(EN) A portable tester for determining the primary winding (18) to secondary winding (22) current ratio of a current transformer (16) while that transformer is in service, including a current probe (40) having a split core positionable around an active power line (14) supplying current to the transformer, an AC to DC converter (44) to convert the AC output of that probe to a first DC signal, another AC to DC converter (56) for converting the secondary output of the current transformer under test to a second DC signal, and a ratio meter (46) for converting the first and second DC signals into a visual display of the ratio of those signals. A mechanism (42) is provided for assuring proper location of the decimal point on the ratio meter. Polarity and burden testing is also provided.