<p style="text-align:justify"><span style="font-size:14px">Main or primary protection schemes are used in the first line of defence. There must be second line of defence provided by certain back-up schemes. There can be a fault in transmission line, a fault in transformer, an induction motor or a generator, the components of switchgear protect the equipment against the damaging consequences of such faults. But then, very obviously, there can be troubles and faults within these components of protective gear (relay coil may be open circuited, in numerical relay, a digital signal processor does not work or some discrete component has failed, trip coil of the breaker has become open-circuited or there is some trouble in operating mechanism or gas pressure has reduced in SF<sub>6</sub> circuit breaker so that it does not trip even after receiving signal from relay, C. T. and/or P. T. are not properly transforming fault current and faulted voltage due to some trouble in them so that finally a relay does not get correct information of fault, everything is healthy but the coil of auxiliary tripping relay has open-circuited or control circuit fuse has failed). In this case the fault will not be cleared. Hence back-up protection is a must. There are three kinds of back-up protections used in practice:</span></p>

<p style="text-align:justify">&nbsp;</p>

<ol>
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong>Relay Back-up:</strong> In this kind of back-up scheme, a protective relay and its C. T. core are duplicated. This is a very costly protection scheme and used for very important and costly equipment. Low forward power protection of large generators uses this relay back-up scheme.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong>Breaker Back-up:</strong> When a feeder breaker fails to trip on a fault in a feeder, the feeder fault becomes virtually a bus fault. In breaker back-up scheme, a busbar protection scheme is initiated in this case and all breakers of the bus (but a breaker that has failed) will trip. This will be discussed at length in buszone protection.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px"><strong>Remote Back-up:</strong> Remote back-up is provided by a relay in the next section towards the source. This scheme of back-up is cheapest because it incurs no cost. We will see this kind of back-up in transmission line protection.</span></p>
	</li>
</ol>