Based on the safety of pilot zero sequence direction protection， the ground faults in parallel lines are discussed. It is concluded that the pilot zero sequence direction relay mal-operate certainly in parallel lines without electricity contact on both sides. For systems（include ones with part parallel lines） with electricity contact on one side or two sides， whether the relay will mal-operate or not depends on the relationship of which one is stronger between electricity and magnetic. For systems with one side or two sides on the same bus， the relay is safe in nature.