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The star point of a primary*-transformer's secondary winding is frequently connected to earth via an impedance, in order to limit the return current in the event of an earth fault close to the transformer. The impedance may be a resistor or an inductor, (choke) depending on the nature of the earth return. In general, if the earth resistance is low, then resistive earthing is used; if the earth resistance is high, then inductive earthing is used.

(*e.g. 33/11 kV)

For resistive earthing, the transformer secondary's star point is often connected to the centre electrode of a galvanized-steel tank which is solidly earthed. The tank is then filled with water, and it is the resistance of the water that provides the required resistance to earth. The actual resistance of the water is modified to the desired value by dissolving sodium carbonate into the water while measuring its resistance. The solution has a negative temperature coefficient of resistance, which means that, in the event of an earth fault, as the fault current increases, the increasing temperature causes the resistance of the water to decrease, thus accelerating the response time of the protective system.

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What is the difference between neutral point and earthing?

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here is no major difference between earthing and Grounding, both means "Connecting an electrical circuit or device to the Earth". This serves various purposes like to drain away unwanted currents, to provide a reference voltage for circuits needing one, to lead lightning away from delicate equipment. Even though there is a micro difference between grounding & earthing.Earthing and Grounding both is refer to zero potential but the system connected to zero potential is differ than Equipment connected to zero potential .If a neutral point of a generator or transformer is connected to zero potential then it is known as grounding. At the same time if the body of the transformer or generator is connected to zero potential then it is known as earthing.


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