Arc flash, end to end.
The complete ClarkTE arc flash hub: what it is, how it's calculated, how to mitigate it, and which ClarkTE service or tool fits your situation.
What is arc flash?
An arc flash is what happens when a fault — phase-to-phase, phase-to-ground, or three-phase — closes through the air between conductors. The fault current ionizes the air, vaporizes the metal in its path, and releases an enormous amount of energy in milliseconds. The thermal portion is the “arc flash” (incident energy in cal/cm²). The pressure wave that comes with it is the “arc blast.” Together, they are responsible for thousands of injuries and dozens of fatalities every year in the U.S. industrial workforce.
The good news: arc flash is engineered, not accidental. The incident energy at every location in your facility is calculable, predictable, and reducible by deliberate design choices. The bad news: most facilities have never actually performed the calculation, and the worker doing energized work has no idea what the incident energy is at the location they are touching. Closing that gap is what an arc flash study delivers.
How arc flash incident energy is calculated
IEEE 1584-2018 is the consensus standard. The calculation engine takes five inputs:
- Available short-circuit current at the bus (computed from a separate short-circuit study)
- Protective device clearing time at that current (read from the upstream relay or breaker time-current curve)
- Working distance (typically 18 inches for low-voltage, 36 inches for medium-voltage)
- Electrode configuration — vertical conductor in a box (VCB), vertical conductor in air (VCA), horizontal conductor in a box (HCB), etc.
- Conductor gap (typical values vary by equipment voltage class)
Output: incident energy in cal/cm² at the working distance, and the arc flash boundary distance. The calculation is per-location — every bus on a one-line diagram gets its own number. A 30-bus facility produces 30 incident energy values, 30 arc flash boundaries, and 30 equipment labels.
Use the arc flash screening calculator → to estimate incident energy for a single bus. For a real study, see the service link below.
Arc flash PPE: matching gear to incident energy
NFPA 70E defines four PPE Categories (formerly called Hazard/Risk Categories or HRC), each with a minimum arc rating in cal/cm²:
| PPE Category | Minimum arc rating | Typical use case |
|---|---|---|
| Category 1 | 4 cal/cm² | Light duty, low-voltage panel-board work |
| Category 2 | 8 cal/cm² | Most 480 V switchgear maintenance |
| Category 3 | 25 cal/cm² | Medium-voltage switchgear, racking |
| Category 4 | 40 cal/cm² | High-energy MV gear; the practical ceiling for arc flash PPE |
Above 40 cal/cm², no commercially available PPE is rated to protect a worker. Locations with calculated incident energy above 40 cal/cm² should be re-engineered (faster relays, arc-resistant gear, remote racking) until they drop below the threshold — or de-energized for any work.
Mitigation: how to reduce arc flash incident energy
The fastest and cheapest way to reduce arc flash incident energy is to clear the fault faster. Five proven levers, ranked by impact:
- Replace electromechanical relays with microprocessor relays. Modern microprocessor relays clear the fault in 1-3 cycles vs 5-10 cycles for older electromechanical units. Often a 2x reduction in incident energy with no other changes. See ClarkTE relay changeouts →
- Add light-detecting arc flash relays. Devices like the SEL-T401L, SEL-751A with arc-flash modules, or ABB REA detect the light from an incipient arc and trip the breaker in 1-3 ms. Dramatic incident energy reduction at locations where they're installed.
- Tighten coordination. Reduce time delays on upstream devices where possible without sacrificing selectivity. Often a 25-50% incident energy reduction with no equipment changes. See ClarkTE coordination studies →
- Install arc-resistant switchgear. Doesn't reduce incident energy at the working distance but redirects the arc blast away from the operator. Required for new construction in many industrial codes.
- Remote racking and remote operation. Keeps the worker outside the arc flash boundary during the highest-risk activity (racking a breaker into the cell). The simplest control with the highest ROI for racking work.
Where ClarkTE fits in your arc flash program
PE-stamped study, equipment labels, and the calculations your auditor expects to see.
The available fault current at every bus — the main input to IEEE 1584.
Tighten upstream coordination to drop incident energy at every downstream device.
Modern relays clear faults faster — often the biggest single lever to reduce arc flash.
Train your workforce on the labels and procedures the study generates.
Estimate the cost-benefit of a real study before signing a contract.
Arc flash FAQ
What is arc flash?
An arc flash is the sudden release of energy from a fault that ionizes the air between conductors. It produces incident energy measured in calories per square centimeter, a pressure wave, molten metal, and a temperature high enough to vaporize copper. Survivable at low incident energies (under 1.2 cal/cm²) with proper PPE; lethal at high incident energies (above 40 cal/cm²) almost regardless of PPE.
How is arc flash incident energy calculated?
IEEE 1584-2018 is the consensus standard. The calculation takes available short-circuit current at the bus, the protective device's clearing time at that current, working distance, electrode configuration (vertical, horizontal, in-box, etc.), and conductor gap. Output is incident energy in cal/cm² at the working distance, and the arc flash boundary distance — the distance from the equipment within which an unprotected worker could receive a second-degree burn.
What is the difference between arc flash and arc blast?
Arc flash is the thermal energy. Arc blast is the pressure wave that comes with it — a fault interrupts thousands of amps in milliseconds, vaporizing copper and air, generating shockwaves that throw workers, knock down equipment doors, and rupture eardrums. PPE protects from the thermal portion; only distance and engineering controls (arc-resistant switchgear, remote racking) protect from the blast.
Are arc flash studies required?
Yes, in practice. NFPA 70E (Standard for Electrical Safety in the Workplace) requires that workers know the incident energy at every location where they perform energized work. The two ways to comply are: (1) the table method (NFPA 70E Table 130.7(C)(15)(A) and similar) — which assumes worst-case conditions and forces the highest PPE category — or (2) an IEEE 1584 study with calculated incident energy and equipment labels. Most facilities choose option 2 because it lets workers use lighter PPE in lower-energy locations.
How do I reduce arc flash incident energy?
Five proven approaches, ranked by impact: (1) faster protective relays — replacing electromechanical with microprocessor relays often cuts clearing time by half and incident energy proportionally; (2) tighter coordination — shorten the time delays on upstream devices where possible; (3) arc-resistant switchgear — redirects the blast away from the operator; (4) remote racking and remote operation — keeps the worker outside the arc flash boundary entirely; (5) light-detecting arc flash relays (50AS / SEL-T401L) that sense an arc and trip the breaker in 1-3 ms instead of waiting for the time-current curve.
What does an arc flash label look like?
Per NFPA 70E and ANSI Z535.4, the label includes: equipment ID, the incident energy in cal/cm² at the working distance, the working distance itself, the arc flash boundary, the limited approach boundary, the restricted approach boundary, the shock hazard voltage, and the date of the study. Some sites add the required PPE category, manufacturer/installer info, and a QR code linking to digital documentation.
Ready to start an arc flash program?
Send your one-line, last short-circuit study, or a list of buses to be labeled. ClarkTE returns an IEEE 1584-2018 study scope, fixed fee, and turnaround estimate within one business day.
Talk with us about an arc flash study