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There’s a disconnect, or rather an improper connection that workers make between categories and values calculated when using the incident energy method. A worker should not correlate categories with incident energy.

A real simple example can be made from comparing an item in the category table of NFPA 70E vs an incident energy calculation meeting the requirements of the table.

See the following table from NFPA 70E 2021

 

Based on the above requirements of the table, to work on this equipment you will need to wear Arc Flash PPE Category 2, see the table below for more PPE requirements:

Category 2 is a minimum arc rating of 8 cal/cm².

If we take the constraints outlined in the table above:
Maximumof 25 kA available fault current; maximum of 0.03 sec (2 cycles) fault clearing time
and calculate inside of software, we will see a different result.

1.6 cal/cm².

This is barely above the minimum threshold for arc flash protection when positioned 18″ away from a source.
Disclaimer:  The above calculations are only an example and should not be construed as representative of any piece of equipment or used to justify using lesser amounts of PPE when utilizing the Category tables in NFPA 70E.  Additionally, only the one example was modeled and is not representative of any other piece of equipment than as described in the table above.

What can be learned here?
The tables are in place to protect you when incident energy calculations can not be performed.

The tables protect you by ensuring you’re wearing enough PPE for the hazard as defined by the constraints.  The calorie level of your clothing has no other correlation to the actual hazard other than “it’s appropriate”.

Here is another example which should help you disconnect categories from incident energy values:  If you utilize the table method to obtain a Category 1 value, you do not have to wear a balaclava.  This does not mean you will be protected when exposed up to 4 cal/cm².  This does mean if your equipment fits the parameters to obtain a Category 1 value, you will be protected when wearing Category 1 PPE.  Comparatively, if you are working on equipment with an incident energy calculation of 1.2-4 cal/cm², you are required to wear a balaclava (“Where the back of the head is inside the arc flash boundary”).

 

It should be noted, that in order to use Table 130.7(C)(15)(a) Arc Flash PPE Categories for Alternating Current (ac) Systems, you must meet the parameters mentioned in each row:  i.e. Maximum 25kA available fault current; maximum of 0.03 sec (2 cycles) fault clearing time; minimum working distance 455 mm (18 in.).  If you are unable to meet or verify these parameters then the table is not allowed for use.  With that in mind, it is going to be very difficult to know the available fault current or the clearing time without performing an evaluation.  Additionally, meeting the requirements of the table is going to be difficult.  Ensuring that the proper clearing time is met with standard equipment is unlikely.  Most fuses and breakers will trip longer than the allowed clearing time at the available fault current:  “Even if the worker can properly identify these values, it is quite possible that an arc-flash event will result in much higher incident energy than expected because Iarc is much lower than Ibf and the protection is much slower at that Iarc.”.  [Valdes, Sullivan, Halle (2022) ANALYSIS OF NFPA 70E, TABLE 130(C)(15)(a) USING IEEE 1584-2018 CONSTANT ENERGY BOUNDARIES. ESW 2022-06, IEEE ESW, Jacksonville FL March 2022].  

This helps show the value and necessity of performing an arc flash evaluation at your facility, its very likely that the outcome will be less PPE required than when utilizing the Category method.  This content can be very confusing for most people who don’t frequently deal with this, contact one of our engineers to help you understand.

 

 

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