Understanding Arc Flash Incident Energy: IEEE 1584-2018 Explained

Why the 2018 Model Replaced the 2002 Model

The IEEE 1584-2002 model was based on approximately 300 tests at a single voltage range and a limited set of electrode configurations. It treated all equipment as having electrodes in a vertical open-air arrangement (VCB) or a vertical arrangement inside a box (VCBB). Real-world equipment does not always match these configurations. A horizontal bus arrangement in a motor control center produces a very different arc plasma geometry than vertical bus bars in a switchgear lineup. The 2002 model could not account for these differences, and field experience showed it sometimes underestimated and sometimes overestimated incident energy depending on the actual equipment configuration.

The 2018 edition addressed these limitations by expanding the test dataset to over 1,800 tests across five electrode configurations, three reference voltage ranges, and multiple enclosure sizes. The mathematical model was rebuilt from the ground up using a different curve-fitting approach. The result is a model that more accurately predicts incident energy across the full range of equipment found in industrial and commercial installations.

One of the most significant changes is the introduction of the enclosure size correction factor. The 2002 model assumed a single "typical" enclosure for each equipment type. The 2018 model accounts for the actual enclosure dimensions, which affect how the arc plasma is focused or dispersed. A shallow enclosure concentrates energy toward the worker, while a deep enclosure allows the plasma to expand away from the opening. This correction alone can change the calculated incident energy by 30 to 50 percent compared to the 2002 result.

Another major change is the requirement to evaluate incident energy at both the full arcing current and a reduced arcing current (using the variation factor, VarCf). The reduced arcing current represents the lower end of the expected arc current range and may result in longer clearing times on upstream protective devices, potentially producing higher incident energy. The 2002 model used a single arcing current value, which could miss the worst-case scenario when the arc current fell below a protective device's instantaneous trip setting.

The Five Electrode Configurations

The 2018 model defines five electrode configurations that represent the physical arrangement of bus bars or conductors inside equipment. Selecting the correct configuration is critical because it determines the arc plasma geometry, which directly affects the incident energy at the working distance. The five configurations are:

VCB (Vertical Conductors inside a Box): Bus bars oriented vertically inside an enclosure. This is the default configuration for most switchgear and panelboard applications where the bus is mounted on the back wall of the enclosure and the worker faces the bus opening. The arc plasma is directed outward toward the worker by the enclosure walls.

VCBB (Vertical Conductors terminated in a Barrier inside a Box): Similar to VCB, but the bus bars terminate at an insulating barrier at the bottom. This configuration applies to equipment where the bus ends at a insulating support or barrier, such as certain switchgear designs. The barrier redirects some of the arc energy, generally producing higher incident energy than VCB at the same current and clearing time.

HCB (Horizontal Conductors inside a Box): Bus bars oriented horizontally inside an enclosure, with the arc initiating between horizontal conductors. This is common in motor control centers (MCCs) where horizontal bus runs behind the bucket compartments. The arc plasma behavior differs from vertical configurations because gravity and magnetic forces act differently on the horizontal arc.

VOA (Vertical Conductors in Open Air): Bus bars oriented vertically with no enclosure. This applies to open-air switchyards, outdoor bus work, and any situation where the arc occurs without an enclosure to focus the energy. Open-air arcs generally produce lower incident energy at the same working distance because the plasma can expand freely in all directions.

HOA (Horizontal Conductors in Open Air): Bus bars oriented horizontally in open air. This applies to overhead bus work and outdoor horizontal conductor arrangements. Like VOA, the lack of enclosure allows the arc to expand freely, but the horizontal orientation changes the plasma dynamics compared to vertical.

When selecting the electrode configuration for a study, match the physical arrangement of the conductors where the arc would initiate. If the equipment does not clearly match one of the five configurations, IEEE 1584-2018 provides guidance on selecting the most conservative (highest incident energy) applicable configuration. For equipment with multiple possible arc initiation points, evaluate each point separately and use the highest result.

Arcing Current and the Variation Factor (VarCf)

The arcing current is not the same as the available bolted fault current. When an arc initiates, the arc itself has impedance that reduces the current flowing through the circuit. The ratio of arcing current to bolted fault current depends on the system voltage, the gap between conductors, and the electrode configuration. At higher voltages, the arc impedance is a smaller fraction of the total circuit impedance, so the arcing current is closer to the bolted fault current. At 480V, the arcing current is typically 50 to 70 percent of the bolted fault current. At 4160V, it may be 85 to 95 percent.

IEEE 1584-2018 provides empirical equations for calculating the arcing current based on the bolted fault current, voltage, gap between conductors, and electrode configuration. The calculation uses logarithmic intermediate variables and a set of coefficients specific to each electrode configuration and voltage range. The model operates across three reference voltage ranges: 600V, 2700V, and 14300V. For voltages between these reference points, the model interpolates between the adjacent ranges.

The variation factor (VarCf) accounts for the random nature of arcing. Arc current is not constant — it fluctuates due to arc lengthening, magnetic blow-out effects, and random re-strikes. The VarCf produces a reduced arcing current that represents the lower bound of the expected arc current range. This reduced current must be checked against the time-current characteristics of the upstream protective device because a lower arc current may fall below the instantaneous trip threshold, forcing the device to clear on its time-delay curve instead. The longer clearing time at reduced current can produce significantly higher incident energy than the full arcing current scenario.

Both the full arcing current and the reduced arcing current must be evaluated, and the higher resulting incident energy governs. The reduced arcing current is calculated as Iarc x (1 - 0.5 x VarCf), where VarCf is the variation correction factor for the electrode configuration. A lower arcing current can produce higher incident energy because it may fall below the instantaneous trip pickup of the upstream protective device, forcing the device to clear on its slower time-delay curve. The longer clearing time at reduced current more than compensates for the lower current, often producing the worst-case incident energy. This dual evaluation is one of the most important changes from the 2002 model, particularly for systems at 480V and 600V.

Incident Energy and Arc Flash Boundary Calculation

Incident energy is calculated at a specific working distance — the distance from the arc source to the worker's face and chest. IEEE 1584-2018 specifies typical working distances for common equipment types: 455 mm (18 inches) for panelboards, 610 mm (24 inches) for motor control centers and low-voltage switchgear, and 910 mm (36 inches) for medium-voltage switchgear. These distances represent where a worker's torso would be positioned while performing the task. Using a shorter working distance increases the calculated incident energy; using a longer distance decreases it.

The incident energy calculation in the 2018 model uses the arcing current, the arc duration (protective device clearing time), the electrode configuration, the enclosure size correction factor, and the working distance. The equations produce incident energy in cal/cm² (or J/cm²), which is then compared to the arc rating of available PPE to determine the required protection level. If the incident energy exceeds 40 cal/cm², NFPA 70E considers the task to have an unacceptable risk level, and the work should not be performed while the equipment is energized.

The arc flash boundary is the distance from the arc source at which the incident energy drops to 1.2 cal/cm² — the threshold for a second-degree burn on unprotected skin. Everyone inside this boundary must wear appropriate arc-rated PPE. Everyone outside this boundary is considered safe from thermal injury (though blast pressure and sound can still cause injury beyond the thermal boundary). The arc flash boundary is calculated by rearranging the incident energy equation to solve for distance at 1.2 cal/cm².

The enclosure size correction factor adjusts the incident energy based on the physical dimensions of the enclosure. The 2018 model provides correction factors for a range of enclosure widths, heights, and depths. A shallow enclosure focuses more energy toward the opening, increasing incident energy. A deep enclosure allows the arc plasma to expand, reducing the energy at the opening. For open-air configurations (VOA, HOA), the enclosure correction factor is 1.0 (no correction). Equipment that does not match the tested enclosure sizes can be evaluated using the smallest tested enclosure that fully contains the equipment's internal dimensions, which produces a conservative result.

Systems Above 15 kV: The Ralph Lee Method

IEEE 1584-2018 is validated for systems from 208V to 15,000V with bolted fault currents from 500A to 106,000A. For systems above 15 kV — typically found in utility substations, large industrial plants, and generator facilities — the IEEE 1584 model does not apply. Instead, the Ralph Lee method (also called the theoretical maximum incident energy method) is used.

The Ralph Lee method, published in 1982, calculates the maximum power delivered to an arc based on the system voltage and bolted fault current. The theoretical maximum power in an arc occurs when the arc voltage equals half the system voltage, which means the arc impedance equals the source impedance. The resulting incident energy equation is:

E = 5.12 x 10⁵ x V x I_bf x t / D²

Where E is incident energy in cal/cm², V is system voltage in kV, I_bf is bolted fault current in kA, t is arc duration in seconds, and D is working distance in mm. This formula gives the result directly in cal/cm² with no further conversion needed. (To get J/cm², multiply by 4.184, or equivalently use the constant 2.142 x 10⁶ in place of 5.12 x 10⁵.) The Lee method produces a theoretical maximum and is generally considered conservative for most equipment configurations. However, it does not account for enclosure focusing effects, so in some enclosed configurations it may actually underestimate the hazard.

For systems between 15 kV and 36 kV, some engineers use the Lee method directly. Others use manufacturer-specific arc flash calculation tools that may incorporate test data for their specific equipment designs. IEEE is working on expanding the 1584 model to cover higher voltages in future editions, but as of the 2018 edition, the 15 kV upper limit remains.

Regardless of the calculation method, the same principles apply: evaluate the arcing current, determine the protective device clearing time, calculate the incident energy at the working distance, and determine the arc flash boundary. The protective device coordination study is just as important above 15 kV as it is at lower voltages, because clearing time is the single most controllable variable in the incident energy equation.

Practical Application: From Study to Label

An arc flash study produces incident energy values and arc flash boundaries for every piece of equipment in the facility where workers might interact with energized parts. The study results must be translated into arc flash warning labels posted on each piece of equipment. NFPA 70E requires that these labels include: the nominal system voltage, the arc flash boundary, at least one of the following — available incident energy and corresponding working distance, minimum arc rating of PPE required, or the required PPE category, and the date of the study.

The study depends on accurate input data. The bolted fault current at each piece of equipment must come from a short-circuit study. The protective device clearing times must come from a coordination study that accounts for the actual protective device settings (not just the nameplate ratings). If breaker trip units have been adjusted in the field, those adjusted settings must be used. A study based on factory default settings that do not match the installed settings can produce dangerously incorrect results.

Studies should be updated whenever there is a change to the electrical system that affects fault current levels or protective device clearing times. Adding a transformer, changing a generator, replacing a breaker trip unit, or modifying the protective device coordination scheme all require a study update. NFPA 70E does not specify a fixed update interval, but industry practice is every five years or whenever a significant system modification occurs. Some facilities with active electrical maintenance programs update their studies continuously using real-time monitoring and automated calculation tools.

The most effective way to reduce incident energy is to reduce the protective device clearing time. Faster clearing times mean less energy released. Zone-selective interlocking (ZSI), maintenance mode settings on breaker trip units, arc flash relays, and current-limiting fuses are all strategies used to reduce clearing times and lower incident energy levels across a facility. A well-coordinated protection scheme can often bring incident energy below 8 cal/cm² (PPE Category 2) for most equipment, making everyday maintenance safer and more practical.