The Standby-Ferro UPS was once the dominant form of UPS in the 3-15kVA (Volt-Ampere) range. This design depends on a special transformer that has three windings (power connections). The primary power path is from AC input, through a transfer switch, through the transformer, and to the output. In the case of a power failure, the transfer switch is opened, and the inverter picks up the output load. In the Standby-Ferro design, the inverter is in the standby mode, and is energized when the input power fails and the transfer switch is opened. The transformer has a special “Ferro-resonant” capability, which provides limited regulation and output waveform “shaping”. The isolation from AC power transients provided by the ferro transformer is as good or better than any filter available, but the ferro transformer itself creates severe output voltage distortion and transients which can be worse than a poor AC connection. Even though it is inherently a standby UPS, the Standby-Ferro generates a great deal of heat because the ferro-resonant transformer is inherently inefficient. Standby-Ferro UPS systems are frequently represented as On-Line units, even though they have a transfer switch, the inverter operates in the standby mode, and they exhibit a transfer characteristic during an AC power failure. High reliability and excellent line filtering are the strengths of the Standby-Ferro design. However, the design has very low efficiency combined with instability when used with some generators and newer power-factor corrected computers, which has caused the popularity of this design to decrease significantly.