The Shunt Active Power Filter is the most wide extended active power filter in the industry. Perhaps it has a poor performance when it is trying to compensate the current of a low impedance nonlinear load, such as rectifiers with smoot dc capacitor. These loads are the most used loads in industry, because are the first power step in adjustable speed drivers and other power electronics devices extended nowadays. Additionally to the description of the operation and general composition of a Shunt Active Power Filter, this paper shows the equations that reveal such issues, and simulation results that validate this analitical approach; also showing that under poor operation conditions, this kind of filter does not reach the expected electric energy savings.