The fuse has a resistance value. In fact, even the wire has resistance, but the resistance of the wire is very small (the nature of the conductor). In general circuit calculations, we usually do not count, but the resistance of the fuse is obviously larger than the resistance of the long wire. In this way, it can play a role in protecting the circuit.
The fuse is marked with "5A 250V", indicating that the maximum current allowed to pass by the fuse is 5A, which is called the rated current; the maximum voltage of 250V allowed to pass, called the rated voltage. If the current or voltage exceeds the rated value, the fuse will melt to break the circuit, which protects the electrical appliance (if there is no fuse, the electrical appliance may be burned out).
The fuse melting and principles are the law of Jiao Er. Law of Jiao Er: The heat generated by the current through the conductor is proportional to the square of the current, which is proportional to the resistance of the conductor, which is proportional to the time for power. Jiao Er Law Mathematics Express: Q = I²RT, the export formula is q = uIT and q = U²2/R × T. All the achievements in the current in the fuse generate heat, that is, all electrical energy is converted into internal energy. At this time, Q = W. Therefore, when i or U greater than the rated value, Q becomes larger, so that the fuse can not be dissipated in time and overheat, and finally melted.
In general, when the circuit current or voltage exceeds the rated value, the fuse will not be disconnected immediately, because the fuse allows the short -term excess working status through a larger current or voltage value. In other words, if you find that the circuit current or the voltage is abnormal, you still have the opportunity to correct errors, but for the accidental error (such as the short circuit of the trunk line), the insurance may not be able to get angry if the insurance is not too late. Pay attention to prevention, no chance to regret.