Friday, September 28, 2007

Video of a fuse in action

A short video, made by myself and a colleague (Alfred) in NIE for our Physics course. The video shows an ammeter indicating the current passing through the fuse and how the fuse reacts to the current.

We chose this video because very few people has actually SEEN a fuse being blown. We studied it in school and yet most of my students have never seen a real fuse in their entire life. Hopefully, this can help them in understand how a fuse works, and what is its purpose in an electrical circuit.

To cut the long story short, when the current passing through the fuse exceeds a certain value, the heat dissipated by the fuse wire (given by Power = I^2R) is sufficient to melt the fuse wire itself. This cause the fuse wire to break and hence opening the circuit, protecting it from the excessive current.

Can you guess the rating of the fuse? To see answer, use your cursor and highlight the space below this:

Perhaps your guess is 1.5 A, as the video suggests. However, it was a fuse with a 0.6A rating. Apparently, things are not as simple as what's stated in Physics textbook. In engineering, there's a buffer of about 2-2.5 times of the rated current. This is to allow for surges or spikes in the current which is considered 'normal' and prevent excessive and annoying fuse breaking. Not to mention there's a time factor involved in breaking the circuit. Remember, you are actually melting the fuse wire. Does ice turn to water suddenly, or gradually (depending on temperature difference)?

Still, when designing a circuit that has a working current of, say 0.8A, it is still ok to use a 1A fuse. In other words, stick to the simple explanation given by your O level textbook and that is to use the fuse with a current rating that is nearest and greater than the current in the circuit you wish to protect.