Littelfuse Arc-Flash Calculator

Overview of Personal Protective Equipment (PPE)

Many types of PPE are rated for incident energy in cal/cm², and values obtained from the Calculator may be used directly, to select PPE. Some PPE is rated in accordance with Hazard Risk Categories from 0 through 4 as shown in NFPA 70E Table 130.7(c)(11) reproduced below; which shows the level of incident energy for each Category. The Hazard Risk Category is determined and shown by the Calculator.

While NFPA 70E’s highest Hazard Risk Category is four, corresponding to a PPE level of 40 cal/cm², PPE exists with ratings up to 100 cal/cm². The Littelfuse Calculator will display these levels. Since so little is presently known about arc blast and the other risk factors at levels above 40 cal/cm², it is recommended that work be limited to testing for energized parts following lockout/tagout procedures. PPE should be used with ratings equal to or greater than the calculated incident energy.

PPE is primarily intended to protect a worker from burns due to exposure to heat from electric arcs. Levels of PPE selected by methods contained in NFPA 70E and IEEE 1584 are intended to reduce Arc-Flash burns to the just curable level for 95% of incidents. PPE may not prevent injury due to arc blast, as mentioned above, and some flash burns may be incurable. Workers and others that may be exposed to arc-flash must be trained in the proper selection and use of PPE and tools. Such protection will include as a minimum: a hard hat, leather gauntlet gloves, safety glasses, and approved flame retardant clothing. In addition, for more severe exposure it could include head covering and full face shields, Nomex coveralls, rubber insulated gloves with leather protectors, leather boots or shoes, hearing protection, etc. Specific requirements for PPE are described in NFPA 70E.

Table 130.7(C)(11) Protective Clothing Characteristics
Typical Protective Clothing Systems
Hazard/Risk Category	Clothing Description (Typical number of clothing layers is given in parantheses)	Required Minimum Arc Rating of PPE [J/cm²(cal/cm²)]
0	Non-melting, flammable materials (i.e., untreated cotton, wool, rayon, or silk, or blends of these materials) with a fabric weight at least 4.5 oz/yd² (1)	N/A
1	FR shirt and FR pants or FR coverall (1)	16.74 (4)
2	Cotton Underwear -- conventional short sleeve and brief/shorts, plus FR shirt and FR pants (1 or 2)	33.47 (8)
3	Cotton Underwear plus FR shirt and FR pants plus FR coverall, or cotton underwear plus two FR coveralls (2 or 3)	104.6 (25)
4	Cotton Underwear plus FR shirt and FR pants plus multilayer flash suit (3 or more)	167.36 (40)

Note: Arc rating is defined in Article 100 and can be either ATPV or E_BT. ATPV is defined in ASTM F 1959-99 as the incident energy on a fabric or material that results in sufficient heat transfer through the fabric or material to cause the onset of a second-degree burn based on the Stoll curve. E_BT is defined in ASTM F 1959-99 as the average of the five highest incident energy exposure values below the Stoll curve where the specimens do not exhibit breakopen. E_BT is reported when ATPV cannot be measured due to FR fabric breakopen.

Reprinted with permission from NFPA 70E-2004, Electrical Safety in the Workplace, Copyright © 2004, National Fire Protection Association, Quincy, MA 02269. This reprinted material is not the complete and official position of the NFPA on the referenced subject, which is represented only by the standard in its entirety.

Visit us on the web! www.littelfuse.com