Arc flash FAQs

An arcing fault is current flowing through the air between phase conductors, or between a phase conductor and neutral or ground. It can release enormous concentrated radiant energy in a fraction of a second: extreme temperatures, a pressure blast, and shrapnel traveling in excess of 700 mph.

Arcs occur when current flows between separated energized conducting surfaces. Common causes include human error (dropped tools, accidental contact, improper procedures), insulation failure from dust and corrosion buildup, sparks from racking breakers or replacing fuses, and even animals bridging conductors.

No. A hazard assessment is required to determine the energy level (in calories) released by a given arc, which then determines the proper PPE needed.

NFPA 70E defines incident energy as the amount of thermal energy impressed on a surface, at a certain distance from the source, generated during an electrical arc event.

Exposure risk depends on several factors:

• How often a worker performs tasks on exposed live equipment
• Task complexity: force required, available space, safety margins, reach
• Training, skill, mental and physical agility, and coordination with a helper
• Tools used
• Condition of the equipment

Exposure frequently causes serious injury and, in some cases, death. Workers have been hurt from 10 feet or more away, with permanent hearing or eyesight loss and severe burns requiring years of rehabilitation. Equipment can be destroyed, causing costly downtime. Treatment for an injured worker can exceed $1 million per case, before litigation, insurance, and lost production are even counted.

NFPA 70E defines it as a source of possible injury or damage to health associated with the release of energy caused by an electric arc.

An engineering study that determines how much current could flow at any point in an electrical system, and how quickly the nearest protective device would need to operate to clear a fault.

Depending on the calculation method, you'll need the enclosure type, gap dimension between exposed conductors, grounding type, phase/connection configuration, and working distance.

The distance from the arc source at which the incident heat energy on the surface of the skin from an arcing fault reaches 1.2 calories per square centimeter.

It's based on voltage, available short-circuit current, and predicted fault duration. NFPA 70E offers three acceptable methods:

• Simplified Table 130.7(C)(15)(a)+(b)
• Analysis based on NFPA 70E Annex D
• Analysis based on IEEE 1584

The boundary around exposed live parts that unqualified persons may not cross unless accompanied by a qualified person.

The area near exposed live parts that only qualified persons may cross, and only while using appropriate shock-prevention techniques and equipment.

NFPA 70E estimates incident energy from a theoretical maximum value of power dissipated during arcing faults, and is generally considered conservative. IEEE 1584 instead uses empirical equations built from statistical analysis of actual laboratory test measurements.

Every method has limitations. NFPA's tables are easy to use but only approximate typical equipment and systems, while detailed analysis yields different, more site-specific results. Whichever standard you use, understand its limitations. Evaluating with multiple methods and comparing results helps identify the best fit for your facility.

The Petroleum and Chemical Industry Committee (PCIC) recommends completing arc flash calculations alongside short circuit calculations and protective device coordination, since arc flash boundaries depend on voltage, available short-circuit current, and predicted fault duration, so sequencing the studies this way produces the most accurate results.

Typically: equipment type, voltage, withstand rating, MVA/KVA, impedance, X/R ratio, and phase/connection configuration.

• Relays: type, CT ratio, pickup (tap) setting, delay type/curve, and time dial setting
• Fuses: type, amp rating, voltage, and peak let-through current
• Circuit breakers: type, fault clearing time, pickup setting, delay curve, and delay setting

NEC 110.16 only requires the label to state that an arc flash hazard exists. NFPA 70E 130.5(H) goes further, requiring:

• Nominal system voltage
• Arc flash boundary
• At least one of: available incident energy and corresponding working distance (or the PPE category from Table 130.7(C)(15)(a)/(b), but not both), minimum arc rating of clothing, or site-specific PPE level

Limited and restricted shock approach boundaries are also suggested hazard-specific parameters.

A number, based on incident energy, representing the level of danger, ranging from Category 1 (little or no risk) to Category 4 (the most dangerous level at which energized work may still be performed).

The energy required to raise one gram of water by one degree Celsius at one atmosphere. Second-degree burns begin at 1.2 calories per square centimeter per second, roughly equivalent to holding a finger over a lighter flame for one second.

Incident energy must be known at every location workers may perform energized work, calculated by a qualified person such as an electrical engineer. Every part of the body that could be exposed needs appropriate coverage, which can include arc-rated clothing, hard hat, face shield, safety glasses, gloves, and footwear, scaled to the magnitude of the arc energy.

Four main regulations govern arc flash:

• OSHA 29 CFR 1910: subpart S (1910.333) covers work practices and references NFPA 70E; subpart R (1910.269) covers power generation, transmission, and distribution.
• NFPA 70 (NEC): contains warning label requirements.
• NFPA 70E: guidance on work practices to safeguard workers near energized parts.
• IEEE 1584: a method for calculating incident energy to define safe working distance and select PPE.

OSHA enforces workplace safety practices: employers must assess hazards, select and use correct PPE, and document the assessment under 1910.132(d). OSHA doesn't directly enforce NFPA 70E, but recognizes it as industry practice, and its field inspectors carry a copy for reference. Electrical inspectors separately enforce NEC labeling requirements.

Arc flash first drew attention in the early 1980s following Ralph Lee's paper, which documented the electric arc blast hazard. As similar research showed too many workers were being injured, the petrochemical industry led the way on protective practices, and standards like NFPA 70E followed to reduce exposure to shock, electrocution, arc flash, and arc blast.

Someone who has demonstrated skills and knowledge related to the construction, operation, and installation of the electrical equipment, and who has received safety training to recognize and avoid the associated hazards.

A state in which the conductor or circuit part has been disconnected from energized parts, locked/tagged per established standards, tested to confirm absence of voltage, and grounded where necessary.

De-energized is always preferable. OSHA 1910.333(a) permits work on live equipment over 50 volts only when de-energizing would introduce additional hazards, or when it's infeasible due to equipment design or operational limits. Even then, full safe work practices, risk assessment, PPE, and proper tools are required.

Preventive maintenance, worker training, and an effective safety program significantly reduce exposure. Equipment should be routinely cleaned and inspected by qualified personnel who can spot loose connections, overheated terminals, discoloration, and pitted contacts. A solid maintenance plan should also include:

• Verifying relays and breakers are set correctly and operate properly
• Sealing open areas of equipment against rodents and birds
• Using corrosion-resistant terminals and insulating exposed metal parts where possible

Every arc flash calculation depends on arc clearing time, which comes from the engineering coordination study and assumes protective devices operate as intended. Without regular maintenance and testing, clearing times can extend unexpectedly due to open or shunted current transformers, open coils, or dirty contacts, throwing off the flash hazard analysis, the boundary, and the recommended PPE. NFPA 70E 205.3 and 205.4 outline the maintenance requirements this protects against.

Incident energy is shaped by system configuration, fault levels, and exposure time. Fault levels can be reduced with current-limiting devices like fuses, breakers, and reactors, or by reconfiguring the system. Faster-acting relays and trip devices, instantaneous relays, and appropriately sized fuses all shorten arcing/exposure time, backed by a protective device coordination study to confirm settings are correct.

NFPA 70E Article 130.5(G) requires review and update whenever the electrical distribution system changes in a way that could affect arc flash hazard levels, and at a minimum every five years. If your study predates 2021 and hasn't been reviewed since, it may not reflect current NFPA 70E requirements. Studies based on the pre-2018 IEEE 1584 calculation model may also carry incorrect incident energy values.

Yes. Adding a transformer, replacing a breaker, changing protective device settings, or modifying your distribution system can all change the available fault current and clearing times at downstream equipment, which directly affects incident energy calculations. Any system change that could affect arc flash hazard levels triggers a reassessment requirement under NFPA 70E.

OSHA doesn't mandate a specific document called an "arc flash study," but OSHA 1910.132(d) requires employers to assess workplace hazards and select appropriate PPE. OSHA field inspectors carry NFPA 70E and treat it as the recognized industry standard for electrical safety compliance. Facilities without a current arc flash assessment, accurate labels, and a documented PPE program are routinely cited. NEC 110.16 independently requires arc flash warning labels on all electrical equipment that may be energized during maintenance.

It depends. If your electrical distribution system hasn't changed and no major regulatory updates have occurred since the study was performed, targeted label updates may be sufficient. However, if your study predates IEEE 1584-2018, the incident energy calculations behind those labels are based on an older model and may not be accurate. ERS can assess your existing study and determine whether a full reassessment or a targeted update is the right call.

Yes. The 2026 National Electrical Code removes the prior 1,000A threshold from NEC 110.16, which previously let some equipment skip arc flash warning labels. Under the 2026 NEC, all electrical equipment that may be energized during service or maintenance requires a label, regardless of ampere rating. Facilities that relied on the prior threshold may now have unlabeled equipment that's out of compliance.

A complete arc flash study from ERS includes:

• A field site survey and data collection
• Single-line diagram development or verification
• Short circuit analysis
• Protective device coordination study
• Arc flash calculations per IEEE 1584-2018
• Incident energy values at all equipment locations
• PPE category designations and flash protection boundaries
• Arc flash warning labels
• A stamped engineering report

The study is performed by NETA-certified technicians in the field and sealed by a registered professional engineer.

Timeline varies by facility size and complexity. A small-to-medium industrial facility typically requires one to three days of field data collection, followed by engineering analysis and report preparation. Larger or more complex facilities with extensive distribution systems take longer. ERS provides a project timeline as part of the proposal.

Yes. ERS performs arc flash studies on energized systems. Field data collection is conducted by NETA-certified technicians following NFPA 70E safe work practices. De-energizing isn't required for the study itself, though some mitigation work identified afterward may require a planned outage.

Existing single-line diagrams (if available), equipment nameplates and ratings, utility fault current data, and access to electrical equipment for field verification. If current single-line diagrams don't exist or are out of date, ERS can develop them as part of the engagement.

Project cost depends on facility size, the number of equipment locations in scope, and whether single-line diagrams need to be developed or updated. ERS provides fixed-price proposals based on a defined scope. Contact us for a quote, most studies can be scoped within 24 to 48 hours of receiving basic facility information.

These terms are often used interchangeably, but they're different activities. The arc flash study (or analysis) is the engineering calculation. It determines incident energy levels at each piece of equipment using IEEE 1584. The arc flash risk assessment is the NFPA 70E-required process of identifying hazards, selecting safe work practices, and determining appropriate PPE based on the study results. A complete program requires both.

At minimum: install arc flash warning labels on all equipment covered by the study, update your electrical safety program to reflect the new incident energy values and PPE requirements, and train workers on the updated requirements. ERS can assist with labeling, PPE program development, training, and, where study results reveal high incident energy levels, mitigation engineering to reduce hazard levels and PPE requirements going forward.

High incident energy is an engineering problem with engineering solutions. Options include:

• Modifying protective device settings to reduce clearing time
• Installing zone-selective interlocking or bus differential protection
• Reducing available fault current through system configuration changes
• Remote racking or remote switching to eliminate worker exposure

ERS's mitigation services evaluate your specific system and identify the most cost-effective ways to reduce incident energy, often significantly.