This standard defines five classes of HRC (Hazard Risk Category), which goes from  HRC 0 to HRC 4. HRC 4 or HRC Level 4 is the highest risk category with ATPV value more than 40cal/cm2. Consequently, protective clothing that withstands HRC 4 should provide maximum safety.

What is an HRC? 
An HRC level is determined by the minimum amount of calories per square centimeter (ATPV or Cal/cm2). Any treated garment must pass through with a 50% probability of a 2nd or 3rd degree burn occurring, which is how the protective level of the treated clothing is determined. The higher the ATPV, the higher the HRC level attained, the greater the protection that is needed.

What is NFPA 70E®?
NFPA 70E requirements for safe work practices to protect personnel by reducing exposure to major electrical hazards. Originally developed at OSHA’s request, NFPA 70E helps companies and employees avoid workplace injuries and fatalities due to shock, electrocution, arc flash, and arc blast, and assists in complying with OSHA 1910 Subpart S and OSHA 1926 Subpart K. Official document scope

What does NFPA 70E® address?
Provisions encompass safety-related work practices, safety-related maintenance requirements, and safety requirements for special equipment. The Standard includes guidance for making hazard identification and risk assessments, selecting appropriate PPE, establishing an electrically safe work condition, and employee training. 

NFPA 70E bases its requirements on several tests, main of which are following:

  • ASTM F1959/F1959M – ATPV (Standard Test Method for Determining the Arc Rating of Materials for Clothing)
  • ASTM D6413-11 – Vertical Test (Standard Test Method for Flame Resistance of Textiles)
  • ASTM F2621-06 – Design Integrity (Standard Practice for Determining Response Characteristics and Design Integrity of Arc Rated Finished Products in an Electric Arc Exposure)

The following HRC level table shows minimum ATPV value that must be calculated to acquire the next level of HRC and typical FR apparel used to accomplish the ATPV/HRC calculation.

Risk Category
Number of
FR protection layers
Samples of layersCommon FR clothing at this levelMinimum ATPV (Cal/cm2)
HRC 0  Cotton UndergarmentsLong Sleeved Shirt (Natural Fiber)Long Pants (Natural Fiber)0
HRC 1Single base layer 
FR protection
FR shirt and FR pants; 
FR coveralls
Cotton UndergarmentsArc Rated Long Sleeved Shirt or FR CoverallsArc Rated Long Pants or FR Coveralls4
HRC 22 or more layers
FR protection
FR under garments (undershirt, underwear) + FR shirt + FR pants;FR under garments +FR coverallsCotton UndergarmentsShort Sleeved T-Shirt (Natural Fibers)Arc Rated (8 cal) Arc Flash Hood or Hard Hat with Arc Rated Face Shield with Balaclava with Coveralls or Jacket & Bibs or 50″ 8
HRC 32-3 or more layers
FR protection
FR under garments (undershirt, underwear) + FR shirt + FR jacket + FR pantsFR coverallsCotton UndergarmentsShort Sleeved T-Shirt (Natural Fibers)Arc Rated (25 cal) Arc Flash Hood with Coveralls or Jacket & Bibs or 50″ Coat with Leggings25
HRC 43-4 or more layers
FR protection
FR under garments (undershirt, underwear) + FR shirt + FR jacket/coat + FR pants + FR coveralls;

FR under garments (undershirt, underwear) + FR shirt + FR pants, multi-layer flash suit
Cotton UndergarmentsShort Sleeved T-Shirt (Natural Fibers)Arc Rated (40 cal) Arc Flash Hood with Coveralls or Jacket & Bibs or 50″ Coat with Leggings40

It is understood that layering generally gives more protection than the sum total of the ATPV values of the individual garments being layered. However, this needs to be tested for each specific garment being layered and thus is never explicitly included in layered calculations, but it is known that layering FR clothing grants you at least some extra protection.

What is NFPA 70E?
The National Fire Protection Agency (NFPA) created NFPA 70E, a Standard for Electrical Safety in the Workplace. NFPA 70E standard was the first nationally recognized standard for electrical safety in the United States, and was the reference document used for the Electrical Safety-Related Work Practices (ESRWP) regulation (OSHA 29 CFR 1910.331 through .335). The first edition was released in 1976 at the request of OSHA to help provide consensus on electrical safety standards. As of 2007 it has been revised seven times with new editions in 2008 and 2009.

Useful Terms and Definitions

Second-degree burns occur at 1.2 1.2 cal/cm2. [energy]
A Cigarette lighter placed under your finger for 1 second equals roughly a 1 calorie burn.
A 100 cal/cm2 blast can reach temperatures of up to 35,000 0F in the center, and 11,000 0F on the perimeter

Arc Flash
An explosive release of energy caused by an electrical arc.
An arc flash event or arc flash blast, is a type of electrical explosion that results from a low impedance connection to ground or another voltage phase also called a “short” in an electrical system. A short circuit can occur anywhere in an electrical system, usually during maintenance work when the unexpected occurs. For instance a tool dropping, a wire slipping out of your hand a mechanical accessory falling loose and so on. A dangerous arc flash can only occur if the fault current is very high – in the range 1000 amps or more. The massive energy released in the fault instantly vaporizes the metal conductors involved, blasting molten metal and expanding plasma outward with extreme force. A typical arc flash incident can be inconsequential but could conceivably easily produce a more severe explosion. The result of the violent event can cause destruction of equipment involved, fire, injury  and even death, not only to the worker but also to anyone standing nearby. 

Electric Arc
The passage of substantial electric current through ionized air.

Arc Rating
A value of the energy necessary to pass through any given fabric to cause with 50% probability a second or third degree burn. This value is measured in calories/cm2. The necessary Arc Rating for an article of clothing is determined by a Hazard/Risk Assessment and the resulting HRC. Usually measured in terms of ATPV or EBT. 

ATPV (Arc Thermal Performance Value)
A reported value from electric arc testing. Basically, it is the measure of how much heat can be exposed to a flame resistant garment before a second degree burn injury is expected to occur. 

Electronically Safe Work Condition
When the conductor or circuit part to be worked on has been disconnected from energized parts, locked/tagged in accordance with established standards, tested to ensure the absence of voltage, and grounded if determined necessary. 

Flame resistant/Abbreviated as FR.
The characteristic of a fabric to resist ignition and to self extinguish if ignited. 

Flame retardant
A chemical substance used to impart flame resistance – not part of the basic fibers chemistry. Flame retardant treatments can diminish overtime or with use.

A sudden, unexpected and intense fire caused by ignition of flammable solids, liquids, gases or dusts. 

Flash Hazard
A dangerous condition caused by the release of energy from an electric arc. 

Flash Protection Boundary
The distance from an exposed live part within which a person could receive a second-degree burn if an electrical arc were to occur. 

HRC (Hazard Risk Category)
The classification of the listed task according to the type of hazard present when performing the task. Zero represents minimal risk, four represents the greatest risk. 
The five Hazard/Risk categories are specified by the chart listed in NFPA 70E. The chart, based on specific job tasks, ranges from HRC 0 (which is low risk and allows for 100% untreated cotton), up to HRC 4 (which is high risk and requires FR clothing with a minimum arc rating of 40). The HRC is used to determine the necessary arc rating of a garment worn during a given job task. 

  • Level 0: Little to no risk
  • Level 4: extreme risk

NFPA (National Fire Protection Association)
NFPA writes voluntary compliance standards related to the Fire Service and other industries. Also works directly with OSHA for establishing legal regulations for electrical safety. 

OSHA bases its electrical safety mandates on NFPA 70E®: Standard for Electrical Safety in the Workplace®. OSHA evaluates compliance with its electrical safety regulations, OSHA 1910 Subpart S and OSHA 1926 Subpart K, using the comprehensive information in NFPA 70E. While OSHA tells you what to do to avoid electrical dangers, this vital Standard tells you how. 

Workplace safety in the United States is evolving due to better awareness and implementation of NFPA 70E: Standard for Electrical Safety in the Workplace®. Yet hundreds of deaths and thousands of disabling injuries still occur each year due to shock, electrocution, arc flash, and arc blast — and most could be prevented through NFPA 70E compliance. The 2012 NFPA 70E responds to the challenges, making it easier to ensure an electrically safe working area and comply with OSHA 1910 Subpart S and OSHA 1926 Subpart K.

Originally developed at OSHA’s request, NFPA 70E responds to new information about the effects of arc flash, arc blast, and direct current (dc) hazards, and recent developments in electrical design and Personal Protective Equipment (PPE).

The rising demand for alternative energy systems such as photovoltaic and wind power present greater dc shock and arc flash hazard exposures to workers. To protect personnel, NFPA 70E includes a new shock protection boundary, hazard/risk table, and incident energy calculation for direct current systems. Revised Article 320 focuses on safe work practices for stationary batteries and battery rooms, such as those used by alternative energy systems.

• Revised requirements delineate the essential difference between “risk assessment” and “hazard identification.” Supporting information is in revised Annex F.

• Hazard/risk tables are expanded to include electrical system characteristics and arc-flash protection boundaries.

• The 2* designation has been eliminated to clarify that all H/R Category 2 tasks require full-head PPE.

• Change on “layering” for a combined arc-rating permits the use of arc-rated garments only.

• Clarified text for arc flash hazard warning labels

Thermal Conductivity
Rate at which heat will flow through a material. 

Thermal Protection
The resistance to flame and associated thermal transfer through the garment. 

TPP (Thermal Protective Performance)
A section of fabric is exposed with a combination of radiant and convective energy. The total energy required to cause second-degree burn injury to human tissue is determined based on heat transfer through the fabric specimen and the Stoll second-degree burn criteria. Single and multiple layer fabric specimens can be tested. 

Hazard Risk Assessment
By OSHA standards it is up to your employer to conduct a Hazard Risk Assessment and determine the required level of protection for the tasks you will be required to perform. However, we recommend that you are aware of the information that NFPA 70E attempts to protect you from and what HRC means so that you can be confident that you are adequately protected.

When conducting a Hazard Risk Assessment the electrical equipment being tested is assessed for the the potential of an explosion or ARC flash, which is also measured in Cal/cm2. Simply stated, the goal is to always have more protection than the potential energy that could be output during an explosion or ARC flash.

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