Whenever there is an imbalance in a circuit, the imbalanced currents will have a negative phase sequence component. A negative phase sequence (or phase imbalance) relay is often provided for the protection of generators and motors against imbalanced loading that may arise due to line-to-line faults. Such a relay has a filter circuit, which is responsive only to the negative sequence components. Since small magnitude over-current can cause dangerous conditions, it becomes necessary to have low settings on such relays. An earth relay can also provide the desired protection but only in cases when there is a fault between any line and earth. For line-to-line faults an earth relay cannot provide necessary protection and hence a negative phase sequence relay is required.
