Arc flash: risk and PPE
Incident energy reduction from faster clearing or greater distance (maintenance switch, remote operation)
- where
- incident energy with existing settings (J/cm²)
- incident energy after the change (J/cm²; shown in cal/cm²)
- clearing times before and after (ms)
- working distances before and after (mm)
- distance exponent
- arc flash boundary before and after (mm)
- incident energy at the boundary (J/cm²)
Explain this equation
Shows the benefit of engineering controls such as a maintenance switch, arc flash relay, bus differential or remote operation, in incident energy and boundary.
E1 and t1 are the existing energy and clearing time, t2 the faster clearing time, D1 and D2 the working distances before and after, x the distance exponent and EB the boundary threshold.
Energy is proportional to clearing time and falls with distance as (D1/D2)^x. The boundary depends only on how much energy the arc releases, so it shrinks with the clearing time but does not move when the worker steps back.
Sense check: cutting clearing time from 500 ms to 80 ms reduces energy by 84 %; with x = 2 the boundary falls by √(500/80), about 2.5 times.
Watch out: the new setting must operate at the reduced arcing current, and the breaker opening time must be included in t2.
Shows the benefit of engineering controls: an energy-reducing maintenance switch, a temporary instantaneous or bus-differential setting, arc flash detection relays, or moving the person further away.
Enter your values to see the result.
Good to know
Arc flash relays (light plus current) typically trip in a few ms and the breaker clears in 50–80 ms, so energy falls almost in proportion. The arc flash boundary depends only on the energy released, not on where the worker stands, so changing distance does not move it. Engineering controls rank above PPE in the hierarchy of controls and are the expected response where risk assessment under EAWR reg 4 and 14 identifies a high incident energy.
Common mistake
Assuming the maintenance setting will operate at the reduced arcing current. Check that the reduced pickup is below the reduced arcing current with margin, and that it is switched back after the work (lock-out or indication).
Limits
Assumes the arcing current and configuration are unchanged and energy is linear in time. Verify the new clearing time includes the breaker opening time. 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)
- 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)
- HSE HSG85 Electricity at work: safe working practices (3rd edition, 2013)
- The Electricity at Work Regulations 1989 (SI 1989/635), regulations 4, 12, 13 and 14
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.