Arc Flash Risk Assessment: IEEE 1584-2018 Calculations, PPE Selection, and NFPA 70E Program Integration
Published: June 2026 Technical Level: Advanced Category: Safety Standards
Abstract
Arc flash risk assessment is the engineering process mandated by NFPA 70E for quantifying the thermal hazard to workers performing tasks on or near energized electrical equipment. The assessment produces two key outputs: the incident energy in cal/cm² at the worker's working distance, which determines the minimum arc rating of required personal protective equipment, and the arc flash boundary — the distance at which the incident energy equals 1.2 cal/cm² — within which arc-rated PPE is mandatory. This paper develops the IEEE 1584-2018 calculation procedure, the NFPA 70E framework for PPE category selection and labeling, and the integration of arc flash study results into an Electrical Safety Program. The paper also addresses the practical engineering judgment required to distinguish between arc flash hazard (available arc energy) and arc flash risk (likelihood of an arc during a specific task), a distinction that NFPA 70E embeds in its risk assessment structure and that is frequently lost in simplified PPE assignment policies.
1. Introduction
Arc flash is a proximity hazard. A worker performing a task within a few feet of energized conductors or equipment can receive a severe thermal burn from an arc flash event without making contact with any energized part. The arc plasma — which reaches temperatures of 20,000 K to 35,000 K — radiates intense heat that can cause second-degree burns in milliseconds at distances of one to three feet. The severity depends on the incident energy at the worker's position, which in turn depends on the available fault current, the arc duration (determined by the upstream protective device clearing time), and the geometry of the equipment.
NFPA 70E's risk assessment framework requires engineers to address both the magnitude of the hazard and the likelihood of an arc flash event for the specific task being performed. These are distinct considerations. The hazard — available arc energy at the equipment — is an inherent characteristic of the electrical system that the engineer calculates using IEEE 1584-2018 and quantifies on the equipment label. The risk — the probability that an arc flash will actually occur during a specific task — depends on the nature of the task, the condition of the equipment, whether covers are removed, and the work methods used. NFPA 70E's acknowledgment that certain tasks on properly maintained, normally operating equipment may not require arc-rated PPE reflects this distinction: the hazard is present, but the risk of the specific task is assessed as acceptably low.
2. IEEE 1584-2018 Incident Energy Calculation
2.1 Available Fault Current and Arcing Current
The first step in arc flash analysis is establishing the available three-phase bolted fault current at each piece of equipment. This requires a current short-circuit study based on an accurate single-line diagram showing source impedances, transformer reactances, conductor impedances, and motor contributions. The accuracy of the incident energy calculation is directly limited by the accuracy of the fault current model: an outdated one-line diagram that does not reflect utility transformer upgrades, cable replacements, or added generation will produce incorrect incident energy values.
The IEEE 1584-2018 arcing current model converts the bolted fault current into the arcing current — the current that actually flows during an arc flash event. Arcing currents are lower than bolted fault currents because the arc impedance limits current flow; they are typically 50 to 85 percent of the bolted fault current at low voltage and 85 to 95 percent at medium voltage. The arcing current is calculated using regression models that are functions of the bolted fault current, the system voltage, the electrode gap (specific to the equipment type), and the electrode configuration class (vertical conductors in a box, horizontal conductors in a box, open air, etc.).
2.2 Incident Energy
With the arcing current established, the incident energy at the working distance is:
Where: is the incident energy in J/cm².
is an enclosure correction factor, greater than 1.0 for boxed configurations where the enclosure concentrates the arc plasma.
is the arcing current in kA.
is the bolted fault current in kA.
is the electrode gap in mm (25 mm for 480 V MCC; 102 mm for 15 kV switchgear).
is the system voltage in kV.
is the arc duration in seconds, equal to the upstream protective device clearing time at .
is the working distance in mm.
is the distance exponent for the electrode configuration (2.0 for open air; lower values for boxed configurations).
through are regression coefficients from IEEE 1584-2018 for the specific electrode configuration class.
Converting to cal/cm²: .
The arc duration — determined by the upstream protective device's time-current characteristic at the arcing current — is the dominant engineering variable. Incident energy is approximately proportional to arc duration: halving the clearing time halves the incident energy. This proportionality is why protective device setting strategies — maintenance mode instantaneous tripping, zone-selective interlocking, arc-quenching devices — are the primary engineering tools for arc flash hazard reduction.
2.3 Arc Flash Boundary
The arc flash boundary is the working distance at which the incident energy equals 1.2 cal/cm² (5 J/cm²), the threshold for second-degree burn on unprotected skin. It is calculated by solving the distance relationship in the IEEE 1584 model:
Where: is the arc flash boundary in mm.
is the incident energy at the 610 mm reference distance in J/cm².
is the distance exponent for the configuration.
Inside the arc flash boundary, arc-rated PPE at the appropriate minimum arc rating is required for all energized work tasks. Outside the boundary, the incident energy is below the burn threshold and arc-rated PPE is not required by NFPA 70E, though other electrical hazards (shock) may still require PPE.
2.4 Worked Example: 480 V Motor Control Center
The complete calculation procedure is best understood through a representative case. Consider a 480 V motor control center with an available bolted fault current of 42 kA, a typical electrode gap of 25 mm for this class of equipment, and a working distance of 457 mm (18 inches) as specified by IEEE 1584 for low-voltage MCCs. The upstream protective device clears the arcing fault in 0.2 seconds at the calculated arcing current.
Applying the IEEE 1584-2018 arcing-current correlation for a 480 V boxed configuration yields an arcing current of approximately 23.5 kA, reflecting the characteristic reduction from the bolted value at low voltage. Substituting this arcing current, the 42 kA bolted current, the 25 mm gap, and the 0.2 second clearing time into the incident-energy expression gives an incident energy at the 457 mm working distance of approximately 33.5 J/cm², which converts to:
This result places the hazard at the upper edge of PPE Category 2, requiring an arc-rated ensemble with a minimum arc rating of 8 cal/cm². Solving the boundary relationship for the distance at which the incident energy falls to the 1.2 cal/cm² second-degree-burn threshold gives an arc flash boundary of approximately 1,200 mm, or about 47 inches. Any worker approaching within 47 inches of the energized MCC during this task must wear the Category 2 ensemble, and the boundary distance must be marked on the equipment label.
The example also demonstrates the leverage of clearing time: if the upstream device were configured with a maintenance-mode instantaneous trip reducing the clearing time to 0.05 seconds, the incident energy would fall by the same factor of four to roughly 2.0 cal/cm², dropping the requirement to Category 1 and shrinking the boundary substantially. This single result is the strongest argument for energy-reducing maintenance switches on equipment that is worked while energized.
The dependence of incident energy on the fault current and clearing time is shown in Figure 1, which plots incident energy against arcing current for a family of clearing times at this voltage and working distance.
Figure 1. Arc-flash incident energy at a 480 V bus and 18-inch working distance, plotted against arcing current for clearing times from 0.1 to 1.0 seconds. The shaded region lies above the 8 cal/cm² PPE Category 2 limit.
The figure makes the two controlling variables visible simultaneously: incident energy rises with arcing current along each curve, and the curves fan out with clearing time, so that a fault cleared in one second produces an order of magnitude more incident energy than the same fault cleared in 0.1 second. A protection engineer reads the practical conclusion directly from the vertical spacing of the curves — reducing clearing time, not reducing fault current, is almost always the faster and cheaper path to moving a given bus out of the shaded Category-3-and-above region.
3. PPE Selection
3.1 Incident Energy Analysis Method
The incident energy analysis method selects PPE directly from the calculated incident energy value. The worker's arc-rated clothing ensemble must have an arc thermal performance value (ATPV) at or above the calculated incident energy in cal/cm². NFPA 70E Table 130.5(G) provides minimum ensemble requirements for four PPE categories:
Category 1 (4 cal/cm² minimum arc rating) consists of arc-rated long-sleeve shirt and pants or arc-rated coverall. Category 2 (8 cal/cm²) adds an arc-rated jacket or arc-rated coverall with higher arc rating. Category 3 (25 cal/cm²) requires a multi-layer system with an arc-rated hood covering the face and head. Category 4 (40 cal/cm²) is a full arc-flash suit with integrated hood and is the maximum category defined by NFPA 70E. For calculated incident energy above 40 cal/cm², NFPA 70E Section 130.5(G) notes that energized work is not recommended; if required, additional engineering controls must be applied to reduce the incident energy to a level where Category 4 PPE provides adequate protection.
The mapping from calculated incident energy to PPE category follows the threshold structure below, which the engineer applies directly to the cal/cm² result of the incident-energy analysis:
| PPE Category | Incident energy range (cal/cm²) | Minimum arc rating (cal/cm²) | Representative ensemble |
|---|---|---|---|
| 1 | 1.2 – 4 | 4 | Arc-rated shirt and pants or coverall, face shield |
| 2 | 4 – 8 | 8 | Add arc-rated jacket; arc-rated balaclava and hood |
| 3 | 8 – 25 | 25 | Multi-layer arc-rated system with full hood |
| 4 | 25 – 40 | 40 | Full arc-flash suit with integrated hood |
| — | > 40 | — | Energized work not recommended; reduce energy first |
The lower bound of 1.2 cal/cm² is the second-degree-burn threshold: below it, arc-rated PPE is not required for the arc-flash hazard, though shock-protection PPE may still apply. The worked example above, at 8.0 cal/cm², sits exactly at the Category 2 / Category 3 boundary, which is the kind of marginal result that justifies recalculating with the maintenance-mode clearing time before committing a worker to a heavier ensemble.
3.2 PPE Category Method
The arc flash PPE category method uses NFPA 70E Table 130.7(C)(15)(a) to assign a PPE category based on equipment type and nominal voltage, without requiring an incident energy calculation. The method is valid only within its defined applicability limits: system voltage at or below 600 V, available fault current not exceeding 42 kA, fault clearing time not exceeding 12 cycles (0.2 seconds), and minimum working distance as tabulated. When any condition is exceeded, the incident energy analysis method must be used.
The PPE category method assigns conservative categories that reflect the worst-case incident energy within the method's applicability range for each equipment type. For a 480 V MCC, the table assigns Category 2 (8 cal/cm²). An incident energy analysis for the same MCC may calculate 4 cal/cm² with fast-clearing upstream protection, in which case Category 1 PPE would be adequate. The PPE category method is therefore conservative by design — appropriate when analysis resources are not available, but potentially over-specifying PPE relative to the actual hazard.
4. Hazard vs. Risk: Task-Based Assessment
NFPA 70E Section 130.5(A) requires that the arc flash risk assessment consider both the severity of injury (determined by incident energy) and the likelihood of an arc flash occurring during the specific task. NFPA 70E Section 130.2(A) acknowledges that for certain tasks on electrical equipment that is in normal operating condition — with covers in place, no exposed energized conductors, and the task limited to visual observation or operation of properly maintained switching devices at their normal design positions — the risk of an arc flash is sufficiently low that the standard does not require arc-rated PPE.
This exception applies narrowly. The equipment must be in normal operating condition (no known defects, no evidence of deterioration, maintained per manufacturer requirements). The task must not involve exposing energized conductors or removing barriers. And the operation must be within the normal design function of the device. A worker reading a panel meter through a viewing window, or operating a properly maintained molded-case circuit breaker by its handle, is performing a task with low arc flash probability. A worker racking a draw-out breaker, removing a deadfront panel, or connecting conductors at energized terminals is performing a task with substantially higher probability of arc flash initiation, and arc-rated PPE at the appropriate level is required.
5. Electrical Safety Program Integration
NFPA 70E Article 110 requires employers to establish, document, and maintain an Electrical Safety Program (ESP) that governs all energized electrical work activities. The arc flash study is one technical input to the ESP, not the ESP itself. The ESP must include documented energized work procedures with task-specific hazard identification, an energized electrical work permit for tasks where de-energized work has been determined to be infeasible, a PPE inspection and maintenance program, training documentation demonstrating that qualified workers understand NFPA 70E requirements, and a periodic audit cycle to verify field compliance.
Arc flash equipment labels — required by NEC 110.16 and NFPA 70E 130.5(H) — are the primary interface between the arc flash study and daily work practice. Each label must show the nominal system voltage, the incident energy at the specified working distance (or the PPE category), the arc flash boundary, the minimum arc rating required, and the date of the assessment. NFPA 70E requires that labels be updated whenever the system configuration changes in a way that materially affects the incident energy results, and at intervals not exceeding five years in any case. Labels older than five years should be treated as suspect and scheduled for update, because even without visible system changes, utility fault current levels and protective device condition can shift the incident energy from the labeled value.
Related Work
The analysis in this paper connects to several companion studies in this library. Readers concerned with the upstream and downstream engineering will find Arc Flash Risk Assessment develops a closely related aspect of the same problem, while Arc Flash Calculation Software extends the treatment into an adjacent domain. For the broader methodological context, Short-Circuit Analysis provides complementary depth.
Conclusion
Arc flash risk assessment produces two engineering outputs that govern worker safety — the incident energy in cal/cm² at the working distance, which sets the minimum arc rating of required PPE, and the arc flash boundary at the 1.2 cal/cm² threshold — and the 480 V motor control center example developed in this paper shows how the IEEE 1584-2018 procedure delivers both from the available fault current and the protective device clearing time. The distinction this paper draws between hazard and risk is the conceptual core of NFPA 70E compliance: the incident energy quantifies the hazard, but the task-based risk assessment determines whether and how the work is performed, and the most effective control is to eliminate the hazard by establishing an electrically safe work condition rather than to mitigate it with PPE. For the practicing engineer, the operative conclusion is that the arc flash study must be integrated into a living electrical safety program — driving labeling, PPE specification, work permitting, and the periodic revalidation triggered by any change in fault current or protective settings — rather than archived as a standalone calculation.
References
[1] NFPA 70E, Standard for Electrical Safety in the Workplace, 2021 edition, NFPA, 2021.
[2] IEEE Standard 1584-2018, Guide for Performing Arc-Flash Hazard Calculations, IEEE, 2018.
[3] NFPA 70, National Electrical Code, Section 110.16, 2023 edition, NFPA, 2023.
[4] OSHA 29 CFR 1910.333, Selection and Use of Work Practices, U.S. Department of Labor.
[5] P. E. Sutherland, "NFPA 70E Shock and Arc Flash Risk Assessment Best Practices," IEEE IAS Annual Meeting, 2016.
[6] J. C. Das, Arc Flash Hazard Analysis and Mitigation, IEEE Press/Wiley, 2012.
[7] R. F. Ammerman et al., "DC Arc Models and Incident Energy Calculations," IEEE Transactions on Industry Applications, vol. 46, no. 5, 2010.
[8] NFPA, Arc Flash and the NFPA 70E Standard, NFPA white paper, 2023.
[9] IEEE Standard 242-2001, Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems, IEEE, 2001.