Reverse rotation is the phenomenon when a fan rotates in the opposite direction in which it is intended and cannot be corrected by the fan motor. In the event of reverse rotation, many Sanyo Denki fans include electrical safety circuitry which will turn off a fan once reverse rotation is detected. This safety feature is implemented to prevent damage to the fan or the electrical equipment powering the fan. Reverse rotation will not happen when a fan is operating under standard conditions so then what causes reverse rotation to occur? The most common cause for this failure is when a stronger airflow or pressure force acts upon a fan impeller while in a startup phase. This force is commonly generated by other fans operating within a system and thus any system with multiple fans may have a risk of reverse rotation. Hot swapping components have the highest risk of reverse rotation since the other fans in an operational system will continue to blow air backwards through a newly replaced fan and prevent it from starting. A new trend in the fan industry is to use 3-phase motor fans which increase the efficiency and high end performance. The extra efficiency however will mean reduced cogging torque of the fan which will make it more susceptible to reverse rotation.