But usually neutral of generator or transformer winding is earthed through resistance/inductance to limit earth fault currents.
With resistance earthing, it is not possible to protect complete winding from earth fault and the % of winding protected depends on the value of the neutral earthing resistor and the relay setting.
Generally, 80 to 85% of generator winding is protected against earth faults using differential protection.
The remaining 20 to 15% winding from the neutral side is left up protected by the differential protection.
To achieve 100% protection against earth faults, separate earth fault protection is provided in addition to differential protection.
On the occurrence of an earth fault, fault current I flow through a part of the generator winding and neutral to the ground circuit.
The corresponding secondary current of the current transformer I flow through the operating coil and restricted earth fault coil of the differential protection.
If the earth fault occurs at point f of the generator winding, voltage Var is available to drive earth fault current li through the neutral to ground connection.
If a fault occurs near to neutral point ie terminal 'a', the driving voltage Var will be very less. Hence earth fault current Ir is having a considerably low value.
Relay setting can not be kept to a very low value (i.e. setting the relay too sensitive to sense the earth fault currents of small magnitudes). Because if too sensitive the relay may respond during through faults or other faults due to inaccuracies of CTs or saturation of CTs etc.
Hence practice is to protect about 85% of the generator winding against phase to earth fault and to leave the 15% portion unprotected by the differential protection against earth faults.
As separate earth fault protection covers the entire winding against earth faults.