Discussion in 'General Technical Questions and Answers' started by KM6LYW, Dec 21, 2018.
It's just the I (current) that blows the fuse, not I squared R.
I squared R is watts.
"R" is the resistance of the fuse wire. If it is zero, the fuse will never blow. Sorry I was not clear.
Exactly. Fuses are rated to blow at a certain current, but they rely on an implicit resistance across the fusing material. Current by itself doesn't generate heat until it passes through a resistance and it's that heat (I^2R) that melts the fusing link.
I was trying to explain this:
"i guess thats why fuses are rated in amps and not watts?"
Throwing in fusible element resistance and the formula for watts doesn't help with an answer, it kind of adds to the confusion.
Watts are heat. Heat melting the fuse element is what causes it to open.
Sorry. I was trying to make the point that fuses are rated in amps because circuit voltage has no role in determining when a (fusable-link) will blow.
I don't dispute that.
Let's hear your explanation why fuses are not rated in watts.
Because watts would not apply. A fuse rated at 8 amps could supply around 100 watts to a circuit before it blew, if the circuit was 12v. But it could supply 1000 watts to a circuit, if that voltage was 120v.
The current through the fuse, in conjunction with the internal resistance of the fuse, is what causes it to blow. The applied voltage is really of no concern, as long as it is below the rated value of the fuse. The current itself doesn't blow the fuse, the fuse has to have a resistance. It is that interaction between current and resistance, not current and voltage, that causes this to happen. The only formula that applies is I squared R, which converts to watts. Watts are heat. Heat blows the fuse. But a fuse is never rated for watts.
I agree with what you said, but the link you provided could have been better. From that link:
"However, when an excessive amount of current flows through the fuse wire, the heating effect of current causes the fuse wire to melt. This is because the fuse wire is chosen such that it has a low melting point. "
Yes, it could be better. But the only way a current can generate heat is by flowing through a resistance, that is why the I squared R is used, to represent the heat generated.