Abstract: A single-phase multi-ratio current transformer, having at least two different current ratio or output levels through the appropriate wiring of its output leads without any intervention of the main current circuit of the system, having a primary current conductor, a secondary circuit having at least two change-over output wires S.sub.2 and S.sub.3, respectively connected to two contacts and a main output terminal connected to a common contact. A plurality of corresponding plug-in configurational devices, each have at least one jumper wire positioned to the common contact to only one of the two contacts, in order to give the current transformer the current ratio or output level corresponding to the selected wire S.sub.2 or S.sub.