A negative sequence current arises when the voltage and current in a power system are unbalanced. This type of current produces a reverse rotating magnetic field in the air gap, which has a synchronous speed but rotates in the opposite direction to the rotor. This reverse rotating field is detected by points on the rotor surface and can cause damage to rotating machinery if left unchecked. It is important to have Negative Sequence Current Ratings for generators as it is expected that they will not be connected 100% Negative-Sequence all the time, with three statements about the power system needing to be true for this to occur. Excessive negative phase sequence current can cause excessive heating, vibration, and bearing wear, leading to long-term damage or even machinery failure.