VFD driver

Hello, we often find VFD, UPS devices in the distribution system. Normally how do you evaluate the fault contribution from these devices? I was told that usually we can use 200% of the FLA, is it right? How do you deal with the by pass? Thanks!

VFDs are inherently current limiting . 200% of FLA is a good number. For a UPS it would depend on the type, a double converion (AC to DC, DC to AC) would be SCR based so the same 200% would be good. Ferroresonant UPSs might be a little more but still pretty low because of the high impedance. Bypass switches, the electomechanical kind, would be like any other conductor. You'd have to calc for when fed from utility and for when fed from UPS, but they would not be current limiting and you would calculate at the point where the line cables land on the switch. If the switch is triac based it would be current limiting but that would only play in downstream of the bypass as the line cables would still be landed on the line side of the switch.

Thanks Haze!