Why neutral of transformer is earthed?

The neutral of the secondary is connected to earth for safety. 

If the neutral were not connected to earth, the voltage relative to earth could change as a result of conditions on the system. In particular, with the right parameters in the circuit, a short circuit to ground can generate sustained voltages that can be several times the open circuit voltage of the transformer. But connecting the neutral to earth establishes the maximum steady-state voltage that can exist between any conductor and earth as the open circuit voltage produced by the transformer.

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