For large transformers with low winding resistance and high inductance, these inrush currents can last for several seconds until the transient has died away (decay time proportional to "X"L/"R") and the regular AC equilibrium is established. To avoid magnetic inrush, only for transformers with an air gap in the core, the inductive load needs to be synchronously connected near a supply voltage peak, in contrast with the zero-voltage switching, which is desirable to minimize sharp-edged current transients with resistive loads such as high-power heaters. But for toroidal transformers only a premagnetising procedure before switching on allows to start those transformers without any inrush-current peak.