In short. An arc flash is an explosive release of energy from an electrical fault in air. The risk depends on fault current, clearing time, working distance and equipment configuration. Mitigation focuses on reducing the clearing time and keeping people away from live equipment.
What sets the hazard
- Fault current available at the equipment.
- Clearing time of the upstream protection. Longer time means more energy.
- Gap, enclosure and electrode configuration.
- Working distance of the person.
Lower fault current is not always safer: it can slow a protective device and lengthen the clearing time. This is common on generator supply, so arc-flash must be studied in each operating mode.
Study outline
- Collect equipment and protection data and verify the single-line diagram.
- Calculate fault currents, including minimum and maximum cases. See fault levels.
- Determine protective device clearing times from coordination curves.
- Calculate incident energy and arc-flash boundary with a recognised method such as IEEE 1584.
- Define PPE categories or, better, mitigation to reduce the energy.
- Label equipment and communicate the results.
Mitigation options
| Option | Effect |
|---|---|
| Maintenance mode switch | Temporarily lowers trip settings during work |
| Zone-selective interlocking / bus protection | Faster clearing of bus faults. See busbar protection |
| Arc detection relays | Trip in a few milliseconds on light detection |
| Current-limiting fuses or breakers | Limit let-through energy |
| Remote racking and operation | Keeps people out of the boundary |
| Arc-resistant switchgear | Directs energy away from operators |
| Work practices | De-energise before working where possible |
Generator-specific points
- Fault current decays, so clearing time at the generator end matters.
- With multiple sources, bus faults are fed from all, so bus protection should trip all.
Illustrative example
A study shows an incomer with a delayed short-time setting produces a high incident energy at the main board. Adding a maintenance mode switch with an instantaneous setting reduces clearing time during work, and the label shows both energy levels, with instructions on which mode applies. A later project adds arc detection to cut the time further.
What arc flash is
An arc fault releases intense heat, light, pressure and molten metal when current jumps through air between conductors. Incident energy depends on fault current, arc duration, working distance and enclosure type. Duration is the factor protection can control most.
Incident energy rises roughly in proportion to arc current × arc duration
Halving the clearing time roughly halves the energy, other things equal.
Mitigation hierarchy
| Level | Measure |
|---|---|
| Eliminate | De-energise before work |
| Reduce energy | Faster protection, maintenance switch settings, zone interlocking, arc detection relays |
| Contain | Arc-resistant switchgear, venting |
| Distance | Remote racking and remote operation |
| PPE and procedure | Last line of defence |
Study and labelling
An arc flash study calculates incident energy at each location using recognised methods such as IEEE 1584 and results in labels showing the hazard level. Studies need updating after system changes.
Generator specific points
Generator fault current decays, so a slow relay may see too little current to trip quickly. Check low-fault cases, since clearing time may be longer than at the utility level.
Practical actions
- Use maintenance-mode instantaneous settings during live work.
- Fit arc detection on critical boards.
- Provide remote operation of large breakers.
- Train staff and follow the site’s electrical safety rules.
Common mistakes
- Study at the utility case only.
- Labels not updated after changes.
- Relying on PPE alone.
What to send us
Send the single-line diagram and protection data. Send your application details. See also setting philosophy.
Questions engineers ask
Does a lower fault current reduce arc-flash energy?
Not always. If it slows clearing, energy can increase.
What is a maintenance mode switch?
A switch that applies faster settings while people work on or near live equipment.
How often should the study be updated?
After system changes and periodically, per site policy and regulations.
Is PPE enough?
It is the last line of defence. Reduce energy and exposure first.
This guide gives general principles. Arc-flash studies must be performed by qualified engineers.