For the purposes of sizing the breakers, or rather their breaking capacity or frame size,  assume the PSSC may be as high as that of  the street supply or local transformer - as the 'on mains' case clearly gives the higher current. In the usual way the currentand  energy limiting effect of fuses may be beneficial in relaxing the size of down stream breakers to 10kA or 6kA types.

However, for the purpose of earth fault trip times and ADS, you will need the lowest fault current case, and that is genset, so then assume that Zs may be as high as the phase voltage divided by about twice the genset full load current, or if you prefer to think that way that the PSC may as low as  200% of genset rating when it is the power source. There is no equivalent energy limiting benefit from the death or glory fuse, on genset as it will most likely never operate. Set your earth trips accordingly, As an example, but  not the only ones, the vigi ones are nice offering independent programming of delay and threshold. However the price reflects this

This will result in a design that trips promptly even on genset, and also that does not catch fire with excessive fault currents when on mains.

Miike