Arc Flash Causes & Triggers Explained
Arc flash is caused when an electrical fault creates a conductive path through the air between conductors or between a conductor and a ground. This fault generates an electric arc with a core temperature of 35,000 °F (roughly four times hotter than the surface of the sun) with radiative heat reaching 7k-10k °F , releasing an explosive burst of energy, light, heat, and pressure in a fraction of a second.
Understanding what causes an arc flash is the critical first step toward preventing one. The triggers generally fall into four categories: electrical fault conditions, equipment failures, environmental contamination, and human error. Below, we break down each category so you can identify the risks present in your facility and take action before an incident occurs.
For a foundational overview of the hazard itself, see our Arc Flash Basics guide.
How Does an Arc Flash Occur?
An arc flash occurs when electrical current leaves its intended path and travels through the air from one conductor to another, or from a conductor to a ground. Air is normally an insulator, but under the right conditions (voltage, air gap, and a triggering event) the air between conductors can ionize and become conductive. Once that happens, an electric arc forms and sustains itself by superheating the surrounding air into plasma.
The result is a violent release of energy known as an arc flash event. According to IEEE 1584-2018, the incident energy released during an arc flash depends on variables like available fault current, the duration of the arc, the distance from the arc, equipment dimensions, and the equipment’s electrode configuration. These variables are the basis for the arc flash hazard calculations required by NFPA 70E.
The dangerous reality is that an arc flash can occur in any electrical system, from 208-volt panels to high-voltage (>600V) switchgear, whenever conditions align to create a fault.
Fault Conditions That Cause Arc Flash
Electrical faults are the direct, initiating cause of nearly every arc flash event. A fault occurs when current flows along an unintended path, and several specific fault types can trigger an arc.
Insulation Failure
Insulation is the primary barrier keeping electrical current on its intended path. Over time, insulation degrades due to heat cycling, chemical exposure, moisture ingress, mechanical damage, or simply age. When insulation breaks down, it can no longer prevent current from arcing between conductors. The National Institute for Occupational Safety and Health (NIOSH) identifies insulation failure as one of the leading precursors to electrical arc events in industrial settings.
In some circuits electrical copper buses are exposed (not covered with insulation) and rely on air as the insulating medium. Dropping a metal tool or metal part between these bare conductors is also a form of (air) insulation failure.
Short Circuit: Phase-to-Phase Faults
A short circuit occurs when two conductors at different potentials make direct or near-direct contact. Phase-to-phase faults, where two or three phases of a power system come into contact, release enormous amounts of energy because of the high available fault current in these systems. In industrial facilities with large transformers and distribution equipment, available fault currents can exceed 50,000 amperes, producing devastating arc flash events if protective devices do not clear the fault quickly enough.
Short Circuit: Ground Faults
A ground fault happens when current finds an unintended path to ground. While ground faults may start with lower energy than phase-to-phase faults, they can still produce dangerous arcing conditions, particularly in systems where ground-fault protection is improperly set or absent. Ground faults that persist due to slow or failed protective relay operation escalate the incident energy significantly.
Insulation failures that create an arc flash will produce Short Circuit Faults (either between phases or phase(s) and ground.
Equipment-Related Causes
The condition and quality of electrical equipment directly influence arc flash risk. Equipment that is old, poorly maintained, or improperly installed creates the conditions under which faults, and therefore arc flash events, are far more likely to occur.
*Aging or Deteriorating Equipment
Electrical equipment does not last forever. Circuit breakers, contactors, bus bars, and switchgear assemblies all degrade over time. Internal components can crack, warp, or corrode. The Electrical Safety Foundation International (ESFI) reports that equipment failure is a contributing factor in a significant portion of workplace electrical incidents each year. Regular maintenance and thermographic (Infrared or IR) scanning can catch deterioration before it leads to an arc flash.
*Loose or Corroded Connections
Normal loading and unloading of electrical equipment result in thermal expansion and contraction of electrical equipment.This can cause previously torqued connection to come loose. Loose electrical connections generate heat through increased resistance at the contact point. This heat accelerates corrosion, which further increases resistance, creating a feedback loop that eventually leads to failure. A corroded bus bar joint or a loose lug on a breaker can become the ignition point for an arc flash. This is why torque verification and infrared inspection of connections are critical elements of a preventive maintenance program.
Improper Installation or Undersized Components
Using components rated below the available fault current of a system creates a dangerous mismatch. Undersized breakers, improperly rated fuses, or incorrectly installed bus work may fail to interrupt a fault quickly enough, or may themselves become the point of failure. Ensuring that equipment ratings match system requirements, as outlined in OSHA’s electrical safety standards (29 CFR 1910 Subpart S), is a baseline requirement for arc flash prevention.
* – Regular thermal imagaing is now mandated by the NFPA 70B Standard.
Environmental and Contamination Triggers
Even properly installed and maintained equipment can be pushed into an arc flash event by external environmental factors. These triggers compromise the insulation and clearances that normally keep electrical systems safe.
Dust, Debris, and Conductive Particles
Accumulation of dust, particularly conductive dust from metal grinding, carbon, or chemical processes, on electrical components can create a path for current to arc across conductors. Facilities that produce airborne particulates need rigorous housekeeping and equipment enclosure strategies to keep conductive contamination away from energized parts. NFPA 70E emphasizes that maintaining equipment in a clean condition is part of establishing an electrically safe work environment.
Moisture and Condensation
Water is a conductor. When condensation forms inside electrical enclosures, which is common in environments with fluctuating temperatures or high humidity, it can bridge the gap between conductors and initiate an arc. Facilities in coastal, tropical, or seasonally variable climates face elevated risk from moisture intrusion, making enclosure integrity and environmental controls important preventive measures.
Animal and Pest Intrusion
Animals, particularly rodents and snakes, entering electrical enclosures is a well-documented cause of arc flash. When an animal contacts energized conductors simultaneously or bridges the gap between a conductor and a grounded enclosure, the result is an immediate fault. Utilities and industrial facilities routinely install pest barriers and sealed enclosures to mitigate this risk.
Human Error and Procedural Failures
Human actions remain a significant cause of arc flash events. Even in well-maintained systems with quality equipment, procedural failures and errors by workers can create the conditions for a catastrophic arc.
Dropped Tools and Accidental Contact
A dropped wrench, screwdriver, or other conductive tool inside an energized panel can bridge conductors and create an immediate short circuit. Accidental contact with energized parts, whether from a tool slipping or a worker reaching into the wrong section of a panel, is one of the most common triggering events for arc flash in the workplace. This is precisely why NFPA 70E requires that workers use insulated tools and maintain appropriate approach boundaries around energized equipment.
To understand how those boundaries work, see our guide on arc flash protection boundaries.
Working on Energized Equipment
Performing maintenance, testing, or inspection on energized electrical equipment exposes workers directly to arc flash risk. While NFPA 70E establishes a hierarchy of risk controls that prioritizes de-energizing equipment, there are situations where energized work is necessary. In those cases, a documented energized work permit, proper PPE selection based on incident energy analysis, and strict procedural controls are required.
Knowing how to read and interpret arc flash labels is essential for any worker who may encounter energized equipment.
Failure to Follow Lockout/Tagout (LOTO) Procedures
Lockout/tagout failures, whether from skipping the procedure entirely, applying it incorrectly, or failing to verify zero-energy state, are a recurring factor in arc flash incidents investigated by OSHA. When equipment that a worker believes is de-energized is actually still live, the risk of accidental contact and a resulting arc flash is severe. OSHA’s lockout/tagout standard (29 CFR 1910.147) exists specifically to prevent this scenario.
Frequently Asked Questions
What Is the Most Common Cause of Arc Flash?
Equipment failure and insulation breakdown are the most common root causes of arc flash. These conditions develop over time through aging, poor maintenance, environmental exposure, and thermal cycling. Human error, particularly accidental contact with energized parts and dropped tools, is the most common immediate trigger that initiates the arc event itself. Accident investigation footage shows arcs initiating while workers were removing covers from electrical equipment and moving electrical wires while trying to trace a circuit.
Can Arc Flash Happen in Low-Voltage Systems?
Yes. Arc flash can occur in systems operating at voltages as low as 208V. While higher voltages increase the likelihood and severity of an arc, the critical factor is available fault current and arc duration, not voltage alone. Many serious arc flash injuries occur in 480V systems, which are common in commercial and industrial buildings.
IEEE 1584-2018 provides calculation methods for systems from 208V to 15,000V. (It is important to note the IEEE1584-2018 has new rules for low voltage circuits. If you have had a study performed pre-2018, please see this reference for moe information.
How Hot Does an Arc Flash Get?
An arc flash can generate extremely high temperatures, with the core of the arc reaching up to 35,000 °F (19,400 °C). However, most workers are exposed to the surrounding plasma and radiated heat, which are typically below 10,000 °F but still more than sufficient to cause severe injury.
Regardless of the exact temperature at a given point, arc flashes will melt materials inside electrical equipment, produce an intense fireball, and eject superheated plasma that can ignite non-arc-rated clothing. The event also creates molten metal spray and splatter, along with additional hazards such as pressure waves, shrapnel, intense light, and sound.
What Is the Difference Between Arc Flash and Arc Blast?
Arc flash refers to the thermal and light energy released by an electric arc. Arc blast refers to the pressure wave that accompanies the arc flash event. When an arc vaporizes copper or aluminum conductors, the rapid expansion of the metal (copper expands by a factor of 67,000 when vaporized) creates an explosive pressure wave that can throw workers off their feet, collapse lungs, and cause hearing damage. Both hazards occur simultaneously during an arc flash event.
Taking the Next Step
Arc flash events are caused by a convergence of fault conditions, equipment deterioration, environmental contamination, and human error. No single factor acts in isolation. A corroded connection in a dusty enclosure accessed by an untrained worker represents multiple overlapping risks.
The most effective defense is a comprehensive approach that includes regular equipment maintenance, arc flash hazard analysis per IEEE 1584, proper labeling, worker training, and compliance with NFPA 70E. Understanding what causes arc flash is the foundation; acting on that understanding is what prevents injuries.
Sources
- IEEE 1584-2018, IEEE Guide for Performing Arc-Flash Hazard Calculations. Institute of Electrical and Electronics Engineers. https://standards.ieee.org/ieee/1584/5765/
- NFPA 70E-2024, Standard for Electrical Safety in the Workplace. National Fire Protection Association. https://www.nfpa.org/codes-and-standards/nfpa-70e-standard-development/70e
- National Institute for Occupational Safety and Health (NIOSH). Electrical Safety in the Workplace. Centers for Disease Control and Prevention. https://www.cdc.gov/niosh/topics/electrical-safety/
- Electrical Safety Foundation International (ESFI). Workplace Electrical Injuries and Fatalities. https://www.esfi.org/workplace-electrical-injuries-and-fatalities/
- Occupational Safety and Health Administration (OSHA). 29 CFR 1910 Subpart S — Electrical. U.S. Department of Labor. https://www.osha.gov/electrical
