When selecting arc flash PPE using the category method; the fault current and clearing time parameters are key to doing it correctly. It’s time we got serious about these parameters
NFPA 70E permits either of two methods be used when selecting arc flash PPE. The PPE category method, which relies on tables, or the incident energy analysis method, which calls for an electrical power system analysis to be performed by electrical engineers. What many don’t realize is that because of the parameters in the PPE tables both methods require a power system analysis.
When using the PPE category method for selection of arc flash PPE, you must ensure that your equipment is within the clearing time and fault current parameters listed in the tables. If your circuit is not within these parameters, the standard states you can not use these tables and instead you must use the incident energy analysis method.
An electrical power system analysis involves electrical engineers using engineering software, and data collected in the field, to create a model of the electrical distribution system of your facility. This data includes transformer information, wires sizes, fuses, breaker model numbers and settings, all overcurrent-protection devices, etc. Collecting, modeling and analyzing this data can be time-consuming and costly; which is why many facilities struggle with getting it done. This study would provide you with your available fault current and clearing times that you must have to use the PPE category tables. Just a bit more work by the engineer and you would have a complete incident energy analysis.
From NFPA 70E Table 130.7(C)(15)(a) a 480-volt panelboard calls for category 2 arc rated PPE. The table further states the parameters of 25,000 amps of available fault current and a two-cycle fault clearing time. If your fault current or clearing time is outside of these parameters, it is possible the Cat 2 PPE recommended will fall short of the protection you need. There is no way of knowing these parameters without doing a power system analysis?
Another example would be an upstream current-limiting fuse protecting the panel you are about to open for troubleshooting. The informational note to Table 130.7(C)(15)(a) states that a current-limiting fuse has a typical clearing time of .5 cycles if it is within the current limiting range. This is an informational note and not part of the standard language however someone might be tempted to use this clearing time as their clearing time without knowing if the fault current is within the current limiting range of the fuse. Current-limiting fuses are very fast acting fuses and can reduce arc flash energy but only if there is enough current to drive the fuse into its current limiting range.
When selecting arc flash PPE the standard doesn’t allow us to assume we are at a .5 cycle clearing time. We have to verify we are and we can’t do that without getting electrical engineers involved.
The most important thing to keep in mind is that this is the same information you would have to collect and almost all of the calculations you need to perform an incident energy analysis. Just a little more work by the engineer and you will have a complete incident energy analysis. At this point, you would have no use for the tables. Most often the incident energy analysis recommends less PPE be worn, gives you a chance to mitigate the arc flash energy and is a better method.
Selecting arc flash PPE by either method is never an exact science. There are too many variables and unforeseen circumstances. The more of these variables you can control the more accurate your selection will be. One thing is for sure, wearing arc rated PPE helps reduce injury from arc flash. We need to do all we can to make sure we are wearing the proper PPE.