The superconducting XFEL machine will be built in a single tunnel. The RF stations provide RF power of up to 10 MW at 1.3 GHz. The modulators will produce high-voltage pulses of up to 120 kV, 140 A with a pulse length of 1.54 ms. These will be installed in a service hall at ground level. The klystrons will be positioned near the cavities inside of the tunnel. The energy will be transported via pulse cables of up to 1.5 km length between the pulse generating units of the modulators located in the halls and the pulse transformers in the tunnel. Since the transformers have a transformer ratio of 1:12, pulses of 10 kV and 1.6 kA must be transmitted. Tests with pulse cables have been made with an existing modulator and klystron of the FLASH-Linac. The results showed that the internal construction of the modulator had to be changed to improve EMI behaviour. In order to prove that the beam in the accelerator is not disturbed by the operation of the cables a set of pulse cables was installed inside the tunnel of the FLASH-Linac in parallel to the beam line. An existing pulse transformer/ klystron unit was supplied via these 1.5 km long cables by a new prototype of a modulator taking into account EMI considerations. The FLASH-Linac has been operated for a longer period of time with this set up proving reliable and stable operation. (4 pages)