I have a question about the utility of this.
There was an informational note added back in 2014 to 110.24 that says the available fault current markings are related to the equipment. The handbook then goes on with commentary about needing to be able to determine PPE per NFPA 70E tables. However, the analysis of changes available from NFPA (page 36) says very clearly that the note was added to clarify that the labeling was for equipment compatibility, and not for anything to do with arc flash worker safety. So it would seem that this requirement is there so that the service electrician being asked to add a circuit can know what interrupting rating he would need for equipment.
The tool mentions it can help one comply with 110.24. If one applies the logic in the analysis of changes, it would seem that they would conclude that they can't use this tool for calculating available fault current, as series rating would come with the requirements of 240.86, which has requirements that include only a licensed PE can do these, or they must be specific pieces of equipment approved for use with specific upstream devices, and that series rating cannot be done with motor circuits. (The calculator allows one to calculate motor contributions, which still can result in a rating lower than the mains service fault current capability). It would seem then, that this tool doesn't enable any compliance with 110.24, and that marking a piece of equipment with a lower than mains fault current rating may confuse future service workers into improperly selecting equipment with a lower fault current rating than the mains.
Alternately, if one uses this to comply with NFPA 70E PPE tables (2015NFPA70E 130.7C15Ab), this could seem to have some issues:
- There really is one category PPE recommendation for panel boards rated up to 25KA. There isn't a lower PPE level for lower AIC ratings. The only table that does have a different table for lower fault current availability is motor control centers. All other categories of equipment are based on the equipment, with a limitation one can use the tables only up to a certain available fault current rating. It would seem one cannot use this tool to downgrade PPE requirements.
- If there were ever an incident after one used this tool to determine that, say a sub panel fed from a switchboard with a 65KA fault current capability, was able due to the length of its conductors to be calculated 25 KA fault current or less (and therefore able to be worked on using PPE suggested in NFPA70E tables), OSHA likely would fine the employer for a serious violation on the basis of the worker not being qualified to make this calculation.
It would seem that this tool is primarily for information purposes only: perhaps to double check the work of the engineer; to come up with reliable recommendations on series rated equipment to be presented to a supervising engineer for approval; or in the absence of any better advice to estimate how much additional fault current a motor could add to the circuit beyond that of the transformer's infinite bus and verify safe PPE levels.
JW April 27 2019, 3:03 am EDT Reply to this comment |