In the current release of etap there is not a way to automatically model ZSI. To determine the benefits of ZSI you must manually choose fault clearing times by either (1) manually entering the fault clearing time in the bus editor (you must enable this in the "AF FCT" tab in the short circuit study case options), or (2) select the "Fixed FCT" box and enter a clearing time on the "Arc Flash" tab of a bus that you want to evaluate. The second option will allow other buses to automatically select protective devices if you want, only the bus which you have checked the "Fixed FCT" box will have a non-automatically determined FCT. This is explained in the "Calculation Methodology" topic in the help file. These are also the methods you would use to evaluate light relays. Version 12.6.