Earth fault protection for power transformers commonly uses differential relays with restricted earth fault elements, residual over current relays, or separate neutral current transformers monitoring ground currents. On the high voltage side, directional earth fault protection may be applied to distinguish internal faults from external network faults. In grounded neutral systems, a CT on the neutral connection measures zero sequence current. Sensitive residual over current or restricted earth fault protection detects low magnitude faults near the neutral point that might not be seen by simple phase over current protection alone.