Internal Arc Classification Explained (IAC AFLR, 16/25/31.5 kA Basics)

Internal arc faults are among the most dangerous events that can occur inside medium-voltage (MV) switchgear. They release extreme pressure, heat, and toxic by-products within milliseconds. For utilities, EPCs, and industrial operators, ensuring maximum personnel and equipment safety has become a core requirement for any new MV installation.

The globally recognized benchmark for this is the Internal Arc Classification (IAC) defined in IEC 62271-200. This article explains what IAC means, what the classifications AFL/AFLR represent, how fault ratings like 16 kA / 25 kA / 31.5 kA are determined, and how modern SF₆-free GIS achieves high-level internal arc performance while improving sustainability.

1. What Is an Internal Arc Fault?

An internal arc occurs when insulation inside the switchgear fails, causing a short circuit and arcing between live parts and against earth potential.
This results in:

• Rapid pressure rise
• Gas and hot particle ejection
• Light and sound shock
• Potential mechanical deformation

In oil-filled or open-air systems, consequences were historically catastrophic. Today, GIS and metal-enclosed switchgear are designed to contain and release this energy safely.

2. What Is Internal Arc Classification (IAC)?

IEC 62271-200 defines IAC as a safety performance class indicating the switchgear’s ability to:

• Withstand the thermal and mechanical effects of an internal arc
• Prevent harm to personnel standing near the equipment
• Direct arc gases safely through controlled exhaust channels

Switchgear is tested under worst-case internal arc conditions.

3. Decoding IAC Designations (AFLR)

IAC ratings include three letters describing the accessibility zones where the switchgear protects personnel.

A – Accessibility Type A: For authorized personnel only (trained operators, technicians).

F – Front: Protection at the front side of the switchgear.

L – Lateral: Protection at the side panels.

R – Rear: Protection at the rear side.

Example: IAC AFLR 31.5 kA / 1 s

Means:

• Switchgear can safely contain an internal arc
• For authorized personnel (no public access)
• At the front, sides, and rear
• At 31.5 kA rated short-circuit current
• For 1 second arc duration

This is one of the highest safety classifications for MV switchgear.

4. Typical Current Ratings: 16 / 25 / 31.5 kA – What Do They Mean?

These values correspond to the rated short-circuit current at which the test is conducted.

• ‍16 kA / 1s: Used in distribution substations, small industrial sites, utilities with low fault levels.

• 25 kA / 1s: Standard for most primary substations, urban grids, industrial plants.

• ‍31.5 kA / 1s: High-performance level required for:
  
  • Dense urban areas
  • Large industrial loads (chemical, automotive, steel)
  • Substations with strong grid coupling
  • Transmission-adjacent MV nodes

A higher rating means greater mechanical strength, faster pressure relief, and more robust internal components and external enclosure.

5. How Is the Internal Arc Test Performed?

The test simulates worst-case conditions:

1. Install the switchgear completely assembled
2. Create a fault point inside a compartment (breaker, busbar, cable)
3. Inject the test current (e.g., 31.5 kA)
4. Maintain for the specified duration (often 1 s)
5. Observe performance based on five IEC criteria:

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Pass Criteria (IEC 62271-200)

1. Doors and covers remain closed
2. No holes created through which flames/hot gases escape
3. No fragments > 60 g ejected
4. Indicators around the switchgear must not ignite
5. Protective grounding must remain intact

A pass demonstrates containment, pressure relief, and operator safety.

6. Why Internal Arc Safety Matters More Today

Key industry trends increase the importance of IAC:

• More compact substations → closer operator interaction
• Increasing short-circuit power in urban grids
• Higher expectations for occupational safety
• Moves from AIS to GIS in enclosed environments
• More renewables → more switching operations → higher stress conditions

Utilities increasingly include minimum IAC AFLR 25 kA requirements in tenders.

7. How SF₆-Free GIS Achieves Strong IAC Performance

Modern SF₆-free GIS using dry air or clean gases achieve IAC levels equal to or higher than traditional SF₆ GIS.

Key design elements include:

• ‍Robust mechanical enclosures: Engineered to withstand high internal pressure peaks.

• Optimized exhaust channels / arc absorbers: Direct pressure waves safely away from the operator.

• Compartmentalization: Busbar, breaker, and cable compartments isolated.

• No toxic decomposition products: SF₆ produces HF, SOF₂, SO₂F₂ during arcs. Dry air produces only clean air-based by-products, improving safety for personnel.

The combination of high arc performance + elimination of SF₆ toxicity risks is a decisive advantage.

8. Operator Safety: SF₆-Free GIS Has a Critical Additional Benefit

Even if an arc is contained, the decomposition gases matte

• SF₆ arc decomposition forms corrosive, toxic substance

• These require:
  
  • PPE level 2–3
  • Neutralization procedures
  • Gas handling equipment
  • Trained personnel‍
  • Dry-air insulated GIS does not create toxic by-products: This is a major safety and OPEX advantage.

9. What to Consider When Evaluating IAC in Tender Documents

Buyers should look for:

• IAC classification (AFL / AFLR)
• Rated fault level (e.g., 25 kA / 1 s)
• Test reports according to IEC 62271-200
• Gas type and pressure
• Arc-proof design of doors, covers, flaps
• Presence of safe exhaust ducts
• Compartment separation

10. Conclusion

Internal Arc Classification is one of the most important safety parameters in MV switchgear. Understanding ratings such as AFLR, 16/25/31.5 kA, and the IEC testing methodology helps utilities and EPCs make better decisions when designing substations.

Modern SF₆-free GIS is fully capable of achieving high IAC performance—while eliminating the toxic by-products of SF₆ arc decomposition and supporting sustainability goals.

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For operators, this means safer substations, simpler maintenance, and reduced environmental impact.