Arc flash: risk and PPE
Arc-rated PPE check: incident energy against garment arc rating and NFPA 70E categories
- where
- arc rating of the clothing system, ATPV or EBT (cal/cm²)
- incident energy at the working distance (cal/cm²)
- NFPA 70E PPE category
- minimum arc rating of category n (cal/cm²)
Explain this equation
Checks whether the arc rating of the clothing system covers the calculated incident energy, and shows the NFPA 70E PPE category with a minimum rating that would.
ATPV (or EBT if lower) is the energy at which the clothing gives a 50 % chance of the onset of a second-degree burn, from open-arc tests. E is the incident energy at the working distance. The categories have minimum ratings of 4, 8, 25 and 40 cal/cm².
The margin is a simple ratio: above 1 the rating exceeds the energy. The category is the lowest whose minimum rating is at least E.
Sense check: 6.2 cal/cm² needs at least category 2 (8 cal/cm²); 30 cal/cm² needs category 4 (40 cal/cm²).
Watch out: above 40 cal/cm² there is no category and live work should not be planned; PPE does not protect against blast.
Checks whether arc-rated clothing is adequate for a calculated incident energy, and shows the NFPA 70E PPE category with a minimum arc rating that would cover it.
Enter your values to see the result.
Good to know
The equivalent category is the lowest NFPA 70E PPE category whose minimum arc rating (1: 4, 2: 8, 3: 25, 4: 40 cal/cm²) is at least the incident energy. Under an incident energy analysis the arc rating of the whole clothing system (including face shield or hood and gloves) must be at least the incident energy at the working distance. The NFPA 70E PPE categories (Table 130.7(C)(15)(c)) have minimum arc ratings of 4, 8, 25 and 40 cal/cm²; strictly they are used with the table method (equipment, fault level and clearing time limits), not with a calculated energy, so the category here is for comparison. Arc ratings are ATPV or EBT from ASTM F1959 or IEC 61482-1-1 (open arc, cal/cm²); IEC 61482-1-2 box-test classes (APC 1 and 2) are not directly comparable (see the box test calculator). Labels should give the incident energy and working distance, arc flash boundary, nominal voltage, approach boundaries and study date.
Common mistake
Choosing PPE by category from a label calculated with a different working distance, or ignoring the hood, gloves and underlayers that make up the rated system. Layering ratings do not simply add.
Limits
Thermal protection only: arc-rated PPE does not protect against blast, shrapnel, noise, toxic products or electric shock. Arc ratings are 50 % probability values at the onset of second-degree burn; a margin above the calculated energy is prudent. UK duty: under the Electricity at Work Regulations 1989 work on or near live conductors is allowed only where reg 14 is met (unreasonable to make dead, reasonable to work live, suitable precautions); regs 12 and 13 require isolation and precautions for dead working, and reg 4(4) requires suitable protective equipment. HSE HSG85 and your DNO or company safety rules (ENA-based distribution safety rules) take precedence. Apply the hierarchy of controls: de-energise first, then engineering controls (faster protection, maintenance switches, remote racking/operation, arc-resistant switchgear), then distance and procedures; arc-rated PPE is the last line of defence.
Sources
- NFPA 70E-2024 Standard for Electrical Safety in the Workplace: Informative Annex D (incident energy and arc flash boundary methods) and Table 130.7(C)(15)(c)
- ASTM F1959/F1959M Standard test method for determining the arc rating of materials for clothing (ATPV, EBT, Stoll response)
- IEC 61482-1-1:2019 Live working – Protective clothing against the thermal hazards of an electric arc – Part 1-1: Open arc method (ATPV, EBT, ELIM)
- The Electricity at Work Regulations 1989 (SI 1989/635), regulations 4, 12, 13 and 14
- HSE HSG85 Electricity at work: safe working practices (3rd edition, 2013)
- HSE HSR25 Memorandum of guidance on the Electricity at Work Regulations 1989
For competent high-voltage engineers. Results are calculations from the published methods shown and depend entirely on the data you enter. They are not a substitute for a system study, manufacturer data, the applicable standards and Engineering Recommendations, or your organisation’s safety rules. Never use a result to justify working on or near live HV equipment.