One of three cooling tower fans at a Power Station experienced a failure of the fibre composite drive shaft that connects the electric drive motor to the fan gearbox assembly. This was the only fan in service at the time of the incident. Preparations were being made to commission the gas turbines following a shutdown. Immediately prior to the incident, a manual bus transfer was conducted leading to a momentary power interruption. This caused a sudden motor speed decrease, applying significant torque to the shaft, causing a torsional failure. Two pieces of the shaft were ejected from the cowling. Both pieces landed on the opposite side of the upper deck of the cooling tower, approximately 30 meters from the point of ejection.