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The paper intends to give a contribution to the investigation, by both computer simulations and laboratory tests, on the criteria for the selection of the SPD protection level (Up) in order to limit the voltage at the equipment input to a value less than or equal to its withstand voltage (Uw). Such criteria are strongly influenced by installation conditions of SPD (i.e. length of the leads connecting SPD to the equipotential bonding bar), the characteristics of the electrical lines (i.e. routing conditions and distance between SPD and equipment to be protected) and different load conditions. In order to analyze the influence of different parameters, computer simulation by using engineering transient software, it is recognized to be a very powerful tool. However, to achieve reliable simulations it is necessary to have adequate numerical model of the equipment to be protected as well as of protective devices. For this aim laboratory tests on real arrangement represent an essential reference step to obtain a satisfactory model. In the present contribution a real arrangement of a circuit including SPD and the equipment to be protected has been built in the HV laboratory of Warsaw University of Technology and such arrangement has been simulated by the computer using a commercial transient software. It is shown that a good agreement has been reached between the results by laboratory tests and by computer simulations. Main concern has been paid to find a comparison of results. Differences between the values and the shape of the voltage appearing on the equipment and on SPD as the length of their connecting wiring is increasing and in different load conditions are shown.