Arc flash: risk and PPE
Incident energy at a different working distance or arc duration
- where
- known incident energy (J/cm²)
- incident energy at the new distance and time (J/cm²)
- arc durations (ms)
- working distances (mm)
- distance exponent
Explain this equation
Rescales a known incident energy to another working distance or arc duration, for example to judge the effect of a longer hot stick or a faster trip.
E1 is the known energy at distance D1 and duration t1; E2 is the result at D2 and t2. x is the distance exponent for the configuration.
Energy is linear in duration because the arc power is roughly constant while it burns, so energy simply accumulates with time. It falls as a power of distance because the radiation and plasma spread out; x = 2 for a point source in open air, lower in enclosures.
Sense check: moving from 610 mm to 910 mm with x = 2 cuts energy by more than half (factor 0.45); halving the time halves the energy.
Watch out: if the arcing current changes, the clearing time changes too and this simple scaling no longer applies.
Rescales a known incident energy to another working distance (power law in D) or arc duration (linear in t), for example to compare a hot stick or extended racking tool with normal working distance.
Enter your values to see the result.
Good to know
All the published models (IEEE 1584-2002 and 2018, Lee, Doughty–Neal, d.c.) are linear in arc duration for a fixed arcing current, so energy scales directly with time. Distance exponents are listed in the boundary calculator; use the one for the configuration and voltage of the original study.
Common mistake
Scaling the time without checking that the arcing current, and therefore the protection time, is unchanged; or scaling to distances below 305 mm, where the empirical models were not tested.
Limits
Valid only where the arcing current and electrode configuration are unchanged. Approximate for 0.6–15 kV in the 2018 model because of interpolation between three exponents. 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
- IEEE Std 1584-2018 IEEE Guide for Performing Arc-Flash Hazard Calculations (equations 1–25, Tables 1–7, Annex D worked examples)
- IEEE Std 1584-2002 IEEE Guide for Performing Arc-Flash Hazard Calculations (superseded; empirical model and Lee method)
- 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)
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.