PowerFactory allows for calculation of faults between lines at different voltages, that means in general faults between different circuit phases through a definition of a "Intercircuit Fault". In order to do so you should follow the next steps:

1. Go to the Edit Short Circuits command located in the Main Toolbox 1,
2. Add a new Short-Circuit event selecting the element "Intercircuit Fault Event (EvtShcll?),
3. In the pop-up window there are options to set fault type, locations and fault resistance and reactance. Set according to your needs and select OK,
4. Select Calculate Short-Circuit command (CTRL + F11), select method 'complete' if not already selected and tick 'Multiple Faults',
5. Select Execute

In the attached file you will find an example where a Short-Circuit is calculated between Phase A of line 'L1 HV' in 110 kV grid and Phase B of line 'L1 MV' in the 10kV grid.