Arc Flash Risk Assessment: Incident Energy, Boundaries and PPE Selection
arc flash risk assessment: An arc flash risk assessment calculates the incident energy a person standing in front of a board could be exposed to; the result fixes both the boundary distances and the class of protective equipment required.
Contents
The most serious injuries around electrical switchboards come not from shock but from arc flash. The arc formed at the instant of a short circuit reaches temperatures that vaporise metal within a tiny fraction of a second and throws molten particles out with a pressure wave. The severity of that event is not a matter of chance: it can be calculated from the fault current, the clearing time of the protective device and the distance of the person from the equipment. A risk assessment is exactly that calculation, turned into operating rules and labels.
What arc flash is and how it differs from an arcing fault
An arcing fault is the passage of current through air after insulation between two conductors breaks down. Arc flash is the thermal and pressure effect of that fault directed at a person. The internal arc withstand of a switchboard describes how the enclosure and compartments behave during such an event and provides protection while the doors are closed. During maintenance, measurement or commissioning, however, that protection disappears as soon as a door is opened, and it is precisely this situation the assessment addresses.
What incident energy depends on
Incident energy is determined by three main variables: the short circuit current at the fault position, the total clearing time of the device that will interrupt it, and the working distance of the person. Clearing time is the strongest of these; as time shortens, energy falls proportionally. What surprises many is that the highest risk is not always at the highest fault current: at intermediate levels the protective device may not enter its instantaneous region, so the clearing time lengthens and total energy comes out higher. The calculation is therefore made across a range of currents rather than for a single worst case.
Boundaries, labelling and documentation
The calculation produces an incident energy value and a corresponding boundary distance for each section of the board. The boundary is the distance at which the second degree burn threshold is reached, and no work is carried out inside it without protective equipment. Presenting the results on a legible label on the board is what allows the person on site to select the right equipment. The label should carry the date of the study and the assumptions used, because the assessment has to be updated when a transformer or a protection setting changes.
Methods of reducing the risk
| Method | Effect | Application note |
|---|---|---|
| Working de-energised | Removes the risk | Always the first choice |
| Shorter clearing time via maintenance mode | Reduces incident energy markedly | Requires an electronic trip unit |
| Optical arc detection relay | Brings clearing time down to milliseconds | Sensor placement is critical |
| Remote operation | Moves the person outside the boundary | Motor mechanism or extension handle |
| Internal arc rated compartments | Protects with the doors closed | Needs verification documentation |
The most effective way to reduce risk is to switch off and work de-energised; where that is not possible, the design is used to intervene. A maintenance mode function temporarily lowers the protection threshold while someone is working in front of the board, shortening clearing time. Optical arc detection relays recognise an arc from light and current together and issue a trip command within milliseconds. Remote operation of a circuit breaker moves the person outside the boundary. A compartmented board with documented internal arc withstand provides the baseline protection with the doors closed.
Protective equipment and the work procedure
Protective equipment is selected in a class that covers the calculated incident energy, and clothing, face shield, gloves and helmet are considered as one system. Equipment is nevertheless the last layer: the correct sequence is to isolate first, then lock and tag, then verify the absence of voltage by measurement. Where live measurement is unavoidable, a work permit, a second person in attendance and equipment of the right class are applied together. If the results of the study are never passed to the field through training, even the best calculation stays on paper.
Key takeaways: arc flash risk assessment
- Clearing time — Clearing time is the strongest driver of incident energy; every millisecond saved lowers the energy.
- Worst case — The highest risk is not always at the highest fault current, so the study spans a range of currents.
- Labelling — If results are not shown on a legible label, the field team is left without a basis for decisions.
- Sequence — Protective equipment is the last layer; isolation, locking, tagging and voltage verification come first.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Where is arc flash risk highest?
Main distribution points with high short circuit current come to mind first, but clearing time is the real driver. At intermediate current levels, where the protective device does not enter its instantaneous region, the time lengthens and incident energy can be higher than expected. That is why each compartment is assessed separately.
Does an internal arc rated board make the assessment unnecessary?
No. Internal arc withstand protects people while the doors are closed. During maintenance, measurement and commissioning that protection is out of service once a door is open, and the risk assessment and equipment selection exist for exactly that scenario.
How does a maintenance mode function work?
On breakers with an electronic trip unit, the protection threshold and delay are temporarily lowered for the period during which work takes place in front of the board. A fault is then cleared much faster and the incident energy falls. When the work is finished the function returns to normal settings.
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- Internal Arc Withstand and Arc Protection: What IEC 61641 Adds
- What Is Selectivity? Protection Coordination in Switchgear
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Devpan designs switchboards with internal arc withstand and compartmentation in mind, and plans protection settings and maintenance mode functions in line with risk assessment results. Share your single line diagram and protection settings and we will review them with you. Related Devpan solution: electrical panel design services.
