If a fuse is subjected to a current greater than the minimum current required to produce melting, the fuse element will melt. The higher the current, the shorter the melting time will be. This inverse relationship is shown graphically by the time-current characteristic (TCC). Figure 1 shows a typical time-current characteristic for a semiconductor fuse with a nominal current rating of 450A. The time it takes for a fuse element to melt is often referred to as the prearcing time, since melting is followed by a period of arcing. Melting of a fuse element is due to the heating effect of the current, which depends on the r.m.s. value of the current actually flowing through the fuse before melting occurs.
Figure 1. TCC for typical 450 A semiconductor fuse
For operation in times less than one a.c. cycle, the melting time of the element is greatly affected by the wave shape of the current. In this case it is necessary to use I²t values for checking the system protection. The standard time current curve shown in figure 1 is for a symmetrical sine wave. The boundary C-C' shown on figure 1 indicates that the fuse will safely interrupt currents at times below this limit. The fuse must not be applied to interrupt current levels which produce melting times longer than this limiting boundary. Sustained overloads which persist for longer times may result in failure of the fuse, and must be cleared by other means. There is a limit to how long a temporary overload can be tolerated by the fuse. This limit is shown by sloping part of the C-C' boundary.
New style gR class fuses do not have a C-C' limit and can be used for overload protection. They are defined in the IEC 60269 standards. They must melt for a 160% overload but should not melt for a 125% overload.
Current limiting effect of the fuse and peak let-through current curve (cut off current)
An important advantage of the current-limiting fuse is its ability to break high fault currents rapidly, which limits the peak current flowing in the circuit, and consequently limits the let-through I²t. For current-limiting fuses the peak let-through current (or cut-off current) is a very important parameter. Fuse data is presented in the form of peak let-through (cut-off) characteristics. These characteristics are published for specified test conditions, typically AC voltage, frequency and power-factor. Figure 2 shows a peak let-through characteristic for a typical semiconductor fuse with a current rating of about 30A.
Figure 2. Peak-let through (cut-off) characteristic
For low prospective (available) currents, the fuse takes several a.c. cycles to melt, and the highest value of current is equal to the peak current in the first half-cycle. This is 1.414⋅IRMS for a symmetric wave and about 2.3-2.5⋅IRMS for an asymmetric wave, depending upon the circuit power factor. These limits are shown by the faint lines in figure 2. However, above a certain level of prospective current (threshold current), melting occurs within the first half-cycle, and current-limiting action occurs. At high prospective currents, the peak current is much lower than the peak prospective value.
For a given r.m.s. prospective current, the peak let-through current varies, depending on the angle of the sourcevoltage wave at which the short-circuit occurs. At the 100kA level, the highest value of let-through current is obtained with a symmetrical short-circuit wave, while in the region just above the threshold current the asymmetrical wave gives the highest value. Published data always shows the highest possible (i.e. worst-case) value.
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