Modeling VFD motors in bypass mode raises calculated short circuit current, but rarely by enough to change an equipment duty rating. Across a 700-plus bus SKM model containing 88 VFD-equipped motors, bypass pushed exactly one panel past its rating; the mean increase was 150 A and the median just 36.49 A.
Electrical engineers conducting arc flash analyses face an important modeling decision: should Variable Frequency Drives (VFDs) be represented in bypass mode to capture worst-case scenarios? This case study examines the practical impact of this choice.
What system was analyzed?
PowerSafe Engineering analyzed a large electrical system with over 700 buses using SKM software. The system included 88 motors equipped with VFDs totaling 2,988 HP, all featuring bypass capabilities. The analysis compared short circuit currents calculated with VFDs in bypass mode versus normal operating mode across all equipment.
What did bypass mode actually change?
The most striking discovery involved only one instance where calculated short circuit current exceeded a panel's rating when VFD bypass was engaged—a situation that would not have occurred with VFDs in normal mode.
When VFDs operated normally without bypass, the maximum short circuit current increase was 13.54% (2,769A) at one specific panel rated for 42k. This change did not affect the panel's duty rating determination.
How large were the fault current differences?
The distribution of fault current differences across all locations revealed:
- Mean difference: 150A increase when switching from normal to bypass mode
- Median: 36.49A (significantly lower than mean, indicating right-skewed distribution)
- Standard deviation: 339A, reflecting high variability
- Range: Outliers as high as 2,776A
This skewed distribution demonstrates that while most locations experienced modest changes, certain configurations showed pronounced differences.
So should VFDs be modeled in bypass mode?
Most differences are relatively small, with a few larger differences, indicating that in most cases the fault current changes modestly between operating states. However, the substantial variability observed suggests that VFD bypass state deserves careful consideration, particularly at locations exhibiting significant fault current variations. Engineers should identify and prioritize assessment of high-impact locations rather than assuming uniform effects throughout systems.