In an electrical distribution system for supplying power from an AC source to an adjustable-speed drive connected in a single-phase manner, a device for substantially eliminating harmonic currents in the supply lines of said system. The device includes a completely-passive parallel resonant circuit having three passive electrical branches connected in parallel and also having an almost infinite impendence at a third harmonic frequency of a fundamental frequency of said AC source to prevent the formation of only said third harmonic frequency so that there is no third harmonic current to remove or dissipate as heat. The three passive electrical branches comprise a first branch consisting of a capacitor, a second branch consisting of a reactor, and a third branch consisting of a resistor. The parallel resonant circuit is electrically connected to at least one supply line.