<p style="text-align:justify"><span style="font-size:14px">It is seen that farther the fault from the source, less is the magnitude of fault current, hence more time can be allowed (and must be allowed as fault can be transient) to clear the fault. The equipment of power system can withstand small fault currents for longer time. On the other hand, if the fault is nearer to the source, fault current magnitude will be high and hence such fault should be cleared quickly. Therefore, the relays, the time of operation of which is inversely proportion to fault current magnitude, are the best to be applied to radial feeder protection.</span></p>

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<p style="text-align:justify"><span style="font-size:14px"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16252230511625223051.jpg" style="height:489px; width:800px" /></span></p>

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<p style="text-align:justify"><span style="font-size:14px">Therefore, the relays R<sub>1</sub>, R<sub>2</sub> and R<sub>3</sub> of figure 7.2 are inverse time (IDMT) overcurrent relays. The distance-time characteristic using such relays is shown in figure7.5. It can be appreciated that the selectivity is yet maintained with lower time of operation for faults near relaying point. Back-up protection is also offered. Reader shall also appreciate the speed of operation of inverse time overcurrent relays vis-&agrave;-vis the definite time equivalents as indicated by shaded area in figure 7.5. The high-set instantaneous unit can be in-built with inverse time unit in this case as well as shown in figure 7.5.</span></p>

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