Explain with necessary diagram the earth fault protection of power and auxiliary circuit.
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i) Earth fault protection for power circuit in Loco: When any earth fault occurs in the traction power circuit the fault current flows through the relay coil as the battery negative is also earthed. This trips the main CB. The relay can be temporarily isolated by connecting the faulty traction circuit through a high earthing resistance to earth.

ii) Earth fault protection for auxiliaries in Loco: Detection of any earth fault in auxiliary circuit of Loco is a very important element of the protection strategy. Since earth fault should not lead to dangerous condition till another fault occurs, this protection is provided with facility of isolation. The protective relay for monitoring earth fault in the auxiliary circuit is connected to the star point of the Arno as shown in figure. An earth fault occurring anywhere in the auxiliary circuit gets monitored by the relay which acts to trip the main CB. Since earth fault is not dangerous, the locomotive can be worked temporarily by isolating the relay which connects the star point to earth through a high resistance.

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