Answer Posted / om s.

while connecting pt each primary phase winding is connected to phase to earth. The neutral point of load is connected to neutral point of secondary’s. Neutral point of primary is solidly earthed.
If primary neutral is not earthed the zero sequence component of voltages (due to earth fault) cannot flow through primary windings. Hence phase to earth voltages of system which contain zero sequence component do not get truly transformed and measured voltage are distorted. Hence the earth fault on system cannot be sensed on secondary side of VT.

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