<p style="text-align:justify"><span style="font-size:14px">A symmetrical triple line to ground fault can occur in case of switching ON a circuit breaker when the earthing switch is inadvertently kept ON. There is a kind of mechanical or electrical interlock between a circuit breaker of a transmission line and its earthing switch such that if the earthing switch is ON, circuit breaker cannot be switched ON. However, a protection engineer always has to consider a probability of failure of that interlocking arrangement and design a protection scheme to look after this fault.</span></p>

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

<p style="text-align:justify"><span style="font-size:14px">Two phases can be bridged together either in generators or motors or in power transformers/ distribution transformers because of failure of insulation between phases, particularly when conductors of different phases are in the same slot of a stator of a generator or motor (this is very unusual as machines are rarely designed with two conductors of different phases in the same slot, hence probability of L-L fault in machines is only in overhung portion).</span></p>

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

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

<p style="text-align:justify"><span style="font-size:14px">In the transmission lines, two phase wires can be shorted by kite strings or tree limbs. Moreover, in monsoon, the two phase conductors may swing (as they are suspended using suspension insulators which are free to move) due to winds and storms. While swinging two conductors (in horizontal line configuration) may come nearer, the dielectric strength of air might have dropped to as bad as </span></p>

<p style="text-align:justify"><span style="font-size:14px">5 kV (peak)/ cm leading to flashover between two phase wires. This is the cause of L-L fault in lines.</span></p>

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

<p style="text-align:justify"><span style="font-size:14px">Line to ground fault (L-g) is most common fault and can occur&nbsp; because of flashover across the line insulator due to lightning or switching overvoltage. Line to ground fault can occur in machines and transformers as well because of failure of insulation between a live conductor and earthed core.</span></p>

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

<p style="text-align:justify"><span style="font-size:14px">L-L-g fault can occur when L-L (say R-Y) and L-g (say Y-g) faults occur simultaneously.</span></p>

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

<p style="text-align:justify"><span style="font-size:14px">These are common and known causes. It is difficult to enlist the causes of faults as the faults occur in the field practices because of innumerable number of causes. Some of them can be listed as follows:</span></p>

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

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

<ul>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Terminal block in an indoor control panel burnt due to overheating (there can be some cause of this overheating) and terminals shorted. The current juice from C. T. secondary to relay via terminal block did not reach relay from one pair of C. T.s in case of differential protection scheme due to this shorting of terminals. The current juice from another pair of C. T.s passed through protective relay, mal-operating the differential protection of generator. Class A trip occurred, there was a complete collapse of generator, GT, boiler, turbine and auxiliaries. There was no internal fault in the generator, but differential protection operated just because of mere terminal block.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:11.0pt">There was a ground fault (L-g) in the cable connected between generator and GT. This L-g (say R-g) fault resulted in increase of V<sub>YB</sub> voltage by approximately </span><span class="math-tex">\(\sqrt3\)</span><span style="font-size:14px">&nbsp;times rated line to line voltage. Because of this overvoltage, there was flashover between Y and B phases in generator bus P. T. panel which was part of differential protection zone. This resulted in class A trip of generator protection scheme.</span></p>
	</li>
</ul>

<ul>
	<li style="text-align:justify">
	<p><span style="font-size:14px">In case of differential protection of transformer, measuring C. T. connections were wrongly wired to a differential relay from one side (say, secondary) of transformer C. T.s. (current transformers). This error was caught while primary injection testing of differential protection scheme, as differential current of certain milliamperes (as against zero) was measured under normal condition. If this problem would not have been caught, the differential protection would certainly have mal-operated due to heavy through fault (for which, it must be stable).</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">While making a circuit breaker OFF at an industry (66 kV consumer) for shut-down and scheduled maintenance, the biased differential protection of 132/66 kV power transformer&nbsp;&nbsp;&nbsp; (of state electricity transmission corporation)&nbsp; mal-operated leading to unwanted tripping of 132/66 kV transformer which was feeding many other consumers obviously. This problem was corrected by changing the setting of second harmonic stabilization.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">At one generating station, rotor earth-fault protection operated because of uneven d. c. current flow in parallel paths of carbon brushes due to small variation in brush dimensions.</span></p>
	</li>
</ul>