I just got the new version of SKM installed, and it's nice to discover the little changes here and there (and not so nice to see that some things still don't work). One of the changes is the ability to add tolerances for utility contribution and impedances, and I'd like to use these for my studies. So far the options are:

Utility pre-fault voltage: doesn't work apparently, the value just resets to zero if you exit the screen. Also no change in study results
Utility contribution: two separate % for + and -
Transformer impedance: one % value +/-
Cable impedance: one % value +/-

These tolerances can be 'activated' when running the comprehensive study or arc flash study, so for example a maximum scenario would use extra contribution and lower impedances. The settings for fault studies and arc flash are not linked, so it takes a little spelunking through the option screens to get them properly setup (and keep them consistent across scenarios).

It can add up to pretty significant differences, so my question is what would you use as baseline? My first suggestion would be:

Voltage (whenever it works) +/- 10% for MV fluctuation in the defined limits (IEC world, YMMV)
Contribution +/- 20%, or depending on what the utility gives as min/max
Transformer impedance: +/- 5%, I usually take the value from the rating plate, so unsure if this is too large still
Cable impedance: +/- 10%, there's a lot of rounding and estimation here in my experience, so willing to make it larger

Would like to hear what you think!