Why Transformer Neutral Connected to Earth

Distribution systems are earthed to create a reference point for the system voltage, to facilitate the detection and discriminative isolation of faults involving contact to earth, and to limit over-voltages under transient conditions. A neutral earthing system is a system in which the neutral is connected to earth, either solidly, or through a resistance or reactance of value sufficient to materially reduce transients, and to give sufficient current for selective earth fault protection devices to operate. Sensitive fault detectors allow the reduction of fault currents to very low values.

Unearthed neutrals were used in the past due to the fact that the first earth fault did not require tripping of the system. An unscheduled shutdown on the first earth fault was particularly undesirable for industries that were based on continuous processes and where continuation of the supply even under single fault conditions was necessary. Although achieving the initial goal, the unearthed system provided no control of transient over-voltages.

Neutral earthing systems are similar to fuses in that they do nothing until something in the system goes wrong. Then, like fuses, they protect personnel and equipment from damage. Damage comes from two factors, how long the fault lasts and how large the fault current is. Earth fault relays trip breakers and limit how long a fault lasts and neutral earthing resistors limit how large the fault current is.

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Solidly earthed systems

In a solidly earthed system, the neutral point is connected directly to earth, either directly or via a virtual neutral transformer. Generally all LV systems are solidly earthed. For MV and HV systems, solid earthing is the cheapest method but has a number of serious drawbacks.

Neutral earthing resistors are used to limit the fault current in transformers When a phase to earth occurs, the fault current is limited only by the soil resistance. This current, which can be very high, can damage the windings. Low voltage networks are generally solidly earthed and resistive neutral earthing is generally only applied to MV and HV lines.

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Created By :- Deepakkumar Yadav

Why is Neutral Connected to Earth in a Transformer?