The zero Sequence current is also known as the dc component of the fault current. It is represented by three vectors in parallel.
The Zero sequence current can be detected by adding the vectors of the three phase currents. As long as there is no zero sequence current, the vector sum of the phase currents is zero. When there is an earth fault, the vector sum of the phase currents is no longer zero. This indicates the presence of the zero sequence current.
The value of the vector sum in that case is the value of the zero sequence current. The zero sequence current can be measured by the zero sequence current transformer.
The zero sequence current transformer also known as a core balance current transformer can be connected to an earth fault relay which will isolate the earth fault.