Which arc-flash model does this use?

It uses the Ralph Lee theoretical maximum-power method published in the IEEE 1584 annex. Lee assumes the worst-case condition where the arc voltage is half the system voltage, which makes it conservative — it tends to over-predict energy at low voltage.

Why not the full IEEE 1584-2018 equations?

The 2018 empirical equations require equipment-specific coefficients, electrode configuration, enclosure size, and a multi-region interpolation that depend on tabulated lab data. The Lee method is the closed-form screening model and is appropriate for a first pass; a stamped study uses the full equations.

What working distance should I use?

Use the standard working distance for the equipment class: about 455 mm (18 in) for low-voltage panels and 600 V switchgear, and larger for medium-voltage gear. The working distance is to the worker's torso and face, not to the arc source.

How is the clearing time found?

Read it from the upstream protective device's time-current curve at the calculated arcing current. Faster clearing dramatically lowers incident energy because energy is directly proportional to time.

Can I rely on this to pick PPE?

No. This is a conservative screening estimate for planning and training. Final PPE selection and equipment labelling must come from a qualified engineer's IEEE 1584-2018 study and your site's electrical safety program.

What is the Arc Flash Incident Energy & Boundary Calculator?

Screen arc-flash incident energy in cal/cm² and the arc-flash boundary from bolted fault current, system voltage, working distance, and clearing time using the conservative Ralph Lee model, then map the result to NFPA 70E PPE categories. It runs free in your browser on Gera Tools, with nothing uploaded.

Arc Flash Incident Energy & Boundary Calculator

Name: Arc Flash Incident Energy & Boundary Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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What incident energy and the arc-flash boundary mean

An arc flash is the explosive release of energy when a fault arcs through air between conductors. Two numbers govern worker safety: the incident energy (how much thermal energy reaches a worker at a given distance, in cal/cm²) and the arc-flash boundary (the distance at which that energy drops to the 1.2 cal/cm² onset of a second-degree burn). Together they determine the arc-rated PPE a worker must wear and how far unprotected people must stay back. This calculator gives a conservative first-pass estimate of both.

How it works

It uses the Ralph Lee theoretical maximum-power method from the IEEE 1584 annex. Lee’s worst case occurs when the arc voltage equals half the system voltage, which yields a closed-form expression for incident energy at distance D:

E (J/cm^2) = 5.12e5 × V × Ibf × t / D^2

where V is in kV, the bolted fault current Ibf is in kA, the clearing time t is in seconds, and D is in millimetres. Dividing by 4.184 converts joules to calories. Setting E equal to the 1.2 cal/cm² burn threshold and solving for D gives the arc-flash boundary:

D_B (mm) = sqrt( 5.12e5 × V × Ibf × t / (1.2 × 4.184) )

The tool then maps the incident energy to an NFPA 70E PPE category — Category 1 up to 4 cal/cm², Category 2 up to 8, Category 3 up to 25, Category 4 up to 40, and “dangerous, work de-energized” above 40.

Example and notes

A 480 V (0.48 kV) panel with 25 kA of bolted fault current and a 0.1 s clearing time at an 18 in (455 mm) working distance yields roughly 3.6 cal/cm² and a boundary near a metre — Category 1 PPE. Halve the clearing time and the energy halves with it, which is why upstream protection coordination is the single biggest lever on arc-flash risk. Treat every number here as a conservative screening value. It is intentionally pessimistic and is never a substitute for an engineered IEEE 1584-2018 study, equipment labelling, and a documented electrical safety program. Energized work always requires a qualified person and a job-specific risk assessment.