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.