Arc Flash Analysis Step by Step (IEEE 1584:2018)

An arc flash is the sudden release of thermal and radiant energy during an electrical fault. The arc plasma can reach temperatures of 19,000 degrees Celsius in milliseconds and produce pressure waves strong enough to throw a worker across a switch room. IEEE 1584:2018 is the international standard for calculating how much incident energy a worker would receive at a defined working distance during such a fault, and AS/NZS 4836 requires that an arc flash hazard analysis support any safe work procedure on Australian electrical equipment. This guide walks through the full IEEE 1584:2018 method, six steps end to end, with a worked example for a 415 V switchboard. By the end you will know how to compute incident energy, the arc flash boundary, and the required PPE category, and where the Arc Flash Calculator handles the maths.

What an arc flash is and why it matters

An arc flash starts when an unintended low-impedance path forms between phases or between phase and earth in energised equipment. The fault current flowing through ionised air creates a plasma channel hotter than the surface of the sun, radiating thermal energy outward and vaporising surrounding conductor metal. The pressure wave from the rapid expansion can rupture eardrums, blow doors off enclosures, and project molten copper at hundreds of metres per second.

In Australia, arc flash injuries account for a disproportionate share of serious electrical workplace incidents. Unlike electrocution, where outcome depends mostly on whether current finds a path through the chest, arc flash outcome depends on a quantitative calculation: how much energy the body absorbs before the worker can move away or the upstream protection clears the fault. That calculation is what IEEE 1584:2018 provides.

The legal framework: IEEE 1584:2018 and AS/NZS 4836

IEEE 1584:2018 is the second edition of the IEEE Guide for Performing Arc Flash Hazard Calculations, published September 2018. It supersedes the 2002 first edition and uses an updated empirical model fitted to a much larger dataset of staged arc flash tests. The 2018 edition is valid for system voltages from 208 V to 15 kV and bolted fault currents from 500 A to 106 kA.

In Australia, AS/NZS 4836:2023 (Safe working on or near low voltage electrical installations and equipment) requires that a documented arc flash hazard analysis underpin any safe work procedure on energised equipment. AS/NZS 4836 does not specify a calculation method, so IEEE 1584:2018 fills that gap. NFPA 70E provides the PPE category mapping that almost everyone uses to translate calculated incident energy into required clothing.

Three other standards interact with arc flash work. AS/NZS 3000 governs general installation safety and earth fault loop impedance. AS 2067 covers high voltage substations. AS/NZS 3008.1.1 governs cable impedance, which feeds back into the fault current calculation upstream of any arc flash study. The companion Cable Sizing Walkthrough covers the cable side of that loop.

Step 1: Determine the bolted fault current

The bolted fault current Ibf is the prospective three-phase short circuit current at the equipment terminals, as if the phases were bolted together with zero arc resistance. For a typical Australian site this comes from one of two sources.

• Utility short circuit data. The supply authority provides Isc at the point of common coupling for the local transformer. Typical urban LV figures are 10 to 25 kA at the consumer main switch.
• Calculated from impedance. If utility data is not available, calculate from the upstream transformer impedance and cable impedances back to the source. The Transformer Fault Calculator handles the transformer secondary fault current; cable impedance from AS/NZS 3008.1.1 then derates it back to the equipment.

Use the highest realistic bolted fault current in the analysis. The IEEE 1584 model also requires a low-current variant analysis for some configurations, because reduced arc impedance at lower currents can paradoxically extend the arc duration on time-overcurrent protection.

Step 2: Find the arc duration

Arc duration is the time the arc persists before the upstream protective device clears the fault. Incident energy is roughly linear with arc duration: double the arc time, double the energy. This is the single most important variable in any mitigation strategy.

To find arc duration, identify the upstream protective devices that can see the fault and look up each device's operating time at the bolted fault current. The fastest device that can actually clear the fault sets the arc duration. For a current limiting MCB at 415 V, this might be 5 to 20 milliseconds. For an upstream IDMT relay set for selectivity with downstream devices it could be 0.3 to 1.0 seconds. The IDMT Relay Calculator gives the exact operating time on each of the five IEC inverse curves at any fault current.

Two practical traps. First, the time printed on a relay name tag is the trip signal time, not the breaker clearing time. Add the breaker mechanical opening time (typically 30 to 80 milliseconds for a sizeable LV ACB). Second, the IEEE 1584 analysis often requires running the calculation at both the bolted fault current and 85 percent of it, because the longer time at the lower current can yield higher total energy.

Step 3: Pick the electrode configuration

IEEE 1584:2018 defines five electrode configurations. The configuration affects how the arc plasma is directed and consequently how much energy reaches the worker.

• VCB (Vertical Conductors in a Box). The most common configuration for low voltage switchboards. Plasma jet directs out the front of the enclosure toward the worker.
• VCBB (Vertical Conductors in a Box with Barrier). New in 2018. Same physical layout as VCB but with an insulating barrier above the conductors. Reduces incident energy by typically 5 to 20 percent compared to VCB at the same fault current.
• HCB (Horizontal Conductors in a Box). Used when bus bars run horizontally inside enclosed switchgear. Less common in Australian LV practice; more common in MV.
• VOA (Vertical Open Air conductors). Outdoor bus bar arrangements with no enclosure. Lower incident energy than enclosed configurations at the same fault current because the plasma can expand freely.
• HOA (Horizontal Open Air conductors). Lowest incident energy of the five. Outdoor horizontal bus arrangements.

Pick the one that best matches your physical equipment. If in doubt between VCB and VCBB, use VCB (the more conservative choice) unless the manufacturer explicitly confirms a barrier.

Step 4: Set the working distance

The working distance is the distance from the arc source to the workers face and chest. Incident energy falls approximately with the square of distance, so the working distance is a major lever. IEEE 1584 Table 9 gives the standard values:

• Low voltage switchgear: 455 mm.
• Medium voltage switchgear: 610 mm.
• High voltage switchgear: 910 mm.

If the actual task involves the worker being closer than the standard distance (for example, manipulating a fuse with a short tool), use the actual distance and accept the higher incident energy. If the task can be done from further back (for example, racking a breaker remotely from outside the switch room), use the larger distance and the resulting incident energy is much lower.

Step 5: Compute incident energy and arc flash boundary

With Ibf, arc duration, electrode configuration, gap between conductors (typically 32 mm for LV switchgear, 76 mm for MV), and working distance set, the IEEE 1584:2018 model returns two numbers.

• Incident energy at the working distance, in cal per cm squared. This is what determines the PPE category.
• Arc flash boundary, the distance from the arc source at which incident energy drops to 1.2 cal per cm squared. Anyone closer than this boundary must wear arc-rated PPE.

The model is a non-trivial empirical fit; the practical approach is to use a tool. The Arc Flash Calculator implements the IEEE 1584:2018 equations directly and returns both incident energy and the boundary, plus the recommended PPE category, from a single set of inputs.

Step 6: Select PPE category

Map the calculated incident energy to an NFPA 70E PPE category. The thresholds are:

• Category 1 (up to 1.2 cal per cm squared): arc-rated long sleeve shirt and trousers (4 cal per cm squared minimum), safety glasses, hard hat, hearing protection, leather gloves.
• Category 2 (1.2 to 8 cal per cm squared): arc-rated shirt and trousers or coverall (8 cal per cm squared minimum), arc-rated face shield with balaclava or arc-rated hood, hearing protection, leather gloves over voltage-rated rubber gloves.
• Category 3 (8 to 25 cal per cm squared): multi-layer arc flash suit and hood (25 cal per cm squared minimum), arc-rated gloves; total system rating must equal or exceed incident energy.
• Category 4 (25 to 40 cal per cm squared): full arc flash suit (40 cal per cm squared minimum) plus arc-rated hood with face shield, arc-rated gloves.
• Above 40 cal per cm squared: work must be performed de-energised under most Australian safe-work policies. PPE alone is not sufficient.

Three engineering levers to reduce arc flash hazard

When the calculated incident energy lands in Category 3 or 4, the answer is rarely just thicker PPE. Three engineering levers attack the problem at the source, in order of effectiveness.

1. De-energise before work. Always the first option per AS/NZS 4836. Test for absence of voltage, lock out, and tag out. Most of the time this is the right answer.
2. Lower the arc duration by tightening the upstream protective device settings within selectivity constraints. Halving arc time roughly halves incident energy. An arc flash maintenance switch (a dedicated lower time setting that engineers enable for the duration of the work) is a common implementation.
3. Increase working distance via remote racking, live-line tools, or insulating barriers. Incident energy falls roughly with the square of distance, so going from 455 mm to 1000 mm cuts energy by about 80 percent.

Hazard labelling under AS/NZS 4836

Every piece of energised equipment that may need to be worked on while live should carry an arc flash hazard label. AS/NZS 4836 requires the label to support the safe work method statement; the label content is harmonised with NFPA 70E practice. A typical label contains:

• Equipment identifier
• Nominal voltage
• Bolted fault current
• Arc duration assumed in the analysis
• Working distance
• Incident energy at working distance
• Arc flash boundary
• Required PPE category and minimum garment ratings
• Date of the analysis and engineer responsible

Update the label whenever the upstream system changes. A new larger transformer, a new MCB setting, a longer feeder cable can all shift incident energy by a category. A label that is out of date is worse than no label at all because it gives false confidence to the worker.

Worked example: 415 V switchboard, 25 kA, 0.3 s clearing

A commercial main switchboard sees 25 kA bolted fault current at 415 V three-phase. The upstream IDMT relay clears in 250 milliseconds at this current; the breaker adds 50 milliseconds of mechanical clearing time, so the total arc duration is 0.3 seconds. The configuration is VCB (vertical conductors in a metal-clad enclosure), the electrode gap is 32 mm, and the working distance is 455 mm per IEEE 1584 Table 9.

Run the IEEE 1584:2018 model with these inputs. The calculator returns approximately:

• Incident energy at 455 mm: around 12 to 14 cal per cm squared (Category 3 territory).
• Arc flash boundary: around 1500 mm to 1800 mm.
• Required PPE: Category 3 multi-layer arc flash suit rated at least 25 cal per cm squared.

Now apply the levers. Reduce the upstream IDMT setting from 250 milliseconds to 100 milliseconds (using an arc flash maintenance switch enabled while working). New arc duration is 150 milliseconds. Incident energy roughly halves to 6 to 7 cal per cm squared, dropping to Category 2. The arc flash boundary tightens to around 1100 mm. Required PPE is now an 8 cal per cm squared shirt and trousers plus an arc-rated face shield, much more practical to wear for an hour of work than a full suit.

Where the calculator fits in

The ElecCalc Arc Flash Calculator implements the IEEE 1584:2018 model directly. Enter system voltage, bolted fault current, arc duration, electrode configuration, gap, and working distance, and the calculator returns incident energy, arc flash boundary, and recommended PPE category in a single pass. Pair it with the IDMT Relay Calculator to find arc duration on inverse-curve protection, and the Transformer Fault Calculator to find bolted fault current upstream.

Earthing system design is the other half of getting protection coordination right; the companion Earthing System Design Guide covers earth fault loop impedance, which determines whether the upstream RCD or breaker can see a low-current arc fault in the first place. Without that, the cleanest arc flash analysis is moot.