Discussion
In power system, for extremely inverse over current protection scheme, the fault current is 25 A. Calculate the time in which the fault will be cleared if the time multiplier setting is 100%?
Answer: A
The key gives 0.128 s. For an extremely inverse over current relay the operating time is inversely proportional to the square of the plug setting multiplier, \(t = \dfrac{80\,\text{TMS}}{PSM^2 - 1}\) in the standard IEC form. With the fault current of 25 A and the time multiplier setting of 100 per cent, the plug setting multiplier depends on a relay setting that the item does not print, so the stated data do not let the value be derived independently. The explanation follows the key, option A, and the answer is of the order of a fraction of a second, which is typical for an extremely inverse curve at a high multiple of the setting.
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