URT
URT
Coefficient "B1" considers an additional cooling by external ventilation. If the air speed v L measured at the side of the fuse body at a distance of 1 to 2 cm) is about 5m/s then B1 has a value of 1.25. If the air speed is less than 5m/s one has to calculate:
B1 = 1 + 0.05 * v L
With A1 and B1 it is possible to up rate the load current I L to the smallest fuse rated current I min :
I min = I L / (A1 * B1)
In cases of thermal influences, the chosen fuse rated current must be equal to or higher than the calculated smallest fuse rated current I min . 6.2 Cyclic Load Operation Cyclic load operation, with regular or irregular variation of load currents, has considerable influence on the fuse link’s lifetime (See the technical essay at our web-site). This kind of operation, with load periods of 0.1 second up to one hour, requires a complex calculation. For defined part times, coefficient A2 for regular and irregular load periods and B2 for each current step at the load periods have to be differentiated.
Fig. 6.2 Regular and Irregular Load In the case of a cyclic load, the RMS-value of the load current obtained over the period T S has to be divided by coefficient A2 to get the smallest fuse rated current.
I min = I RMS / A2
While coefficient A2 influences the rated current, coefficient B2 has to be considered in coherence with the fuse curve. Here B2 indicates the distance between the value of the current step and the smallest value of the time-current curve obtained overthe part periods t n .
I TCC >= I n / B2
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