RMU Internal Arc Classification: What It Means for Site Design

RMU internal arc classification is a type-tested designation that states how a ring main unit’s enclosure protects people if an arcing fault occurs inside it. The designation answers three questions at once: who is expected near the equipment (accessibility type), which sides of the enclosure are protected (classified sides), and the arc current and duration the design was tested to contain.

Reading the label is the easy half. The harder half is honouring it: venting, room strength, clearances, access rules, and protection clearing time all have to match the tested scenario, or the classification describes a laboratory event rather than your site.

Part 1. What is an RMU internal arc classification?

An arc fault inside sealed switchgear releases energy as pressure, heat, and hot gas within a very small volume. Internal arc classification exists so that this scenario is tested rather than argued about: IEC 62271-200, the standard covering AC metal-enclosed switchgear above 1 kV up to and including 52 kV, defines the classification, and the technical review of internal arc testing describes its purpose plainly — verifying the effectiveness of the design in protecting persons.

Because a ring main unit is a metal-enclosed assembly, the classification applies to it exactly as it applies to larger switchboards. The designation belongs to a tested design, not to a product category: two RMUs of the same voltage class can carry different classifications, and an offered unit’s value comes from its type-test documentation, never from an article.

For buyers, the classification is one of the few datasheet lines that directly changes civil and layout work. That is why it deserves the same attention as ratings when preparing a specification.

Part 2. How do you read an IAC designation?

A published switchgear manufacturer FAQ on internal arc accessibility decodes the designation elements. Its worked example reads IAC AFL 20 kA 0,5 s as: accessibility type A, protected front and lateral sides, tested at a 20 kA arc current for 0,5 seconds.

Read the four elements together, not in isolation. A designation with front-only protection changes wall placement; a type-B classification signals a design intended for public surroundings; and the current-duration pair is a promise that only holds inside those tested limits. Datasheet-reading habits for adjacent equipment classes are covered in the metal clad switchgear buyer checklist, and the same discipline applies here.

Part 3. What does the internal arc test actually verify?

During the type test, an arc is deliberately initiated inside a compartment while instrumented indicators stand in for people around the enclosure. The published explainer of internal arc fault classification summarizes the acceptance criteria in categories a buyer can remember:

• Correctly secured doors and covers must stay closed; deformation is tolerated, ejection is not.
• The enclosure must not fragment, and accessible sides must not develop holes.
• Indicators positioned around the accessible sides must not ignite from escaping gases.
• The test arrangement itself is standardized — the internal arc testing review describes a defined room simulation with floor, ceiling, and walls, because gas behaviour depends on the surroundings.

Equally important is what the test does not verify. It does not prove the unit will never suffer an internal fault, it does not cover sides that were not classified, and it does not test your room, your duct, or your protection settings. Those remain project responsibilities that the following Parts turn into concrete inputs.

Part 4. Where do arc gases go, and who decides?

Arc gases leave the enclosure through designed pressure-relief paths, and the receiving space must be planned. Practitioner discussions show how real this question is: one engineering-forum thread debates at length whether the arc exhaust duct on top of switchgear should exit the building, with responders calling the byproducts hazardous to personnel and warning that room overpressure can damage the structure.

Manufacturer guidance points the same way. One OEM’s published white paper on venting exhaust gases from arc-resistant equipment recommends exhausting arc gases outside the building where conditions allow, because doing so separates people from the pressure, temperature, and potentially toxic gases; where the building cannot support it, the user must apply other protective measures.

Questions to settle before the layout freezes

• Does the offered RMU vent internally to the room, through a duct, or into a dedicated exhaust volume — and what do its installation instructions require?
• If gases vent into the room: can the room withstand the transient pressure, and is the discharge zone kept clear of people and equipment?
• If gases vent outside: where does the discharge point sit relative to walkways, and who controls access to it?
• Which structural, sealing, and clearance conditions in the OEM instructions become civil-works obligations?

The decision owner is the project — but the decision inputs come from the offered design’s documentation, which is why this article keeps them as RFQ items rather than assumed answers.

Part 5. Which site-design inputs follow from the classification?

The classification converts into a short list of site-design inputs. Each row below pairs the designation element with the work it creates.

Two of these deserve emphasis. First, the duration element creates a protection dependency: an industry white paper on internal arc in switchgear notes that for the classified duration to be effective, the installed protection needs to clear the fault within that time — a coordination-study output, not a datasheet default. Second, siting context matters: where the RMU lives inside a prefabricated enclosure at a publicly accessible location, the station level is governed by IEC 62271-202, which addresses personnel protection at the enclosure level. Layout practice for such stations is covered in the prefabricated substation layout guide.

Part 6. Which conditions limit or void the protection?

An internal arc classification describes the equipment in its normal, closed, correctly secured condition. Practitioners state the consequence bluntly in a forum discussion of arc protection during maintenance work: when the switchgear is open, it loses its benefit as arc-resistant — the tests are run with all doors closed and covers properly bolted.

That single fact drives several operating rules worth writing into the project file:

1. Treat open-door work as a different risk situation with its own procedures, regardless of the classification on the nameplate.
2. Keep covers, bolts, and pressure-relief paths exactly as the OEM instructions define them; a missing bolt or a blocked relief path was not part of the test.
3. Record which sides were classified, and do not let later site changes put walkways on an unclassified side.
4. Revisit the classification’s current and duration assumptions whenever the network fault level or protection settings change.

None of this diminishes the value of the classification. It simply keeps the rating tied to the conditions under which it was earned.

Part 7. RFQ input checklist for internal arc requirements

State the requirement and the site facts; ask the supplier for the tested evidence. This checklist keeps both sides of that exchange in one place.

RFQ input list

• Accessibility situation: who can approach the installed unit, from which sides, and whether the location is restricted, public, or out of reach.
• Required classified sides, derived from the planned layout — including any side that will face a walkway.
• Network fault level at the installation point and the expected protection clearing time, supplied as project data for comparison with the tested current and duration.
• Siting: indoor room, outdoor kiosk, or prefabricated station — with the governing enclosure context named.
• Venting expectations: internal to the room, ducted, or external discharge, plus any building constraints the supplier must know about.
• Room construction constraints that could conflict with the offered design’s installation instructions.
• A request for the offered internal arc classification with its type-test documentation and the installation conditions attached to it.
• Interlocking, access procedures, and any pressure-relief or exhaust hardware included in the scope.
• Documentation list: type-test evidence, installation instructions, drawings showing relief paths, and operating procedures.

The checklist deliberately excludes an arc-class value. The classification arrives with the offered design’s documents; the project’s fault study and layout decide whether it fits.

Part 8. Which JUBANG equipment fits this discussion?

Buyers holding an internal-arc requirement set for compact ring nodes can take it to JUBANG’s product pages directly. The GT-HRM-12 environmental protection gas insulated ring main unit page is the entry point for sealed compact RMU duties, and the XGW□-12 box-type switching substation (ring main box) page covers outdoor kiosk-style ring nodes where public surroundings shape the requirement. Indoor withdrawable switchgear line-ups are deliberately not recommended in this article: they raise a different room-design conversation, and mixing the two would blur the compact-node focus here.

Fit Boundary

• A good fit: utilities, EPC contractors, and facility owners siting compact RMU nodes who can state the accessibility situation, layout, and network fault data.
• Not the right path: projects that need a full arc-hazard study before equipment selection, rooms that cannot honour an offered design’s installation conditions without civil redesign, and withdrawable feeder line-up requirements.
• Confirm first: the offered internal arc classification, the test conditions and classified sides behind it, the venting arrangement, and the installation instructions — all in writing from the offered design.

This article assigns no internal arc classification, tested current, duration, or compliance status to any JUBANG product; the offered design’s type-test documentation governs those values.

Next step: send the internal arc requirement set — accessibility situation, required sides, fault data, siting, and venting constraints — and request the offered classification’s documentation for your ring main unit project.

FAQ

What is internal arc classification on a ring main unit?

It is a type-tested designation, defined in the metal-enclosed switchgear standard, stating how the enclosure protects people during an internal arc fault: the permitted accessibility, the protected sides, and the tested arc current and duration.

What does IAC AFLR mean on a switchgear datasheet?

Accessibility type A (authorized personnel), with front, lateral, and rear sides classified, followed by the tested current and duration. A published manufacturer example reads IAC AFL 20 kA 0,5 s as front-and-lateral protection at 20 kA for 0,5 seconds.

What do accessibility types A, B, and C mean?

Type A restricts access to authorized personnel; type B allows unrestricted access, including the general public; type C covers equipment installed out of reach. The type signals which surroundings the design was tested to face.

Does the internal arc rating apply when a door is open?

No. The classification describes the closed, correctly secured condition — the tests are run with doors closed and covers bolted. Open-compartment work is a different risk situation with its own procedures.

Where do the arc gases go during an internal arc fault?

Through the design’s pressure-relief paths: into the room, into a duct, or to an external discharge point. Manufacturer guidance favours discharging outside the building where conditions allow; the offered design’s installation instructions define the requirement.

Does an internal arc classification depend on protection clearing time?

Yes. The tested duration only protects people if the installed protection clears an internal fault within that time, so the coordination study must confirm the clearing time at the installation point.

How does internal arc classification affect switchgear room design?

It sets access-control needs, wall and barrier placement against unclassified sides, room-strength or duct requirements for gas discharge, and the clearance conditions in the OEM installation instructions.

Is internal arc classification the same for outdoor kiosk substations?

The switchgear inside carries its own classification, while the prefabricated station around it is governed at the enclosure level, including personnel protection for publicly accessible locations. Both layers belong in the requirement set for kiosk installations.

References

1. Metal-enclosed switchgear standard defining the classification: IEC 62271-200:2021, AC metal-enclosed switchgear and controlgear
2. Prefabricated substation counterpart: IEC 62271-202:2022, AC prefabricated substations
3. Designation decode with worked example: What is meant by accessibility type AFLR, Schneider Electric FAQ
4. Test purpose and arrangement: Internal arc testing of MV switchgear, Electrical Engineering Portal
5. Acceptance criteria summary: Internal arc fault classification in MV switchgear, Engineers Hangout
6. Containment and clearing-time dependency: Internal arc and arc hazards in switchgear, Roxtec white paper
7. Venting practice guidance: Venting of exhaust gases from arc-resistant equipment, Siemens white paper
8. Practitioner venting debate: Arc exhaust chamber and duct, Eng-Tips forum
9. Practitioner discussion of open-door reality: Arc flash detection systems, Eng-Tips forum