Fault Currents Outside the Model Range in Arc Flash

Question: How does Arc Flash handle bolted fault currents that are out of the model range?

Answer:

In IEEE 1584 method, PTW uses the Ralph Lee Equation for any parameter that is outside of the range of model (Voltage, Fault Current, Gap):



Sample:

(1a) PTW Calculation:



(1b) Hand Calculation:




In the NFPA (Doughty Neal) Method, a scale factor is used when the bolted fault current is out of the model range:

Basically, the scale factor is the (Bolted fault/lower or upper limit). The factor was obtained in the Tutorial B presentation in PCIC. (see attached)