That's the question. General consensus I've heard is that motors fed with VFDs equipped with a bypass need to be calculated as if there was no VFD, since the bypass could be used. In such a case, do you model the VFD at all? Do you skip it, or create multiple scenarios? Personally, if I need the AIE to account for the VFD not being implemented, then there's not much point in modeling it at all unless the customer specifically asks for it.

For me it just depends on what I am doing the study for. Since this is the AF forum, I assume you mean for short circuit and arc flash studies for the purpose of fault contribution. If there is a drive, I do model it whether it has a bypass or not. I used to do as you prescribe, if it has a bypass I check the box to allow 100% let-through current and/or the bypass box. I have changed that thinking over time though. If the drive is on a motor that is part of a system or process that I know will not work if the drive is in bypass then I don't allow fault contribution. This is often the case. Say for instance the drive is in bypass/60 hz and it cannot respond to the control inputs and might over-pressurize the system with air or fluid and damage equipment or actually cause a failure in the process, then odds are very high no-one is going to do that intentionally or at least not for very long before something breaks. Why would you have a drive with a bypass in that situation you might ask? Because some engineers just spec all drives with a bypass without giving it any thought as they are EE's and the drives are spec'd in Div 26 or old Div 16 not by the ME's or the process engineers. When I was still doing a lot of design myself I changed from me spec'ing the drives to having the ME's or process guys spec the drive and/or having the drives supplied by whomever was supplying the equipment that the drive controlled to avoid any compatibility issues and over-spending on drive features in my specs that were not required and would never be used.

I also consider the motor sizes and scrutinize larger motor systems more closely before deciding to allow let-through or not.

If I run across regenerative drives, those can allow fault contribution and I do check that box on those.

Oh, I also do not label VFD's with arc flash labels unless the customer requires it and then I make it clear in my reports that IE inside the drives is incalcuable and that only the line side IE is reported which may not be very accurate.