NFPA 70E defines four categories of arc flash personal protective equipment (PPE), each with a minimum arc rating for the clothing: 4, 8, 25 and 40 cal/cm² for categories 1, 2, 3 and 4. The category is chosen from the standard's tables only if the equipment is within their limits of short-circuit current and clearing time; otherwise, or when more precision is wanted, the incident energy is calculated with IEEE 1584 and clothing with an equal or higher rating is selected. The two methods are not mixed on the same piece of equipment.
Why the categories exist
An arc flash releases thermal energy in milliseconds that can cause severe burns to a person near the equipment. Protection is measured by the incident energy, in cal/cm², that the skin would receive at the working distance. Arc-resistant clothing is tested and rated by the energy it withstands before the wearer would suffer a second-degree burn or before the fabric breaks open.
So that nobody has to calculate the energy of every piece of equipment before every task, NFPA 70E offers a simplified path: tables. Depending on the type of equipment, its voltage, its short-circuit current and its clearing time, the tables assign a PPE category (personal protective equipment) with a defined protection package.
The four categories
| PPE category (NFPA 70E) | Minimum arc rating of the clothing |
|---|---|
| 1 | 4 cal/cm² |
| 2 | 8 cal/cm² |
| 3 | 25 cal/cm² |
| 4 | 40 cal/cm² |
Each category includes, besides the clothing, the head, face and eye, hearing, hand and foot protection that the standard’s PPE table lists. From a certain category upward, face protection moves from a face shield to an arc flash hood.
The old category 0 was removed in the 2015 edition. The threshold of 1.2 cal/cm² (5 J/cm²) is used as the second-degree burn limit and defines the arc flash boundary. See arc flash study.
Two methods that are not mixed
- Is the equipment within the current and clearing-time limits of the NFPA 70E tables?
- Yes: Is the exact value per piece of equipment preferred?
- Yes: Incident energy analysis with IEEE 1584
- No: PPE category method
- No: Incident energy analysis with IEEE 1584
- Yes: Is the exact value per piece of equipment preferred?
Category method. Fast, but valid only within the parameters the table assumes: the maximum available short-circuit current and clearing time for each type of equipment. If the equipment has more current, or the protection is slower, the table does not apply. The catch is that to know this you have to know the current and the time, which is almost the same as doing the study.
Incident energy analysis. It calculates the energy of each piece of equipment with IEEE 1584-2018 from the short-circuit current, the actual protection and the equipment configuration. Clothing with an arc rating equal to or higher than the calculated energy is selected. It gives the exact value and usually avoids both insufficient and excessive PPE.
NFPA 70E does not allow using both on the same piece of equipment: the label carries the incident energy or the category, never both.
How clothing is rated
Arc-resistant clothing carries an arc rating on its label, in cal/cm², obtained from exposure tests with controlled arcs. The value is expressed as ATPV or as EBT, depending on which of the two limits is reached first in the test. In the IEC world, the standard for protective clothing against the thermal hazards of an electric arc is IEC 61482-2, with its own test methods and classes. When buying clothing, the right comparison is the garment’s rating against the incident energy of the equipment or against the minimum of the category.
Layers count: a set of several garments can have a higher rating than each one separately, but only if the set was tested that way. Underwear and the layers under the outer garment must be made of materials that do not melt.
The other school: APC protection classes (Germany and IEC 61482-1-2)
In Europe, clothing is often chosen with a different approach, which is worth knowing when garments are imported or when working with a European parent company. The publication DGUV Information 203-077 (September 2020 edition, in German), from the German accident insurance, guides the selection of PPE against the thermal effects of an arc and relies on two tests from IEC 61482-1:
- Method 1 (IEC 61482-1-1, open arc): determines the arc ratings of fabrics and clothing (ELIM, ATPV or EBT). It is the method that delivers a rating in energy, not a class.
- Method 2 (IEC 61482-1-2, box test): classifies the garment into one of two arc protection classes (APC), with a 500 ms arc struck in a 400 V AC test circuit.
| Class | Prospective test current | Arc duration | Test arc energy | Mean exposure energy |
|---|---|---|---|---|
| APC 1 | 4 kA | 500 ms | 168 kJ | 146 kJ/m² (about 3.5 cal/cm²) |
| APC 2 | 7 kA | 500 ms | 320 kJ | 427 kJ/m² (about 10.2 cal/cm²) |
The conversion used by the guide itself is 1 cal/cm² = 41.868 kJ/m². The two classes are not direct equivalents of an NFPA 70E category: they are a different method, with a different test geometry.
The decision logic of DGUV 203-077 compares the expected arc energy (WLB, in kJ) with three thresholds:
- If it is below 50 kJ, the guide indicates that no arc flash PPE is needed: this is the value from which a skin burn from direct exposure cannot be ruled out.
- If it does not exceed the protection level of an APC 1 garment, an APC 1 or APC 2 garment will do.
- If it does not exceed that of an APC 2, an APC 2 garment will do.
- If it exceeds it, the arc energy and the probability of injury must be reduced by other measures before working on the equipment.
The equivalent protection level of the garment is further adjusted for the working distance (the test is run at 300 mm) and for a transmission factor that depends on the volume of the installation where the work takes place. The guide orders the measures with the TOP principle: technical first, then organizational and finally personal. It is the same hierarchy NFPA 70E champions: PPE is the last barrier.
What PPE includes besides clothing
Arc flash PPE is a set, and the protection of the set is fixed by its weakest link:
- Outer garments with an arc rating: shirt and pants, coverall or full arc flash suit in the higher categories.
- Head and face protection: dielectric hard hat, arc-rated face shield with balaclava, or arc flash hood.
- Hand protection: leather or arc-rated gloves and, where there is a shock hazard, rubber insulating gloves with leather protectors.
- Hearing and eye protection, because the arc produces a pressure blast and intense light.
- Leather footwear.
Clothing must be kept clean and free of grease or fuel contamination, which can burn on the garment, and withdrawn from service if damaged. Inner layers must be made of fibers that do not melt, such as cotton, wool or arc-rated materials.
Common mistakes
- Using category 2 for everything because «it is the one used on low voltage», without knowing whether the equipment is within the limits of the table.
- High-energy equipment with category 4 PPE: if the calculated energy exceeds 40 cal/cm², no PPE will do and the work must be done de-energized.
- Excessive PPE: heavy clothing where the energy is low reduces mobility and visibility, and raises the risk of other accidents and of heat stress.
- Labels that do not match the current relay settings.
- Forgetting that PPE is the last barrier. NFPA 70E and the hierarchy of controls put eliminating the hazard (de-energizing) and reducing it through engineering first.
Lowering the category with engineering
Incident energy depends mostly on clearing time. Faster protection settings, a maintenance mode, arc flash detection relays or differential protection can bring a piece of equipment from category 4 down to 2, or to a level where everyday arc-rated work clothing is enough.
We run your arc flash study
Our team calculates the incident energy of each switchboard, switchgear lineup and MCC in your plant with IEEE 1584 on the actual settings of your protection, tells you the protection required for each piece of equipment and proposes how to reduce it where it is high. The study is done remotely, worldwide, from your single-line diagrams and relay settings. See our engineering design and studies service and protection coordination, and write to us through the contact page.
Related guides
References
- NFPA 70E, Standard for Electrical Safety in the Workplace, 2024 edition (Table 130.7(C)(15)(c), PPE categories).
- IEEE 1584-2018, IEEE Guide for Performing Arc-Flash Hazard Calculations.
- IEC 61482-2, Live working – Protective clothing against the thermal hazards of an electric arc – Part 2: Requirements.
- DGUV Information 203-077, Thermische Gefährdung durch Störlichtbögen (2020 edition, in German), with the APC classes of IEC 61482-1-2.
- ASTM F1506, specification for arc-rated work clothing; ASTM F1959, test method for arc rating.
Frequently asked questions
What does arc flash category 2 mean?
It is the NFPA 70E PPE category that requires clothing with a minimum arc rating of 8 cal/cm², together with the rest of the equipment the standard's table lists for that category, such as suitable face and head protection, gloves and footwear.
Is there a PPE category 0?
Not anymore. Earlier editions of NFPA 70E used it for non-melting clothing with no arc rating. It was removed in the 2015 edition; today the categories run from 1 to 4.
What do I do if the incident energy is above 40 cal/cm²?
There is no PPE category for that level. The right approach is not to work the equipment energized and to de-energize it, or to reduce the incident energy with setting changes, a maintenance mode, arc flash relays or other measures before working on it.