Internal Arc Withstand and Arc Protection: What IEC 61641 Adds
Internal arc withstand: How internal arc faults develop, what IEC 61641 verifies and which protection measures are genuinely effective.
ContentsInternal arc withstand means that an arc fault between phases or from phase to earth inside a switchboard is managed without injuring personnel or bursting the enclosure. An arc is the most destructive form of a short circuit: within a few milliseconds it produces temperatures of thousands of degrees, a sudden pressure rise, molten metal spatter and intense light. Conventional short circuit withstand does not describe this event; internal arc requires a separate verification.
How an arc starts
On site the causes of arc faults are surprisingly ordinary: a loosened busbar bolt, a tool left behind, rodent or bird ingress, surface tracking caused by moisture and contamination, a switching error, or the prolonged heating of an undersized joint. None of these is instantaneous; almost all of them leave a thermal signature beforehand. That is why preventive maintenance is the most effective way to reduce arc risk.
What IEC 61641 verifies
IEC 61641 defines a test guide for assessing how an assembly behaves under internal arcing conditions. During the test a controlled arc is initiated inside the board, and the criteria assessed are whether cotton indicators placed at defined points outside ignite, whether doors and panels open, and whether the continuity of the protective conductor is maintained. The result is documented as a withstand duration together with a set of criteria. The key point is that this verification does not show that an arc will not occur, but that its consequences stay under control if it does.
Effective measures and their limits
| Measure | What it delivers | Limitation |
|---|---|---|
| Insulated (coated) busbars | Largely prevents arc initiation between phases | Insulation is interrupted at joints |
| Separation (Form 3b to 4b) | Limits arc propagation to neighbouring units | Reduces thermal performance |
| Pressure relief duct | Directs hot gas away from personnel | Duct route and outlet must be planned |
| Arc detection relay | Cuts clearing time to a few milliseconds | Requires correct sensor placement |
| Arc eliminator (crowbar) | Quenches arc energy through a deliberate short circuit | High capital cost and added maintenance |
| Periodic thermal imaging | Catches pre arc heating in advance | Needs a regular programme and records |
Key takeaways: internal arc withstand
- How an arc starts — On site the causes of arc faults are surprisingly ordinary: a loosened busbar bolt, a tool left behind, rodent or bird ingress, surface tracking caused by moisture and contamination, a…
- What IEC 61641 verifies — IEC 61641 defines a test guide for assessing how an assembly behaves under internal arcing conditions.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Is the internal arc test mandatory for every switchboard?
It is not a mandatory verification under IEC 61439; IEC 61641 has the status of a guide. It is nevertheless demanded by specification in critical plants, in environments where personnel frequently work in front of boards, and at points with high prospective short circuit current.
Is PPE alone enough instead of arc protection?
No. Personal protective equipment is the last layer and it does not reduce the energy of the event. The correct order is to reduce the risk by design, then lower the energy through detection and fast clearing, and only then manage the residual risk with PPE. The role of separation in that chain is covered in our article on separation forms.
Related articles
- Form 1, 2b, 3b and 4b Internal Separation Explained
- Short Circuit Withstand in Switchboards: Icw, Ipk and Conditional Rating
- What is Short Circuit Withstand (ICW)? Short Circuit Protection in Electrical Panels.
- All technical articles
At Devpan we assess arc risk at the design stage and plan insulated busbars, the separation level, the pressure relief route and, where justified, an arc detection solution together. Share your short circuit data and the way personnel work in front of the boards and we can clarify which layers are genuinely necessary.
