Part 4 of the arc flash series. On the transformer the arcing current trips the instantaneous element in 70 ms (2.7 cal/cm²). On a standby generator the current falls below it and the arc lasts 1.8 s (23 cal/cm²).
| Equipment | 415 V LV switchgear: gap 32 mm, enclosure 508 × 508 × 508 mm, working distance 610 mm |
| Bolted fault current | 37 kA on the transformer; 12 kA on the standby generator |
| Protection | IEC Standard Inverse, pickup 2,500 A, TMS 0.3; instantaneous 15 kA (20 ms); breaker 50 ms |
Arcing current 23.3 kA is above the 15 kA instantaneous setting.
Set the instantaneous element to 6 kA, above downstream devices' let-through and motor starting but below the minimum arcing current.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Transformer supply | ≤ 8 cal/cm² | 2.74 cal/cm² | ✓ PASS |
| Generator supply, inst 15 kA | ≤ 8 cal/cm² | 23.0 cal/cm² | ✗ FAIL |
| Generator supply, inst 6 kA | ≤ 8 cal/cm² | 0.91 cal/cm² | ✓ PASS |
Key insight: The highest incident energy often comes from the lowest fault current. Every operating mode — normal, one transformer out, generator only — must be studied.
Every input in this example is editable in the live calculator — no signup needed.
Open Arc Flash calculator →The protection sees the current that actually flows during the arc, which at LV can be half the bolted value.
Yes. Many numeric relays switch to a generator setting group automatically when the incomer opens and the generator closes.