Incident energy at a low voltage board comes down to two inputs. How much current the arc draws, and how long the upstream device lets it burn. Everything else adjusts the result at the margins.
The question
A 400 V main switchboard has 20 kA of bolted fault current at its busbar. The upstream device clears in 0.2 s. A technician works at 455 mm from the enclosure. The design needs the incident energy at that distance, the arc flash boundary and the PPE category for the label.
The inputs
- System voltage: 0.4 kV
- Bolted fault current: 20 kA
- Clearing time: 0.2 s
- Equipment: switchgear, enclosed, grounded
- Working distance: 455 mm
Bolted fault current comes from the transformer and network study. Clearing time comes from the upstream protection at the arcing current, not at the bolted current.
The clause
The empirical model sits in IEEE 1584-2002 Cl 5.2, Cl 5.3 and Cl 5.5. Cl 2.5.5 of AS/NZS 3000:2018 covers the fault protection requirements for the installation. Cite both by number on the study.
The steps
The equipment type sets the electrode gap. For enclosed switchgear at 400 V the tool uses a gap of 32 mm.
The arcing current follows from the bolted fault current, the voltage and the gap. Not all of the 20 kA flows through an arc. The tool returns an arcing current of 9.64 kA, under half of the bolted figure.
Normalised incident energy comes next. It is the energy for a reference time and distance. The tool returns 2.697 for this case.
Two corrections turn that into the site figure. The calculation factor for an enclosed 400 V case is 1.5. The distance exponent for this equipment type is 1.473.
The tool scales the normalised energy by the clearing time and the calculation factor. It then scales it to the working distance. The result is 6.23 cal/cm² at 455 mm.
The result
6.23 cal/cm² puts the board in PPE category 2. The arc flash boundary is 1395.97 mm. That boundary is where the energy falls to the onset of a second degree burn. Anyone closer than 1395.97 mm stands inside it.
Clearing time drives the answer harder than any other input. Halve the 0.2 s to 0.1 s and the energy roughly halves with it. The arc has less time to deliver power at the same current. That is why a maintenance setting on the upstream relay is worth more than any change to the board.
Lower fault current does not always help. A weaker source lowers the arcing current. That pushes the upstream device further up its curve and lengthens the clearing time. The product of the two can rise. Run both cases through the tool rather than assuming the worst case is the highest bolted current.
The working distance matters more than it looks. The distance exponent of 1.473 means the energy falls faster than the inverse of distance. Moving from 455 mm to 600 mm cuts the energy noticeably. Set the distance to match how the technician actually works at that board.
What this model covers
This result uses the IEEE 1584-2002 empirical model. The 2018 edition of that standard changed the arcing current and energy equations and added electrode configuration as an input. The tool states the 2002 limitation with every result. Treat the figure as a design aid for the study. Confirm the label against the edition that governs the study.
This page is a design aid. Verify every value against the current edition of the standard.