Because of electrical and magnetically coupling between primary and secondary windings of auto transformers, the primary and secondary earth fault currents are mixed in neutral point of transformer. The difference phase angle between secondary current and induced primary current is approximately 180 degree, therefore the amount of through earth fault current is the difference of Ip and Is , where the Ip and Is are the primary and secondary earth fault current respectively. So the earth fault protection system in neutral point can not detect actual primary or secondary earth fault currents.
