Glossary of Arc Flash Terms

Arc flash terminology can be a foreign language. We’ve compiled this glossary of arc flash terms to help you understand the arc flash testing process. We’ve also defined key equations and summarized important regulations.

Arc Flash. Also known as an arc blast, an arc flash is an explosive release of energy caused by a current passing between two points. Arc flashes usually result from dust, damage, improper installation or accidental contact with electrical systems.

The four types of arcs are:

  • Arc in a box
  • Ejected
  • Open air
  • Tracking

Arc flashes usually include high temperatures (up to 35,000 F) and a pressure wave. They can result in burn injuries or death. The NFPA estimates that approximately 2,000 people are admitted to burn centers with injuries from arc flash events each year.

Arc Flash Injuries per year

Arc Flash Audit. An analysis of the risk of an arc flash incident. Audits include a systematic assessment of potential hazards and suggestions for reducing the associated risk. Many audits measure compliance to the various standards that govern arc flash risks, including OSHA, NFPA and NESC codes.

Arc Flash Boundary. The maximum distance from an energy incident at which a worker requires protective equipment. The arc flash boundary corresponds to a maximum energy level of 1.2 cal/cm squared.

Arc Flash Calculations. A nine step process for calculating the risks of potential arc flash incidents. Governed by IEEE 1584 (Guide for Performing Arc Flash Hazard Calculations).

Arc Flash Hazard Assessment. The process of modeling, or calculating, the incident energy levels to which workers might be exposed. It’s commonly calculated by engineers or other trained personnel because of the difficulty of collecting accurate data. Most hazard assessments use the process of calculating arc flash hazards outlined in IEEE 1584.

Arc Flash Analysis / Arc Flash Study. See arc flash audit.

Arc Rating. The rating applied to personal protective equipment. Arc ratings refer to the value of energy needed to pass through a layer of clothing and cause a second or third degree burn with reasonable (at least 50 percent) probability. Minimum arc ratings required for personal protective equipment are determined by the associated HRC.

Arc Thermal Performance Value (ATPV). The maximum arc flash protection provided by a material. Usually measured in calories per square centimeter.

ANSI. Acronym for the American National Standards Institute. ANSI is a non-profit that sets voluntary standards for products, personnel and processes in the U.S.

ANSI/ISEA 125. American National Standard for Conformity Assessment of Safety and Personal Protective Equipment. This standard offers a model that manufacturers can use to assess the performance level of personal protective equipment.

ANSI 107. A voluntary standard for fire resistance for high-visibility clothing. Includes standards for design, performance and labeling of fire resistant high-visibility clothing. This standard applies to construction, utility and maintenance workers who work in low light areas.

ASTM. Acronym for the American Society of the International Association for Testing and Materials. ASTM is an international organization that sets voluntary standards for products and personnel.

Calorie. The amount of energy required to raise the temperature of one gram of water by one degree Celsius at one atmosphere of pressure.

Cal/cm². Calories per square centimeter. This is an equation is usually used to express an arc rating, and it’s often applied to personalized protective equipment. 1.2 cal/cm² generates enough heat to cause a second degree burn. 40 cal/cm² can cause serious injury or death and requires multiple layers of protection.

CGSB. Acronym for the Canadian General Standards Board. The CGSM is a government organization that sets health and safety standards in Canada.

CSA. Acronym for the Canadian Standards Association. The CSA is a non-profit organization that creates voluntary standards in many areas, including health and safety.

De-energized. Has no electrical connection or charge.

Electrical Safety. Awareness of conditions in which making contact with equipment can cause an electrical shock or arc flash burn. Recognizing when equipment failure can cause shocks, arc flashes or fires is also considered part of electrical safety.

Energized Work. Work with components that are electrically connected or otherwise have a charge or current.

Energy Break Open Threshold (EBT). The energy needed to cause materials to break open and expose the wearer directly to incident energy. Like ATPV, the EBT is calculated by calories per centimeter squared. A “break open” is considered a tear or hole of at least .5 inches squared.

Flash Fire. An intense fire characterized by a rapidly moving flame front, a high temperature and a relatively short duration, commonly less than three seconds. Flash fires are common when fuel and air combust. The heated air can cause smoke burns and damage to the lungs.

what is a flash fire

Flashover. An electrical arc that occurs between exposed conductors. Also known as arc flash.

Flame Resistant. Flame resistant, or fire resistant, materials are textiles that resist catching fire. Both terms are used interchangeably, and the acronym FR is common. These materials do not drip or melt when exposed to high temperatures. Examples of flame resistant materials include Nomex and Indura FR.

Flame Retardant. Flame retardant materials are textiles that have been chemically treated to self extinguish when exposed to flames. Treated cotton is a common flame retardant material. Flame resistant and fire resistant are used interchangeably.

Hazard/Risk Category (HRC). Hazard/Risk Categories (HRC) are defined in NFPA 70E. Each of the five categories correspond to a maximum incident energy level. HRC categories are used to define the minimum standards of personal protective equipment that should be used.

The NFPA provides a table of common working situations and the corresponding HRC category.

Hazard/Risk Category 0 (HRC 0). HRC 0 defines the activities that have no risk associated with them. These activities do not require the use of PPE. Examples include:

Hazard Risk Category 1 (HRC 1). HRC 1 is a hazard category that includes only tasks that have a low risk of incident energy. These tasks require PPE that has a minimum arc rating of 4. Appropriate PPE for HRC 1 tasks include flame resistant shirt and pants (one layer). Tasks that fall into the HRC 1 category include:

  • Removing or installing circuit breakers and fused switches
  • Operating circuit breakers with the covers off
  • Removing a cable trough or tray cover

Hazard Risk Category 2 (HRC 2). HRC 2 is the second of the five hazard/risk categories that require personalized protective equipment. Tasks that fall into this category have a moderate level of risk and require a PPE with a minimum arc rating of 8. This corresponds to one or two layers of clothing, including flame resistant shirt and pants and cotton underwear. Tasks in this category include:

  • Testing voltages working on energized parts
  • Working on control circuits with energized parts of more than 120 volts exposed

Hazard Risk Category 3 (HRC 3). HRC 3 tasks have a moderately high risk of an associated energy incident. These tasks require the use of PPE that has a minimum arc rating of 25, which is generally achieved by two to three layers of clothing. Associated PPE include:

  • Cotton underwear
  • Fire retardant shirt and pants


  • Fire retardant coveralls or cotton underwear
  • Two fire retardant coveralls

HRC 3 activities include working on control circuits with energized parts carrying more than 120 volts or racking starters from cubicles with the doors open.

Hazard Risk Category 4 (HRC 4). Hazard Risk Category 4 is reserved for tasks with the highest level of risk of an associated energy incident. These tasks require the use of three or more layers of protective clothing, such as cotton underwear, flame retardant shirt and pants and a multilayer flash suit. PPE for HRC 4 tasks must have a minimum arc rating of 40. These tasks include:

Hazard Study. See arc flash hazard assessment.

High Voltage. Accounting to the NEC, high voltage is equipment with a voltage of more than 600. The IEEE defines high voltage as equipment with more than 1000V.

what is the definition of high voltage according to NCE and IEEE standards

IEC. Acronym for the International Electrotechnical Commission. The IEC creates international standards and assessments for electrical equipment.

IEEE. Acronym for the Institute of Electronics and Electrical Engineers, a professional organization for electrical engineers. IEEE also creates voluntary standards for electrical products.

Incident Energy. Refers to the radiation that a worker encounters when an arc flash occurs. Usually measured in calories per square centimeter. Incident energy of at least 1.2 cal/cm2 can result in second degree burns. Calculating the incident energy requires knowing how long an arc lasts (timing), distance from the source of the arc and the current.

ISO 9001. An international standard governing the fundamentals of quality management systems. It includes a focus on customer service and continuous improvement. The ISO 9001 standard is one of the most widely used quality management systems in the world. Products and organizations can be certified as ISO 9001 compliant by several third party organizations.

Limited Approach Boundary. The radius around equipment in which an arc flash can create a shock hazard.

Line Side. Energy that feeds into the main circuit breaker. The line side has the highest incident energy. Line side energy is not turned off by turning off the main circuit breaker within a panel.

Load Side. Energy that comes from the circuit breaker panel and feeds into outlets or appliances. Load side energy is usually protected by the outgoing feeders and can be turned off at the main circuit breaker. There is lower incident energy associated with arc flashes on the load side than there is on the line side.

Low Voltage. Equipment with less than 600 volts.

National Electric Code (NEC). The NEC is a regional code that governs the safe installation of electrical wiring and equipment. This code is published by the National Fire Protection Agency and focuses on residential, commercial and industrial wiring. The code is voluntarily adopted by cities and states and can be amended to match local legislation. It’s also known as NFPA 70.

National Electric Safety Code (NESC). A safety code published by the IEEE. Sets standards for installing, operating and maintaining electrical and communication utilities. It is a voluntary standard that has been adopted by a majority of states. Not to be confused with the NEC.

NFPA. Acronym for the National Fire Protection Agency. A US trade association that creates and maintains standards for fire safety.

NFPA 70E. Standard for Electrical Safety in the Workplace. Updated every three years. This standard governs safety standards for avoiding electrocution, arc blast, arc flash and shock. Includes guidance for assessing risks, identifying hazards, maintenance and personal protective equipment. It also requires employers to conduct regular audits and inspections and to maintain proper records.

NFPA 2112. Standard governing flame resistant protective equipment and clothing for electrical personnel. Sets performance standards and testing methods for personal protective equipment.

Occupational Health and Safety Administration (OSHA). The Occupational Health and Safety Administration (OSHA) is a federal agency that creates health and safety standards for the workplace. It’s part of the Department of Labor, and its mission is to ensure businesses provide safe and healthy workplaces. OSHA also investigates businesses to ensure their compliance with health and safety standards.

Personal Protective Equipment (PPE). Personal Protective Equipment (PPE) refers to equipment and clothing that are designed to protect employees from hazards in the workplace. Hard hats, gloves, full body suits and ear plugs are all examples of PPE. OSHA regulations require employers to conduct a hazard assessment of the workspace and train employees who need to use personal protective equipment.

Prohibited Approach Boundary. A radius around live equipment that represents a higher shock hazard. Placing oneself within the prohibited approach boundary is considered the same as making contact with the equipment itself.

definition of prohibited approach bounding

Restricted Approach Boundary. A radius around live equipment that represents an increased risk of shock. Within the restricted approach boundary, inadvertent movements can contribute to shock.

Working Distance. The minimum distance at which a worker will be when an arc flash occurs. The working distance is based on the type of equipment and its voltage. Potential energy output will always be highest at the working distance. Working distance is used to calculate the worst case incident energy.

Looking for more information about electrical safety? Check out our arc flash training course to help you assess hazards and prevent arc flashes on the job.