In investigations of magnetic flux distribution in electromagnetic equipment it is possible to apply a method, using magnetooptic effects. The paper presents results of measurements of the value of the leakage flux appearing in air channels of the magnetic cores being modeled, of the investigated electromagnetic equipment (electrical machine, electric apparatus or transformer). The magnetooptic sensor (fiber optical) is placed in the channel of the core. The paper includes results of verifying numerical calculations of the magnetic leakage flux density in the air channel. Examples of design solutions of magnetic systems are given, where is possible to apply measuring systems fulfilling the role of overcurrent apparatus.