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<p style="text-align:justify"><span style="font-size:14px">Refer figure 7.34(a). There is a fault at F. Overcurrent relay R<sub>2</sub> senses the fault and operates after (say) 0.1 sec. Overcurrent relay R<sub>1</sub> acts as a back-up and is set to operate after 0.35 sec (discrimination margin 0.25 sec as we have discussed earlier). Hence, the first disadvantage of time-graded discriminative system is the time delay. Overcurrent relay R<sub>1</sub> will operate after 0.35 second if R<sub>2</sub> fails. If there is a fault at the bus, R<sub>1</sub> will operate and breaker CB<sub>1</sub> will trip to clear the fault after 0.35 second only, not earlier. This is the second disadvantage of time grading. But this was essential with the use of conventional relays. The numerical relay on the other hand, is equipped with the facility such that this discrimination time margin can be drastically reduced. They are provided with &ldquo;start&rdquo; and &ldquo;block&rdquo; functions. The relay takes sample of the current every 1 millisecond (typical sampling time). If it uses 3-sample technique, it will sense the fault within 3 milliseconds (more accurate are half and full cycle Fourier algorithms in which case fault will be diagnosed after half a cycle and one cycle respectively). Thus, fault is detected after one cycle (maximum), i.e., 20 milliseconds. Hence, for fault at F, the relay R<sub>2</sub> will sense the fault after 20 milliseconds (it is going to operate after 0.1 sec, i.e., 100 milliseconds). Therefore after 20 milliseconds, the start output is excited (logic state 1). This output can be routed to a signaling output relay (which is a d. c. operated attracted armature analog relay). The contact of this signal relay R<sub>2</sub>-start [figure 7.34(b)] can be used to block operation of relay R<sub>1</sub> through binary input. Thus, if R<sub>2</sub> senses the fault, operation of R<sub>1</sub> is blocked. This is also known as reverse interlocking. Therefore, now, a time delay for the operation of an auxiliary tripping relay (about 40 milliseconds) is enough as a discrimination margin between R<sub>1</sub> and R<sub>2</sub>. R<sub>1</sub> will, in this case, be set to operate after 0.14 sec. If there is a bus fault now, Relay R<sub>1</sub> will operate after 0.14 sec only, thus very fast operation. Thus, there is a revolutionary change in the rules of setting and the protection offered is very fast and hence probably no damage to the equipment as against possibility of certain damages to electrical equipment due to slow protective system offered by earlier conventional relays.</span></p>