kencybart wrote:
It is important to remember that the HRC and PPE requirements are also based on verifying the NOTES that apply to each task or equipment class. In most cases, calculations and verification must still be performed to confirm that available fault currents and clearing times of overcurrent protective devices are within the maximum limits specified in the NOTES.
You are correct - the footnotes are important but let’s look at 2 of the footnotes in detail for discussion purposes. Specifically Page 31 of the 2009 Edition of NFPA 70E. 600V Motor Control Centers.
Footnote 2 - upper limit 65 kA for 2 cycles.
Using this data for incident energy calculations based on IEEE 1584, I get around 3.5 cal/cm^2. Using the HRC table, some locations are category 0, 1 or 2*
If the PPE selection for this data was made based on calculations, it would lead to a minimum 3.5 cal/cm^2 PPE which most say is Category 1 (4 cal/cm^2 minimum)
Footnote 3 - Upper limit 42 kA at 20 cycles.
Using this data for incident energy calculations I get around 24 cal/cm^2. (clearing time is very important) Using the tables where footnote 3 applies and Category 4 and Category 1 are both referenced.
If the PPE selection for this data was made based on calculations, it would lead to a minimum 24 cal/cm^2 PPE which most say is Category 3 (25 cal/cm^2 minimum)
The NFPA 70E Tables are a great way to come up with PPE in the absence of calculations and should be used where calculations have not been performed. However the Ei calculations provide a more precise idea of the actual hazard based on specific conditions.