This site has been blocked by the network administrator. A miniature time-delay 250 V fuse standard handbook for electrical engineers pdf free download will interrupt a 0.
3 A current at after 100 s, or a 15 A current in 0. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby interrupting the current. Fuses have been used as essential safety devices from the early days of electrical engineering. Today there are thousands of different fuse designs which have specific current and voltage ratings, breaking capacity and response times, depending on the application. The time and current operating characteristics of fuses are chosen to provide adequate protection without needless interruption.
Wiring regulations usually define a maximum fuse current rating for particular circuits. A variety of wire or foil fusible elements were in use to protect telegraph cables and lighting installations as early as 1864. 1890 as part of his electric distribution system. The resistance of the element generates heat due to the current flow. The fuse ideally would carry its rated current indefinitely, and melt quickly on a small excess.
The element must not be damaged by minor harmless surges of current, and must not oxidize or change its behavior after possibly years of service. The fuse elements may be shaped to increase heating effect. In large fuses, current may be divided between multiple strips of metal. A dual-element fuse may contain a metal strip that melts instantly on a short-circuit, and also contain a low-melting solder joint that responds to long-term overload of low values compared to a short-circuit. Fuse elements may be supported by steel or nichrome wires, so that no strain is placed on the element, but a spring may be included to increase the speed of parting of the element fragments. The fuse element may be surrounded by air, or by materials intended to speed the quenching of the arc.
A maximum current that the fuse can continuously conduct without interrupting the circuit. The speed at which a fuse blows depends on how much current flows through it and the material of which the fuse is made. The operating time is not a fixed interval, but decreases as the current increases. Fuses have different characteristics of operating time compared to current. A standard fuse may require twice its rated current to open in one second, a fast-blow fuse may require twice its rated current to blow in 0. 1 seconds, and a slow-blow fuse may require twice its rated current for tens of seconds to blow. Fuse selection depends on the load’s characteristics.
The fastest blowing fuses are designed for the most sensitive electrical equipment, where even a short exposure to an overload current could be very damaging. Normal fast-blow fuses are the most general purpose fuses. These types of fuse are used on equipment such as motors, which can draw larger than normal currents for up to several seconds while coming up to speed. Manufacturers can provide a plot of current vs time, often plotted on logarithmic scales, to characterize the device and to allow comparison with the characteristics of protective devices upstream and downstream of the fuse.
This term is normally used in short circuit conditions and the values are used to perform co-ordination studies in electrical networks. The energy is mainly dependent on current and time for fuses as well as the available fault level and system voltage. The breaking capacity is the maximum current that can safely be interrupted by the fuse. Miniature fuses may have an interrupting rating only 10 times their rated current. North American practice, to interrupt 10,000 amperes.