where -3Io is the residual current from the bus side CTs, IN is the transformer neutral current, and H is the phase angle between the currents.
The ground-differential sensitivity is very low and will operate for an IN current of 0.2 A. Figure 15 illustrates the zone of operation where the 87T relay cannot detect ground faults.
In many cases, the 51G neutral timeovercurrent relay provides time-delay protection for faults in this zone.