Part 3 of the arc flash series. A main ACB with a 430 ms short-time delay gives Category 3. An energy-reducing maintenance switch that clears in 60 ms brings it to Category 1 while people work.
| Equipment | 415 V LV switchgear: gap 32 mm, enclosure 508 × 508 × 508 mm, working distance 610 mm |
| Bolted fault current | 50 kA (arcing current 27.8 kA) |
| Normal clearing | 430 ms (short-time delay 0.4 s + breaker) |
| Maintenance mode | Instantaneous trip enabled, total 60 ms |
A switch (often with an indicator light and padlock) temporarily enables a fast instantaneous or arc-flash setting while work is done. Selectivity is given up only during that time.
The switch must be on before the work starts and confirmed (light, display), it must be on the upstream device feeding the equipment being worked on, and it must be returned to normal afterwards. Labels should show both energies and say which applies.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Normal mode | ≤ 8 cal/cm² | 20.6 cal/cm² | ✗ FAIL |
| Maintenance mode | ≤ 8 cal/cm² | 2.87 cal/cm² | ✓ PASS |
Key insight: The maintenance setting protects the equipment the upstream breaker feeds, not the breaker's own line side. Faults on the line side are cleared by the device further upstream.
Every input in this example is editable in the live calculator — no signup needed.
Open Arc Flash calculator →NFPA 70 (NEC 240.87) requires an energy-reducing method for breakers rated 1,200 A or more; maintenance switches are one of the accepted methods.
Zone-selective interlocking, arc-flash relays with light sensors, bus differential protection, or remote racking and operation.