In short. An arc-flash study calculates the thermal energy a worker could receive from an arcing fault at each piece of equipment. It needs verified fault currents and protection clearing times for every operating mode. Results drive labels, work procedures and engineering controls such as faster clearing.
Study architecture
- Scope and standard. Choose the method, commonly IEEE 1584, and the equipment included.
- Data. Single-line diagram, equipment type and enclosure dimensions, electrode configuration, working distance, gap, voltage, conductor data.
- Short-circuit study. Bolted fault currents for each mode. See short-circuit architecture.
- Arcing current. Calculated from bolted current and configuration; arcing current is lower than bolted current, so devices may be slower.
- Clearing time. From coordination curves at the arcing current, including minimum arcing current. See coordination workflow.
- Incident energy and boundary. Energy at working distance and distance at which energy falls to a threshold.
- Results review. Identify high-energy locations and drivers.
- Mitigation scenarios. Test changes and recalculate.
- Labels and documentation.
What drives incident energy
| Driver | Effect |
|---|---|
| Arcing current | Higher current, more energy, but also faster clearing |
| Clearing time | Energy rises roughly in proportion to time |
| Working distance | Energy falls rapidly with distance |
| Enclosure and electrode configuration | Enclosed gear directs energy toward the worker |
| Voltage and gap | Affect arc stability and current |
Generator-specific traps
- On generator supply, fault current is low and decays. Overcurrent devices may take far longer to clear, giving a higher energy than on the utility. Study each mode.
- Minimum arcing current can be below the instantaneous pickup, pushing clearing to a delayed region.
- Multiple sources feeding one bus all contribute until each breaker opens.
- Inverter sources limit current, which can leave the arc to be cleared by a slower upstream device. See inverter behaviour.
Mitigation hierarchy
- Eliminate: de-energise before work.
- Reduce energy: maintenance-mode settings, zone-selective interlocking, bus protection, arc detection. See arc-flash mitigation.
- Increase distance: remote racking and switching.
- Administrative: procedures, labels, training.
- PPE as the last line.
Illustrative example
A main LV board shows a high incident energy because the incomer’s short-time delay is 0.4 s. Adding a maintenance-mode input that drops the delay to instantaneous during work cuts the clearing time, and the study reports the reduced energy for the maintenance setting, with labels that show both. On generator supply, the low fault current makes the energy higher still, so a separate generator-mode result is shown.
What an arc-flash study delivers
The study estimates incident energy (in cal/cm²) and arc-flash boundary at each work location, using recognised methods such as IEEE 1584. Results give labels, PPE categories and a basis for reducing energy.
Inputs
| Input | Effect on result |
|---|---|
| Bolted fault current | Higher current, more energy, but may clear faster |
| Clearing time | Directly proportional to energy, the key lever |
| Voltage and gap | Arc sustainability |
| Electrode configuration and enclosure size | Directs energy towards the worker |
| Working distance | Energy falls with distance |
Two scenarios per location
- Maximum fault current, shortest clearing time.
- Reduced arcing current (minimum case), where the protective device may trip slowly. This case often produces the highest energy.
Generator specifics
A generator-only fault produces lower and decaying current. Overcurrent devices may take longer to trip, so incident energy can be higher than in the utility case. Study generator-supplied operation separately.
Reduction strategies
- Maintenance-mode instantaneous settings.
- Zone-selective interlocking.
- Arc detection relays on critical boards.
- Remote racking and operation.
Maintenance of results
Update after any change in source, settings or configuration. See arc-flash mitigation.
Common mistakes
- One value per board regardless of source.
- Labels not updated after setting changes.
- Relying on PPE to justify long clearing times.
What to send us
Send the single-line diagram, equipment details and protection data. Send your application details.
Questions engineers ask
What is the arc-flash boundary?
The distance from the arc at which a person could receive a second-degree burn without protection.
Why does a lower fault current sometimes raise energy?
The protective device may take longer to trip, and energy depends on time.
How often should studies be refreshed?
After system changes and periodically, per site policy and regulation.
Is software required?
Practical studies use software, but the inputs and interpretation need engineering judgement.
This guide gives general principles. Arc-flash studies must be done by qualified engineers.