Part 5 of the arc flash series. An 11 kV board, a 0.4 kV LV board and an MCC studied together. The LV board is the worst point because its incomer faults are cleared by the 11 kV relay.
| Network | 250 MVA grid + 2.5 MVA generator at 11 kV → 1.6 MVA, 6 % transformer → 0.4 kV LV board → 50 m cable → MCC with a 160 kW motor |
| 11 kV board | 15 kV switchgear, clearing 300 ms |
| LV board | LV switchgear, cleared by the transformer's 11 kV relay (IEC SI, pickup 100 A, TMS 0.15, breaker 50 ms) |
| MCC | Shallow LV MCC, cleared by the feeder breaker in 100 ms |
| Basis | Fault currents at nominal voltage (c = 1), maximum and minimum cases |
| Bus | Clearing | E (cal/cm²) | Boundary | Indicative PPE |
|---|---|---|---|---|
| 11 kV switchboard | 300 ms | 4.70 | 2.18 m | Category 2 |
| 0.4 kV LV board | 526 ms | 21.2 | 3.68 m | Category 3 |
| MCC | 100 ms | 2.54 | 0.73 m | Category 1 |
A fault on the LV board's incoming terminals is upstream of the LV incomer breaker, so only the 11 kV relay can clear it. Seen from 11 kV the current is small (about 1.4 kA), so the IDMT relay takes over half a second.
The calculator reports that 199 ms would bring the LV board to Category 2. A maintenance setting on the 11 kV relay clearing in 50 ms gives 2.01 cal/cm².
Each bus gets a label with the incident energy (or PPE category), the arc-flash boundary, working distance, voltage and the limited and restricted approach boundaries, as NFPA 70E 130.5(H) requires.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| 11 kV switchboard | ≤ 8 cal/cm² | 4.70 cal/cm² | ✓ PASS |
| 0.4 kV LV board | ≤ 8 cal/cm² | 21.2 cal/cm² | ✗ FAIL |
| MCC | ≤ 8 cal/cm² | 2.54 cal/cm² | ✓ PASS |
Key insight: The worst location is usually the main LV board fed directly from a transformer, because its incomer terminals rely on HV protection that cannot be fast without losing selectivity.
Every input in this example is editable in the live calculator — no signup needed.
Open Arc Flash Study (IEEE 1584-2018) calculator →IEEE 1584 works from the available bolted fault current at the operating voltage; the IEC 60909 maximum factor would overstate it. The minimum case is also studied because lower current can mean slower clearing.
Yes. Open the Arc Flash Study calculator to see the same network and change any value.