This is from a Q&A posted on the PTW website.
In PTW, you can run a unbalanced short circuit study using the Unbalanced module.
Then enter the SLG fault current or using the 3P fault from UBSC results to the AF screen. PTW automatically unlinks the SC and link TCC. You can enter the adjusted fault current based on the UBSC results and Arc Flash program reads the trip time automatically. The results will be conservative, but having an answer is better than having no answer at all.
In the ArcFlash drop down menu check "Include Non-3-Phase Systems".
http://www.skm.com/Newsletter_April_2012.html