Arc flash: risk and PPE
Arc blast pressure at a distance (Lee 1987)
- where
- blast pressure (lb/ft²)
- arcing current (kA)
- distance from the arc (ft)
- distance from the arc (m)
- blast pressure (kPa)
- force on the frontal area (N)
- exposed frontal area (m²)
Explain this equation
Estimates the pressure wave from a three-phase arc in open air, as a guide to blast and ejection hazards rather than burns.
The arc heats the air and vaporises metal very quickly. Copper expands about 67 000 times as it turns to vapour, which drives a pressure front outwards. Iarc sets the rate of energy input, and D is the distance from the arc.
Lee’s empirical fit has pressure in lb/ft² rising linearly with current and falling as D^0.9. The constant 0.047880259 converts lb/ft² to kPa, and force is pressure × frontal area A.
Sense check: 20 kA at 2 ft gives about 124 lb/ft² (6 kPa), or a force of a few kN on a person’s front. Doubling current doubles the pressure.
Watch out: not valid inside closed equipment and may overstate measured pressures; arc-rated clothing does not protect against blast.
Ralph Lee’s 1987 estimate of the pressure wave from an arc in open air, for judging the blast and ejection hazard alongside the thermal one.
Enter your values to see the result.
Good to know
Units as published: arcing current in kA and distance in feet give pressure in lb/ft²; the calculator converts metres and kPa. Some secondary sources quote the constant as 11.5. The pressure is an impulse lasting milliseconds; it can knock a person off a ladder, rupture eardrums (around 35 kPa for a sustained wave) and throw panels and molten metal. Reducing arc duration does little to the peak pressure, which is set by current.
Common mistake
Treating arc-rated clothing as protection against blast, shrapnel or hearing damage. It is not; positioning, remote operation and arc-resistant switchgear are the controls.
Limits
Empirical, open air, three-phase arcs; not valid inside sealed enclosures, where pressure rise depends on volume and venting (see IEC 62271-200 internal arc classification and CIGRE work). Later reviews suggest Lee’s model can overstate measured pressures. Use as an order-of-magnitude indication only. 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
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.