An arc flash label is the visible end of a long analytical chain. Behind every incident energy value is a model of the power system, a set of assumptions about how equipment is built and operated, and a calculation method. In North America, that method is almost always IEEE 1584, IEEE Guide for Performing Arc-Flash Hazard Calculations. The 2018 edition was the most significant revision since the guide was first published, and its effects are still working their way through facility studies.
Why the 2018 Edition Matters
The 2018 model was developed from a substantially expanded test program. It introduced factors that earlier editions did not address directly, and in many cases it produces results that differ meaningfully from IEEE 1584-2002. The main changes include:
- Electrode configurations: five configurations describe how conductors are oriented and whether they terminate in an insulating barrier: VCB, VCBB, HCB, VOA, and HOA.
- Enclosure size correction: the dimensions of the enclosure influence how energy is directed toward the worker.
- Arcing current variation: a reduced arcing current is calculated to account for variability, because a lower current can lengthen protective device clearing time and increase energy.
- Defined model range: the empirical model applies to three-phase AC systems from 208 V to 15 kV within specified limits of bolted fault current, conductor gap, and working distance.
Horizontal configurations, in particular, can produce considerably higher incident energy at the working distance than the vertical configurations that earlier studies effectively assumed.
The guide also changed the treatment of low-voltage, low-fault-current equipment. Rather than relying on simplified exemptions, engineers are expected to apply the model, or documented engineering judgment, across the full range of equipment in scope. Organizations moving from a 2002-based study to a 2018-based study should plan for label changes and communicate them clearly to the workforce.
The Study Process, Step by Step
A credible study follows a disciplined sequence:
- Collect system and installation data, including utility source information, transformers, cables, protective devices, and equipment types.
- Determine the system modes of operation, such as tie breakers open or closed, generator operation, and maintenance switching.
- Calculate bolted fault currents at each location.
- Calculate arcing currents, including the reduced arcing current.
- Determine protective device characteristics and arc duration for both arcing current values.
- Document system voltages, equipment classes, electrode configurations, gaps, and enclosure sizes.
- Select working distances appropriate to the tasks performed.
- Calculate incident energy and the arc flash boundary, the distance at which incident energy falls to 1.2 cal/cm².
The final step is not a number. It is a set of labels, PPE requirements, and recommendations that people in the field can use.
Data Quality Drives Result Quality
The most sophisticated model cannot compensate for inaccurate inputs. Common data problems include unverified utility fault contributions, missing cable lengths, protective device settings that differ from the as-designed values, and equipment that was replaced without updating the model. A field walkdown by qualified personnel is essential. So is a defined process for updating the study when the system changes.
NFPA 70E requires that the arc flash risk assessment be reviewed periodically, at intervals not exceeding five years, and updated when a major modification or renovation occurs. That requirement assumes the study is a living document.
Practical data collection also depends on safety. Gathering nameplate data, gaps, and enclosure dimensions often requires opening equipment. Those tasks must be planned with their own risk assessment, and when information cannot be obtained safely, the study should record conservative assumptions rather than guesses presented as facts. Protective device settings should be read from the devices themselves or from controlled settings records, not taken from original design drawings.
From Calculation to Field Practice
| Study output | Field use |
|---|---|
| Incident energy at working distance | Selection of arc-rated clothing and PPE |
| Arc flash boundary | Establishing approach limits and barricades |
| Nominal voltage and shock boundaries | Selecting rubber insulating equipment and approach distances |
| Device clearing times | Identifying candidates for incident energy reduction |
Many studies reveal locations where energy is high because an upstream device clears slowly. Engineering options include maintenance switching, zone-selective interlocking, differential protection, arc flash relays with optical sensing, and current-limiting devices. These are higher-order controls in the NFPA 70E hierarchy and deserve serious consideration before PPE is specified.
Common Pitfalls
- Applying the empirical model outside its stated range without engineering judgment or documented alternatives.
- Labeling the line side of a main device with the protection provided by that same device.
- Ignoring alternative operating modes that produce higher energy.
- Treating labels as permanent when settings, transformers, or utility sources change.
A study is a model of reality. Its value depends on how faithfully it reflects the equipment in the field today.
Key Takeaways
- IEEE 1584-2018 adds electrode configuration, enclosure size, and arcing current variation to the calculation.
- Results can differ significantly from 2002-era studies, particularly for horizontal configurations.
- Accurate field data and operating-mode analysis are as important as the calculation itself.
- Studies must be reviewed at least every five years under NFPA 70E and updated after major changes.
- Use study findings to drive engineering controls, not just PPE selection.
ASEEE offers arc flash hazard analysis and power system analysis programs within its Electrical Safety and Power Systems disciplines. Review the current offerings on our courses page, and see our FAQ for enrollment and certification details.
