Traceable calibrations of current sensors with nanosecond rise-times and amplitude of several hundreds of amperes are up to now not offered worldwide. Therefore, increased efforts were made to provide these calibration capabilities for very fast current signals. In this paper, some current sensors as well as the calibration setup are presented. For this purpose, a current signal generator that provides higher frequencies than the rise time of the best current sensor is needed. It also requires a peak amplitude of at least several tens of amperes in order to achieve an acceptable output voltage of the sensors. For the droop rate measurements, a constant current level is desirable that remains stable for at least several microseconds. All these demands are fulfilled by the cable generator that is presented in this paper. The produced signal is a square wave signal with a peak amplitude determined only by the cable impedance and the charging voltage. The main focus has been on the used switch because it has a major impact to the performance of the cable generator. Two different approaches for the switch are researched. The reed relays for lower voltages and more reliable triggering and the spark gap for voltages above 10 kV and therefore higher currents. Also, different spark gap designs with pressurized air gaps, noble gas filled gaps as well as pressurized SF 6 and vacuum gaps have been conferred. In the end, an entire calibration system for very fast current signals is presented including all essential components.