The ground-fault relay monitors the excitation winding circuit
and provides early detection of a ground-fault or insulation
deterioration in any part of it. A single ground-fault that occurs
in a field winding will not cause immediate damage since the
ground-fault current is limited. A second ground-fault could
however result in a catastrophic machine and exciter damage.
It is therefore important to detect the first ground-fault to
enable shut-down for repair.