Arc Flash FAQ

NFPA 70E requires an arc flash risk assessment for all tasks involving interaction with energised equipment. Most jurisdictions mandate studies for facilities with equipment rated above 240V and available fault current exceeding 240A or 10.7 kA. Insurance companies and corporate safety policies may require studies at lower thresholds. Studies should be updated every five years or when system changes occur.

NFPA 70E defines four PPE categories based on incident energy: Category 1 (up to 4 cal/cm²) requires arc-rated long-sleeve shirt and pants; Category 2 (up to 8 cal/cm²) adds a face shield; Category 3 (up to 25 cal/cm²) requires an arc flash suit hood; Category 4 (up to 40 cal/cm²) requires a full multi-layer arc flash suit. Above 40 cal/cm², live work is generally prohibited.

Clearing time has the most significant impact on arc flash incident energy — the relationship is approximately linear. Halving the protective device clearing time roughly halves the incident energy. A fault cleared in 0.05 seconds produces one-tenth the energy of a 0.5-second clearing time. Fast-acting current-limiting fuses and instantaneous-trip circuit breakers dramatically reduce arc flash hazard levels.

The arc flash boundary is the distance from the arc source where incident energy drops to 1.2 cal/cm² — the threshold for second-degree burns on unprotected skin. Workers inside this boundary must wear arc-rated PPE. Typical boundaries range from 0.3 metres for low-energy circuits to over 6 metres for high-fault switchboards with slow protection.

Arc flash labels must display: nominal system voltage, arc flash boundary distance, available incident energy and corresponding working distance, required PPE level or minimum arc rating, and the date of the study. Labels must be applied to all equipment likely to require servicing while energised and updated when system parameters change.

The most effective methods are: reduce clearing time using current-limiting fuses or instantaneous-trip settings; install arc flash detection systems that trip in under 5ms; increase working distance by using remote racking and remote operation; reduce available fault current with current-limiting reactors; implement maintenance mode settings on protective devices; or design for de-energised work procedures.

IEEE 1584-2018 introduced five electrode configurations (VCB, VCBB, HCB, VOA, HOA) replacing the single generic model. It expanded the valid range to 208–15,000V and 500–106,000A bolted fault current. The new model includes enclosure size correction and provides more accurate results, particularly for vertical conductor configurations where the previous model significantly underestimated incident energy.

Yes, significantly. The IEEE 1584-2018 model includes enclosure width, height, and depth as inputs. Smaller enclosures concentrate arc energy more intensely toward the worker, increasing incident energy at the working distance. Larger enclosures allow energy to dissipate over a greater area. The enclosure correction factor can vary incident energy by a factor of 2 or more between typical switchboard sizes.

Yes. Arc flash can occur at any voltage where sufficient fault current exists to sustain an arc. The minimum threshold is approximately 208V AC and 240A fault current per IEEE 1584. Even at 230V, a fault current of 10kA with a 0.5-second clearing time produces dangerous incident energy. Low-voltage motor control centres with high fault levels are common locations for serious arc flash events.

IEEE 1584-2018 defines five configurations describing how conductors are arranged where arcs form. VCB (vertical in box) and VCBB (vertical with barrier) produce the highest incident energy because the enclosure focuses thermal energy toward the worker. HOA (horizontal in open air) produces the lowest. Selecting the wrong configuration can underestimate incident energy by 200–300%, leading to inadequate PPE specification.

NFPA 70E recommends updating arc flash studies whenever significant changes occur to the electrical system — including utility fault level changes, transformer additions, protective device modifications, or major load changes. As a minimum, studies should be reviewed every five years. Many facilities update continuously using real-time monitoring systems that track system parameters affecting arc flash hazard levels.