i2t characteristics of fuse wire
i2t characteristics of fuse wire. Fuses operate very rapidly under short circuit conditions when the current levels are extremely high clearance times being typically only of few milliseconds. Under these conditions the current waves shapes depend on the parameters of the protected circuit the instant in the voltage cycle at which the fault occurs and the current limiting effect of the fuse link itself. only a small portion of a cycle of power frequency current flows before interruption occurs and transient components are present. Such currents cannot be assigned a single value which may be used to determine the corresponding operating time from the time-current characteristics. Because of this use is made of a quantity termed i2t which is the time integral of the square of the instantaneous current which passes through a fuselink between the incidence of a circuit fault and the instant at which the fuse Arc is extinguished.
if it could be assumed that the resistance of the components of the protected circuit was constant throughout the operating Period of the fuse link, then the value of i2t would be proportional to the energy fed to the circuit and also the heating produced by it. In practice, however, the resistances must rise some significantly. nevertheless the i2t value is often described as the let-through energy, a term which is not strictly correct because it does not contain a resistance value and is not there for in energy units J but in a2s.
To assist in the application of fuses many manufacturers of components determine the i2t withstand limits of their products and publish them. The i2t values needed to operate fuse links at very high currents tend to be independent of the waveshapes and manufacturers publish these values as well as providing time-current characteristics to enable appropriate fuselinks to be selected by users.
In practice, fuse link manufacturers provide two sides of i2t values one set associated with the total clearance time and another set with lower values for the i2t let through during the pre-arcing period. Both sides are needed to enable coordination to be achieved between fuse links which are effectively in series in networks.
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