Part 2 of the arc flash series. Incident energy is proportional to arc duration. The same 30 kA fault gives 1.6, 9.9 or 32.9 cal/cm² depending only on how fast the protection clears it.
| Equipment | 415 V LV switchgear: gap 32 mm, enclosure 508 × 508 × 508 mm, working distance 610 mm |
| Bolted fault current | 30 kA (arcing current 19.9 kA, VCB) |
| Clearing times compared | 50 ms (instantaneous), 300 ms (short-time delay), 1,000 ms (IDMT at low current) |
For a given arcing current and distance, doubling the arc time doubles the energy.
| Clearing time | E (cal/cm²) | Boundary (mm) | Indicative PPE |
|---|---|---|---|
| 50 ms | 1.64 | 742 | Category 1 |
| 300 ms | 9.87 | 2,278 | Category 3 |
| 1,000 ms | 32.9 | 4,840 | Category 4 |
Short-time delays added for selectivity (often 0.3–0.5 s), upstream relays protecting LV boards from the HV side, and arcing currents falling below an instantaneous setting (see Part 4).
| Check | Requirement | Actual | Status |
|---|---|---|---|
| 50 ms | ≤ 8 cal/cm² | 1.64 cal/cm² | ✓ PASS |
| 300 ms | ≤ 8 cal/cm² | 9.87 cal/cm² | ✗ FAIL |
| 1,000 ms | ≤ 40 cal/cm² | 32.9 cal/cm² | ✓ PASS |
Key insight: Selectivity and arc flash pull in opposite directions: every time delay added for coordination adds energy. Arc-flash relays, zone interlocking and maintenance switches resolve the conflict.
Every input in this example is editable in the live calculator — no signup needed.
Open Arc Flash calculator →No. The boundary grows more slowly than the energy, because energy falls off with distance raised to a power.
IEEE 1584-2018 notes that 2 s is a reasonable maximum when a person can move away from the arc; the calculator uses this as a cap.