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Travelling electromagnetic waves in power cables are strongly influenced by frequency-dependent attenuation. For many applications in overvoltage protection, knowledge of the attenuation is very important. Especially in long cable systems the attenuation can be so high that lightning or switching voltages will be reduced to an innocuous level. Another area is the power line communication which uses the energy power grid for signal transmission in a frequency range up to 30 MHz. Here an indepth analysis of the attenuation of the transmission line is very important. In particular, the admittance of the cable, composed of insulating and semiconducting materials, is very important for the attenuation of high frequencies. Simulation results agree well with measurements for a 20 kV XLPE cable, if the semiconducting layers between the conductor and screen are thoroughly modelled. The material parameters of the commonly used carbon-polyethylene compound and conductive paper layers were measured for the calculations. Measurements at a test setup show the attenuation for several plys of conducting paper layers on the inner conductor of a coaxial pipe system. The effect of different numbers of plys on the impulse rise time was investigated.