What is an Electric Arc?
An electric arc refers to the phenomenon where electric current flows between two electrodes through ionized air. Arcs can be intentionally created and have various applications, such as welding; natural arcs include lightning. However, accidental arcs in power distribution systems or electrical equipment can cause significant and hazardous damage.
Causes of Arc Flash
Arc flash incidents primarily result from three categories of factors:
(1) Environmental Factors: Such as humidity, dust accumulation, or accidental contact by insects or small animals. (2) Equipment Factors: Poor-quality electrical panels, lack of regular maintenance, or equipment malfunction. (3) Human Factors: Accidental contact with live components by maintenance tools, tools left inside panels, or operational errors during procedures.
Hazards of Arc Flash
Arc flash remains one of the most critical hidden risks for electrical workers, second only to electric shock. During a short-circuit arc in electrical equipment, temperatures at the arc point can reach 19,400°C—four times the surface temperature of the sun. This extreme heat vaporizes or melts copper electrodes and surrounding materials, triggering explosive gas expansion. Toxic gases, molten metal, and fragmented components are ejected with intense light and noise, posing risks of burns, impact injuries, vision loss, and hearing damage. Comprehensive protection requires face shields, earplugs, arc-rated clothing, insulated gloves, and insulated footwear or heavy-duty boots.
Arc Flash Hazard Distance
When an arc flash occurs, explosive waves radiate outward from the arc core. Establishing a defined arc flash hazard distance is critical. Personnel must strictly adhere to this distance to mitigate harm.
Arc Flash Boundary
- Within this boundary, individuals are exposed to energy levels sufficient to cause second-degree burns.
Limited Approach Boundary
- Only qualified and trained personnel may enter this zone. Unqualified personnel require escort by qualified personnel for access.
Restricted Approach Boundary
- Entry is permitted only for trained personnel wearing full arc-rated protective gear. Unqualified individuals are prohibited from entering, even partially or with handheld objects.
Working Distance
- The distance between a worker’s face/body and the potential arc flash point.
NFPA 70E
NFPA 70E, developed by the U.S. National Fire Protection Association, is the leading electrical safety standard. It provides detailed guidance on arc hazard assessments, arc energy calculations, PPE requirements, and protective clothing selection, serving as the gold standard for theoretical and practical electrical safety.
Selection of Arc-Rated PPE
(1) Energy Calculation Method: Calculate arc energy using IEEE 1584 formulas, then select PPE per NFPA 70E tables. (2) Table Lookup Method: NFPA 70E provides reference tables based on voltage levels, short-circuit currents, and arc duration. If parameters match, PPE can be directly selected.
Note: The tables below are for reference only. Always consult the latest NFPA 70E edition for authoritative guidance.
AC Systems
| PPE Category (PPE Cat.) | Min. Arc Rating | Applicable Equipment (AC) | Working Distance | Arc Flash Boundary |
|---|---|---|---|---|
| 1 | 4 cal/cm² | Distribution boards or equipment ≤240V, short-circuit current ≤25kA, duration ≤0.03s (2 cycles) | 0.455 m | 0.485 m |
| 2 | 8 cal/cm² | Distribution boards or equipment 240V–600V, short-circuit current ≤25kA, duration ≤0.03s (2 cycles) | 0.455 m | 0.9 m |
| 2 | 8 cal/cm² | Motor Control Centers (600V), short-circuit current ≤65kA, duration ≤0.03s (2 cycles) | 0.455 m | 1.5 m |
| 2 | 8 cal/cm² | Other 600V equipment, short-circuit current ≤65kA, duration ≤0.03s (2 cycles) | 0.455 m | 1.5 m |
DC Systems
| PPE Category (PPE Cat.) | Min. Arc Rating | Applicable Equipment (DC) | Working Distance | Arc Flash Boundary |
|---|---|---|---|---|
| 2 | 8 cal/cm² | 100V–250V batteries, DC panels, or power sources, short-circuit current ≤4kA, duration ≤2s | 0.455 m | 0.9 m |
| 2 | 8 cal/cm² | 100V–250V batteries, DC panels, or power sources, short-circuit current 4kA–7kA, duration ≤2s | 0.455 m | 1.2 m |
| 2 | 8 cal/cm² | 250V–600V batteries, DC panels, or power sources, short-circuit current ≤1.5kA, duration ≤2s | 0.455 m | 0.9 m |
| 2 | 8 cal/cm² | 250V–600V batteries, DC panels, or power sources, short-circuit current 1.5kA–3kA, duration ≤2s | 0.455 m | 1.2 m |
