An arc flash is a sudden, explosive release of energy caused by an electrical arc passing through air between conductors or from a conductor to ground. It produces intense heat, bright light, pressure waves, molten metal and flying debris. Arc flash can cause severe burns, hearing loss, eye injuries and death, often in a fraction of a second.
Arc flash incidents commonly happen during work on or near energized electrical equipment, such as opening a panel, racking a breaker or testing for voltage. In the United States, NFPA 70E, Standard for Electrical Safety in the Workplace, is the main consensus standard for managing arc flash risk.
What Causes an Arc Flash?
An arc flash starts when current jumps through the air instead of flowing along its intended path. Common triggers include:
- Dropped tools or loose parts contacting energized conductors
- Improper test equipment or testing technique
- Dust, corrosion or moisture creating a conductive path
- Failed insulation or poorly maintained equipment
- Accidentally closing a breaker onto a fault
An arc blast is the pressure wave created by the rapidly expanding air and vaporized metal. It can throw workers and debris and is a related but separate hazard.
OSHA and NFPA 70E Requirements
OSHA's electrical safety-related work practice standards, 29 CFR 1910.331 through 1910.335, require that live parts be deenergized before an employee works on or near them, unless deenergizing introduces additional hazards or is infeasible. Under 29 CFR 1910.335, employees working where there are potential electrical hazards must be provided with and use appropriate protective equipment. For electric power generation, transmission and distribution work, 29 CFR 1910.269 includes specific requirements for estimating incident heat energy and providing arc-rated protection.
OSHA does not adopt NFPA 70E by reference, but the agency often points to it as a recognized way to meet its requirements, and it can be used as evidence of industry practice. NFPA 70E is revised on a three-year cycle.
Key Arc Flash Concepts
Electrically safe work condition
NFPA 70E's primary protective strategy is to establish an electrically safe work condition before work begins. That means identifying all sources, disconnecting them, applying lockout/tagout, testing for absence of voltage and grounding where needed. Energized work is permitted only when justified, such as when deenergizing creates a greater hazard or is infeasible.
Arc flash risk assessment
NFPA 70E requires an arc flash risk assessment to determine whether an arc flash hazard exists, estimate the likelihood and severity of injury, and select protective measures, including PPE. The assessment must be updated when a major modification or renovation occurs and reviewed periodically at intervals not to exceed 5 years.
Arc flash boundary and incident energy
Incident energy is the amount of thermal energy a worker could receive at a given working distance, measured in calories per square centimeter (cal/cm²). The arc flash boundary is the distance from the source at which incident energy equals 1.2 cal/cm², the level associated with the onset of a second-degree burn on bare skin. Anyone inside the boundary needs arc-rated protection.
PPE categories and labels
Employers can select PPE using either an incident energy analysis or the PPE category method in NFPA 70E's tables. The category method uses four PPE categories, with minimum arc ratings of 4, 8, 25 and 40 cal/cm². Electrical equipment likely to be examined or serviced while energized must be field-marked with a label that includes information such as nominal voltage, the arc flash boundary, and either the available incident energy and working distance or the PPE category.
How to Manage Arc Flash Risk Step by Step
- Develop an electrical safety program. Document policies, responsibilities, procedures and training for electrical work.
- Perform arc flash risk assessments. Conduct an incident energy analysis or apply the PPE category tables for each piece of equipment.
- Label equipment. Apply and maintain arc flash labels with current data.
- De-energize whenever possible. Make an electrically safe work condition the default.
- Justify and permit energized work. When energized work is necessary, use an energized electrical work permit and a documented job briefing.
- Provide and inspect PPE. Supply arc-rated clothing, face and hand protection rated for the hazard, and inspect it before each use.
- Train qualified workers. Train employees on the hazards, procedures and PPE, and retrain at intervals not exceeding 3 years under NFPA 70E.
- Maintain equipment. Poor maintenance can increase clearing times and incident energy.
Common Mistakes to Avoid
- Treating energized work as routine instead of an exception that must be justified.
- Relying on outdated arc flash labels after system changes.
- Wearing everyday cotton or synthetic clothing that can ignite or melt under arc-rated layers.
- Skipping the job briefing or failing to define approach boundaries.
- Assuming low-voltage panels cannot produce a dangerous arc flash.
How SMS360 Helps
SMS360's Work Permits module helps teams manage permits for high-risk tasks, such as energized electrical work, digitally. Supervisors can create permits, document hazards and controls, assign workers and route permits for approval from a mobile device. Completed permits are stored as searchable records, and dashboards show open permits across sites.
Frequently Asked Questions
What is the difference between an arc flash and an electric shock?
An electric shock happens when current passes through the body after contact with an energized part. An arc flash happens when current travels through the air, releasing intense heat and light that can burn a worker without any direct contact. Both hazards must be assessed, and NFPA 70E addresses them with separate shock and arc flash risk assessments.
What is the arc flash boundary?
The arc flash boundary is the distance from an arc source at which the incident energy equals 1.2 cal/cm². At that level, unprotected skin could receive a second-degree burn. Workers inside the arc flash boundary must wear arc-rated PPE appropriate for the hazard.
Does OSHA require NFPA 70E compliance?
OSHA has not incorporated NFPA 70E by reference, so it is not directly enforceable as a regulation. However, OSHA requires employers to protect workers from electrical hazards, including arc flash, under standards such as 29 CFR 1910.333 and 1910.335 and the General Duty Clause, and NFPA 70E is widely used to show how those requirements can be met.
What are the arc flash PPE categories?
NFPA 70E defines four PPE categories for its table method. The minimum arc ratings of clothing are 4 cal/cm² for category 1, 8 cal/cm² for category 2, 25 cal/cm² for category 3, and 40 cal/cm² for category 4. Each category also specifies required face, head, hand and foot protection.
