All of the technical devices that are powered by the frequency converter, apart from their great advantages, also bring considerable pitfalls and risks. When designing a machinery, it is necessary to consider the negative effects of characteristics of the frequency converter on the rotating parts, especially on the bearings of motors, gearboxes, gears, seals, etc. These devices and their parts are usually exposed to the effects of capacitive and shaft currents that have a considerable impact on the reliability of machines and equipment. This article focuses on the reasons for the use of insulated bearing (or insulated coupling) for asynchronous motors and on the influence of the measures taken to eliminate the adverse effects on the rotating parts. The following is an example of the use of technical diagnostics to determine the condition of some parts of the machinery, the example is evaluated, conclusions are drawn and analysed.