Abstract: Transformers, like series reactors, have the quality to limit the short-circuit currents to values determined by the transformer's impedance which consist of the resistance and the inductive reactance. Disclosed are a device and a method, that under the short-circuit conditions of secondary windings, with applied rated voltage to primary windings of a step-down transformer or with turns ratio 1:1, is the way of significant reducing of short-circuit currents in primary and secondary windings, as well in any output circuits connected to the transformer. Disclosed is, a discovery of a self loss of a reactive impedance of the transformer windings, when the secondary windings of the transformer are short circuited, and a method of reducing of the short-circuit current of a short-circuited or ground-faulted transformer.