Closing the covers does not shorten the flash protection boundary (FPB). The boundary is measured from the arc location inside the enclosure, and the incident-energy calculation assumes the cover is off. The 10-15 ft boundaries at panels fed by 225 kVA transformers, and the 150 ft boundary at a 208 V panelboard fed by a 45 kVA transformer, come from one input: fault clearing time on a thermal-trip breaker at 1-2 kA. Correct that input, then set access and PPE rules by task.
Reading the symptoms in a low-fault-current arc flash study
Every symptom in this class of study traces back to available fault current sitting below the breaker's instantaneous pickup. The table separates the observation from the cause and the decisive check.
| Observation | Cause | Where to read the deciding quantity |
|---|---|---|
| Level 4 incident energy at load panels downstream of a 225 kVA transformer, even with a 2 s cap | Fault current of 1-2 kA falls in the overload (long-time) region of the breaker curve, so clearing runs to the cap | Breaker time-current curve (TCC) at the arcing current, not the bolted current |
| FPB of 10-15 ft in hallways and corridors | Incident energy scales with arc duration; boundary is the distance where energy falls to the onset-of-burn threshold | Calculation report: clearing time, arcing current, working distance, boundary |
| FPB of 150 ft, clearing times up to 50 s, 1.2 kA, 45 kVA source | Thermal-magnetic breaker on its thermal element at a current only slightly above its trip rating | Breaker TCC lower-current end; transformer kVA and impedance for available fault current |
Clearing time as the deciding quantity
Incident energy at a fixed working distance is proportional to arc power multiplied by arc duration. At 1-2 kA the arc power is small, but a thermal-magnetic breaker that never reaches its instantaneous pickup lets the arc burn for the full long-time delay. A 50 s clearing time against a 2 s cap is a 25:1 ratio in incident energy at the same current and distance (50 / 2 = 25). The boundary distance shrinks more slowly than the energy: it scales as the energy ratio raised to 1/x, where x is the distance exponent for the equipment class in the calculation method. Read x from the tool, not from memory. At 1.2 kA with 50 s, the boundary is a function of duration almost entirely.
Two consequences follow. First, a breaker that trips on its overload element is the wrong protective device to credit for arc clearing, so the study result says more about breaker selection than about the panel. Second, ground fault protection is not a valid basis for reducing the calculated arc energy, because the arc may begin as, or evolve into, a phase-to-phase event that ground fault elements never see.
The clearing-time cap of 2 s is a judgment limit tied to how quickly a worker can move away from the arc. It is not a measured arc duration. Apply it only where the worker can egress; a worker in a confined or awkward position does not get that assumption.
Why the enclosure surface is not the boundary
The calculation models an arc in a box with the cover removed and does not account for relays, breakers, or other hardware that may direct or deflect the arc. The result may not be the worst case, because the variables are too numerous to bound. Three points settle the covers-on question:
- An arc flash calculation assumes something has already gone wrong and an explosive event has occurred. A closed door does not change the assumed event.
- The interrupting or short-circuit rating states that the equipment can withstand and interrupt the fault, including the magnetic forces. It does not state that the equipment contains the arc blast. Containment belongs to arc-resistant construction, and that rating has its own test conditions and limits.
- Pressure and fireball from an arc inside a closed enclosure find an exit. The door may blow open, or the release may come from a loose screw hole at face height or a conduit opening at the top. The exit path cannot be predicted.
Do not terminate the boundary at the enclosure surface for covers-on equipment. Treat the boundary as centered on the arc source inside the enclosure.
Access and PPE rules by task, separate from the boundary distance
A boundary of 10-15 ft in a corridor does not mean office staff must dress in PPE to walk past. Require the posted PPE level when either condition is true:
- Someone is changing the state of the device (operating a breaker or switch, racking a device).
- There are exposed energized parts, for example covers removed or doors open with live parts accessible.
Write this rule into the arc flash program so the labels remain credible. The 3.5 ft limited approach boundary at 208 VAC is a shock boundary for unqualified persons near exposed energized conductors. It does not close a corridor when the equipment is enclosed and undisturbed. Confirm the boundary values and task definitions against the edition your facility follows.
Transformer size threshold and study scope
Both transformer sizes in this study sit on either side of a scope allowance, so decide the path per panel:
| Source transformer | Decision |
|---|---|
| 225 kVA | Above the 125 kVA threshold. Include in the study and correct the clearing time and arcing-current inputs. |
If you exempt a panel from the calculation, apply the standard's table-based approach for its category and record the basis on the study. The table row selected depends on the voltage class and task; read the row from the edition in force rather than from a summary.
Procedure for re-running a low-fault-current panelboard
- Recalculate available bolted fault current from the transformer kVA, impedance, and upstream source. Record it per panel.
- Compute the arcing current for each panel. The arcing current is below the bolted current, and low-current arcs can land in a very different breaker curve region.
- Plot both the full arcing current and the reduced-arcing-current variant on the breaker TCC. Use the longer clearing time of the two, as the calculation method directs.
- Identify whether the breaker responds in the instantaneous or the thermal region. If thermal, note the trip time and the tolerance band on the TCC.
- Apply the 2 s cap only where the worker can move away. Document the location and posture assumption for each panel.
- Re-run incident energy and boundary with the chosen time and compare with the uncapped result.
- Evaluate mitigation: a breaker or fuse with faster response at the arcing current, a maintenance-mode setting where available, or moving load to a device that reaches instantaneous pickup.
- Label, and write the task-based access rule from the previous section into the program.
Checks before releasing labels and boundaries
| Quantity | Limit or expectation | Where to read it |
|---|---|---|
| Arcing current vs breaker pickup | State whether it is above or below instantaneous pickup | Breaker TCC and settings |
| Clearing time used | Traceable to a curve point or the documented 2 s cap | Calculation report |
| Working distance | Matches the actual task position | Calculation inputs |
| Boundary vs geometry | Does not end at the enclosure surface | Site plan and label |
| Transformer kVA basis | Above or below 125 kVA scope allowance stated per panel | Transformer nameplate, study scope |
A result that changes by an order of magnitude when the clearing time changes is telling you the breaker, not the panel, is the finding. Report that sensitivity in the study.
Pitfalls on 208 V panels fed by small transformers
- Using bolted fault current to read the breaker curve. Arcing current is lower and can sit in the thermal region.
- Crediting ground fault protection to lower the phase-fault incident energy.
- Treating the interrupting rating as proof of arc containment.
- Posting a 150 ft boundary without stating the clearing time behind it. The number invites disregard and erodes trust in every other label.
- Confusing the limited approach (shock) boundary with the flash protection boundary, then closing corridors for the wrong reason.
- Applying the 2 s cap uniformly, including to positions where egress is restricted.
FAQ
What happens if the panel covers are on during an arc flash event?
The energy release still occurs inside the enclosure, and the pressure exits through the door, screw holes, or conduit openings at unpredictable locations. The calculation assumes the cover is off, so do not shorten the boundary for covers-on equipment.
What happens if the breaker trips on its thermal element at 1.2 kA?
Clearing time runs to tens of seconds, and incident energy grows in proportion, which is how a 45 kVA-fed 208 V panelboard reaches a 150 ft boundary at up to 50 s. Read the trip time from the breaker TCC at the arcing current and evaluate a faster device or setting.
What happens if an unqualified person walks through the flash protection boundary?
With covers on and nobody changing device state, the practical rule is that no PPE is needed to pass. PPE at the posted level applies when someone is operating the device or there are exposed energized parts.
What happens if I cap fault clearing time at 2 seconds?
The incident energy and boundary drop sharply, but the cap is a judgment about worker egress, not a measured arc duration. Apply it only where the worker can move away, and document the assumption per panel.
What happens if the study result still looks unreasonable after correcting clearing time and arcing current?
Stop and escalate: send the breaker TCC, arcing-current case, and calculation inputs to the breaker manufacturer's or study software vendor's official technical support. Have a licensed engineer or the authority having jurisdiction confirm the edition, the 125 kVA scope allowance, and the clearing-time assumption before labels are posted.