SWG has largely fallen into disuse but is still used for certain applications like guitar strings. The voltage drop should be taken into account, particularly when using a fuse in low-voltage applications. A maximum current that the fuse can continuously conduct without interrupting the circuit. A fuse rated for 1 A at 25 °C may conduct up to 10% or 20% more current at −40 °C and may open at 80% of its rated value at 100 °C. For coordination of fuse operation with upstream or downstream devices, both melting I2t and clearing I2t are specified. The I2t rating is related to the amount of energy let through by the fuse element when it clears the electrical fault. 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.

For example, a glass tube fuse rated at 32 volts would not reliably interrupt current from a voltage source of 120 or 230 V. If a 32 V fuse attempts to interrupt the 120 or 230 V source, an arc may result. 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 breaking capacity is the maximum current that can safely be interrupted by the fuse. Current rating of the fuse. The operating time is not a fixed interval but decreases as the current increases. A time-delay fuse (also known as an anti-surge or slow-blow fuse) is designed to allow a current which is above the rated value of the fuse to flow for a short period of time without the fuse blowing. Fuse bodies may be made of ceramic, glass, plastic, fiberglass, molded mica laminates, or molded compressed fibre depending on application and voltage class.

Rated voltage should be higher than the maximum voltage source it would have to disconnect. 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. Time-current characteristic; i.e. fuse speed. 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. 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. Since the I2t 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.

Once current is applied, resistance and voltage drop of a fuse will constantly grow with the rise of its operating temperature until the fuse finally reaches thermal equilibrium. Fuses can be built with different sized enclosures to prevent interchange of different ratings of fuse. Most fuses are marked on the body or end caps with markings that indicate their ratings. Some cartridge fuses are manufactured with end caps of different sizes to prevent accidental insertion of the wrong fuse rating in a holder, giving them a bottle shape. Fuse selection depends on the load's characteristics. The fuse ideally would carry its rated current indefinitely, and melt quickly on a small excess. A fuse consists of a metal strip or wire fuse element, of small cross-section compared to the circuit conductors, mounted between a pair of electrical terminals, and (usually) enclosed by a non-combustible housing. The clearing I2t is proportional to the total energy let through by the fuse when clearing a fault.

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