However, the fuse designer also has to consider what happens after a fuse blows: the melted ends of the once-continuous wire will be separated by an air gap, with full supply voltage between the ends. If the fuse isn’t made long enough on a high-voltage circuit, a spark may be able to jump from one of the melted wire ends to the other, completing the circuit again:

Electrical systems must meet applicable code requirements including those for overcurrent protection before electric utilities are allowed to provide electric power to a facility. What is Quality Overcurrent Protection? Synergistic Research is not merely a manufacturer but a thought leader that sets a high bar in the audio industry, emphasizing meticulous craftsmanship and the scientific method in its product design process. Every product undergoes rigorous, objective and subjective testing before it reaches the consumer, ensuring unparalleled quality and performance. against this short circuit. Thus, the fan may be saved. Many times electronic components can receive more voltage than rated for and still

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It should be noted that some automobiles use inexpensive devices known as fusible links for overcurrent protection in the battery charging circuit, due to the expense of a properly-rated fuse and holder. A fusible link is a primitive fuse, being nothing more than a short piece of rubber-insulated wire designed to melt open in the event of overcurrent, with no hard sheathing of any kind. Such crude and potentially dangerous devices are never used in industry or even residential power use, mainly due to the greater voltage and current levels encountered. As far as this author is concerned, their application even in automotive circuits is questionable. For example, if it shows T20A 250V or S20A 250V then it is a slow blow fuse. T=Timed and S=Slow (depends what the manufacturer decided to use) The two designations mean the same thing. The I 2t rating is related to the amount of energy let through by the fuse element when it clears the electrical fault. This term is normally used in short circuit conditions and the values are used to perform co-ordination studies in electrical networks. I 2t parameters are provided by charts in manufacturer data sheets for each fuse family. For coordination of fuse operation with upstream or downstream devices, both melting I 2t and clearing I 2t are specified. The melting I 2t is proportional to the amount of energy required to begin melting the fuse element. The clearing I 2t is proportional to the total energy let through by the fuse when clearing a fault. The energy is mainly dependent on current and time for fuses as well as the available fault level and system voltage. Since the I 2t rating of the fuse is proportional to the energy it lets through, it is a measure of the thermal damage from the heat and magnetic forces that will be produced by a fault end. In general, the time required for a fuse to blow can be given I2t where I is the current, and t is the time. A slow blow fuse requires higher I2t than fast-blow fuses to blow up.

A miniature time-delay 250 V fuse that will interrupt a 0.3 A current at after 100 s, or a 15 A current in 0.1 s. 32mm (1 1/4") long. Where several fuses are connected in series at the various levels of a power distribution system, it is desirable to blow (clear) only the fuse (or other overcurrent device) electrically closest to the fault. This process is called "coordination" or "discrimination" and may require the time-current characteristics of two fuses to be plotted on a common current basis. Fuses are selected so that the minor branch fuse disconnects its circuit well before the supplying, major fuse starts to melt. In this way, only the faulty circuit is interrupted with minimal disturbance to other circuits fed by a common supplying fuse. But what does a fuse do? For simplicity, a fuse uses a wire or connection that gets burned and disconnects the circuit during an overcurrent situation. The wire or the connection is done in such a way that it requires specified current to pass through it for a specified duration of time. Therefore, if a fuse is rated for 1A, the current needs to cross the current limit of 1A to blow up the fuse but the overcurrent duration of time is dependent on what type of fuse it is. There are many types of fuse and we have already discussed them, among them, the slow blow fuse (a.k.a. time delay fuse) and fast-acting fuse (a.k.a fast blow fuse) are the most popular choice for electronic circuits like SMPS circuits or other power circuits. In this article, we will discuss Fast Blow Fuse vs Time Delay Fuse.The third method employs the 'M' effect. In the 1930s Prof. A.W.Metcalf (hence the 'M') researched a phenomenon where the tin alloy used to solder the ends of the fuse seemed to affect the time to blow, reducing it in a strange way. He found that a spot (the 'M' spot) of solder on a silver wire element did not affect the short circuit performance, but it did reduce the time to blow on a sustained lower current. In this case, at the lower temperature of the wire, the solder diffused into and alloyed with the silver to create a region of high resistance in the spot, which would glow red hot, with the wire rupturing next to it. This, with suitably chosen alloys, nicely gives the characteristic needed for a surge resistant fuse. A problem with this type of fuse is that occasional currents just above the rated value may cause some unwanted diffusion to occur, altering the fuse characteristics without visible change. A fuse is nothing more than a short length of wire designed to melt and separate in the event of excessive current. Fuses are always connected in series with the component(s) to be protected from overcurrent, so that when the fuse blows (opens) it will open the entire circuit and stop current through the component(s). A fuse connected in one branch of a parallel circuit, of course, would not affect current through any of the other branches. A classic example of a slow-blow fuse application is in electric motor protection, where inrush currents of up to ten times normal operating current are commonly experienced every time the motor is started from a dead stop. If fast-blowing fuses were to be used in an application like this, the motor could never get started because the normal inrush current levels would blow the fuse(s) immediately! The design of a slow-blow fuse is such that the fuse element has more mass (but no more ampacity) than an equivalent fast-blow fuse, meaning that it will heat up slower (but to the same ultimate temperature) for any given amount of current. Some fuses allow a special purpose micro switch or relay unit to be fixed to the fuse body. When the fuse element blows, the indicating pin extends to activate the micro switch or relay, which, in turn, triggers an event.