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<p style="text-align:justify"><span style="font-size:14px">The power and control circuits of tripping a breaker in case of fault are shown in figure 6.2 (a) and 6.2 (b). Whenever a fault occurs on the feeder, C. T. transmits the fault current to current coil of a protective relay. (For a relay such as directional or distance relay, P. T. transmits the faulted voltage to the potential coil of the relay). The protective relay operates as per its characteristics (instantaneous, definite time or inverse time) and its contact closes [R-1 in figure 6.2 (b)].</span></p>

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<p style="text-align:justify"><span style="font-size:14px">Referring to figure 6.2 (b), R-1 closing, coil of tripping relay (86) energizes. The current capacity of contact R-1 is 1 or 2 amp. The trip coil (52-TC is a trip coil of circuit breaker) current is of the order of 15 to 20 amp d. c. Hence R-1 operates 86 and the contact 86-1 is used to trip the breaker as contact 86-1 can be designed to carry and break large current (20 amp d. c.). The measuring relay contact cannot be designed to carry and break such a large current as this heavy contact will reduce the sensitivity of the relay. The tripping relay also does the multiple functions such as annunciation, alarm, interlocks etc. [(not shown in figure 6.2 (b)].</span></p>

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<p style="text-align:justify"><span style="font-size:14px">52-a is a mechanical switch associated with breaker. Usually, 8 NO and 8 NC contacts (switches) are provided by breaker manufacturer. These contacts are designed to carry and break large current. Hence when breaker is closed, NO contact 52-a is closed. On occurrence of fault,</span></p>

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<ul>
	<li style="text-align:justify">
	<p><span style="font-size:14px">R operates.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">R-1 closes.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">86 energized.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">86-1 closes.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">52-TC is energized.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Breaker trips.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Fault is said to be cleared.</span></p>
	</li>
</ul>

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<p style="text-align:justify"><span style="font-size:14px">Thus, the faulty section is isolated from the rest of the healthy system. Protective relay contacts are usually self-reset contacts, hence on tripping of a breaker, current through breaker becomes zero, R resets and R-1 opens. But 86 is usually a lock-out relay (hand-reset). Hence 86-1 remains closed until manually reset. If 52-a were not provided, 52-TC will remain energized. Trip coils are intermittent duty coils and are not designed to remain energized continuously. Hence the function of 52-a is to disconnect trip coil circuit, because when breaker trips 52-a will open. </span></p>

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<p style="text-align:justify"><span style="font-size:14px">While reading any control circuit, it should be remembered that it is an international norm of drawing that all relays are shown deenergized, all breakers are shown open, all isolators are shown open, all switches are shown open in the drawing. Also, every electrical component of protective relaying system and protective relays are numbered as per international codes for drawing. &nbsp;</span></p>