The current is the switching, control or monitoring variable in electrical devices and systems. Current transformers are used for this current measurement, which record currents that are sometimes up to 1000A high. These are converted into small values of a few mA using the transformation ratio. One of the main applications is in the low-frequency range with 16.7Hz (railway frequency), 50/60Hz (mains frequency in households) and 400Hz (on-board power supply, e.g. in aviation). The second important application field is in the high frequency range from 20kHz ... 500kHz. Here the currents in switching power supplies / power supplies (SNT) / switch mode power supply (SPS) are measured.