To maintain losses and cooling efforts of growing capacity and power rating of battery energy storage systems the battery system voltage will have to be increased significantly. Hence novel high voltage battery energy storage systems (HV-BESS) are the subject of development in various cases with high power demands in mobile and static applications. In contrast to low voltage battery energy storage systems (LV-BESS), a special characteristic of these high voltage systems is the large amount of battery cells connected in series and the tendency to fewer parallel strings. The use of high-power high-voltage battery systems must not decrease the reliability of the HV-BESS, therefore a continued operation after a single fault is essential. This requirement leads to the necessity of re-evaluating faults in a HV-BESS, including transient electrical stress. The usability and feasibility of a HV-BESS up to 10 kV is evaluated in the “KV-BATT”-project. Due to the decreased count of parallel cells, short current amplitudes in HV-BESS are less critical than in LV-BESS on the one hand. On the other hand high differential voltages may occur at open circuit situations, e. g. as a consequence of internal short circuits or sudden contact faults. This events are able to excite transient voltages. The fixed cell voltage of Li-Ion cells leads to large spatial strings, where stray capacitance and inductance has to be taken into account in these transient situations and a insulation coordination is necessary. To qualify the insulation coordination in HV-BESS, simulations are performed to quantify possible transient voltage amplitudes.