. The voltage rating is irrelevant while the fuse is not blown.

High voltage fuses do have different features such as sand or other materials that melts or buries the contacts when the fuse blows to reduce arcing or explosions.

As you found, fuse blow times are inexact and vary. If you look at the datasheets of some fast-blow and slow-blow fuses, you will find charts of how long it takes for the fuse to blow at a rated current. At 100% of the rated current, the fuse could take a very long time to blow (could run for minutes on end). At 200% or 500% of the rated current, it blows much faster. Don't expect a fuse to behave ideally and instantly blow within 1ms as soon as you exceed the rated current.

I cannot explain why it took 1-2 seconds to blow at 1600% overload though.