The fifth harmonic is an AC electrical or acoustic wave component that vibrates at exactly five times the frequency of the primary fundamental wave. In standard 60 Hertz power systems, the fifth harmonic operates at 300 Hertz. In three-phase electrical power engineering, the fifth harmonic is classified as a negative sequence harmonic, meaning it rotates in the opposite direction of the fundamental wave inside electric motors. This counter-rotational magnetic field produces severe mechanical torque pulsations, vibration, and rapid thermal overheating in connected machinery, making its mitigation a vital priority for industrial plant operators.