Internal Arc Classification (IAC) in MV Switchgear: What Engineers and EPC Contractors Need to Know
Internal arc performance is one of the most important safety considerations when specifying medium voltage switchgear.
A switchboard may have the correct voltage rating, continuous current, short-circuit withstand rating and circuit breaker capacity, but those electrical ratings alone do not explain what happens around the enclosure if an internal arc fault occurs.
This is where Internal Arc Classification (IAC) becomes important.
For electrical engineers, EPC contractors, utilities, mining operators, renewable energy developers and industrial asset owners, understanding internal arc classification switchgear terminology helps ensure that personnel safety requirements, switchroom layouts, pressure-relief arrangements and procurement specifications are aligned before equipment is ordered.
IEC 62271-200:2021 applies to prefabricated AC metal-enclosed switchgear and controlgear above 1 kV and up to and including 52 kV, for both indoor and outdoor installations. The current IEC publication also incorporates specific provisions relating to Internal Arc Classification and internal arc testing.
This guide explains what IAC means, how classifications such as AFL and AFLR should be interpreted, why accessibility matters, how installation conditions affect performance, and what engineers should include in an MV switchgear specification.
What Is an Internal Arc Fault?
An internal arc fault is an electrical fault occurring inside a switchgear enclosure where an arc develops between live conductors, between phases, or between a live conductor and earth.
An internal arc can release substantial energy within a very short period. The resulting event can generate high pressure, hot gases, heat and particles inside the switchgear enclosure.
For this reason, internal arc safety is not simply about whether the circuit breaker can interrupt the system fault current. The design must also consider how the switchgear enclosure behaves while the fault exists and how fault energy is managed around personnel-accessible areas.
IAC provides a standardized way of declaring tested internal arc performance for the relevant switchgear arrangement under specified conditions. IEC 62271-200 contains the framework for Internal Arc Classification of applicable metal-enclosed switchgear.
Why Internal Arc Classification Matters
Personnel may be near MV switchgear during:
Normal switching
Inspection
Circuit breaker operation
Racking operations
Isolation
Testing
Maintenance preparation
Fault investigation
Commissioning
An internal fault occurring during these activities can expose personnel to serious hazards if fault pressure and hot gases are not appropriately contained or directed.
For this reason, engineers should not treat phrases such as “arc proof,” “arc resistant,” or “internal arc protected” as sufficient specification language.
The actual requirement should identify the appropriate IAC designation, accessible sides, fault current and fault duration.
Leistung Energie's own MV specification guidance similarly recommends defining the required classification, such as AFL or AFLR, together with internal arc current, duration, pressure relief and the accessible sides of the equipment.
Understanding the IAC Designation
An IAC designation can contain several pieces of information.
A typical specification might read:
IAC A-FLR 25 kA 1 sEach part communicates something different about the tested configuration.
IAC
IAC means Internal Arc Classification.
It indicates that the switchgear has been assessed for internal arc performance under the relevant provisions of the applicable IEC switchgear standard.
A – Accessibility Type
The letter immediately after IAC identifies the accessibility type.
IEC-based terminology defines three accessibility types:
Type A – access restricted to authorised personnel.
Type B – unrestricted accessibility, including access by the general public.
Type C – accessibility restricted by installation out of reach above an area accessible to the general public.
For accessibility Types A and B, tested accessible sides can then be identified using F, L and R.
For most industrial, utility, mining and renewable energy substations where switchgear is located inside restricted electrical rooms, Type A is commonly relevant, but the required accessibility classification should always be determined from the actual project layout and operating environment.
What Do F, L and R Mean?
For IAC accessibility Types A and B, the enclosure sides that satisfy the internal arc test criteria are designated:
F – Front
L – Lateral
R – Rear
Therefore:
IAC A-F indicates Type A accessibility with classified performance from the front.
IAC A-FL indicates Type A accessibility from the front and lateral sides.
IAC A-FLR indicates Type A accessibility from the front, lateral and rear sides.
Schneider Electric's explanation of IEC 62271-200 terminology confirms that F, L and R designate the front, lateral and rear sides respectively for accessibility Types A and B.
This distinction matters significantly during switchroom design.
If personnel can access the rear of the switchgear during normal service, specifying only AFL when the actual layout requires rear accessibility may leave a mismatch between equipment classification and the installation philosophy.
What Do the kA and Time Values Mean?
An IAC classification also states the internal arc fault current and duration for which the switchgear configuration was tested.
For example:
IAC A-FLR 25 kA 1 smeans the switchgear has the declared IAC classification for an internal arc test current of 25 kA for 1 second under the applicable test arrangement.
The current and time should therefore not be omitted from an EPC specification.
Writing only:
Internal arc: AFLRdoes not fully define the requirement.
The specification should instead define:
Accessibility type
Accessible sides
Arc fault current
Arc duration
The required current and duration should be coordinated with the actual system fault level and expected protection clearing time rather than copied automatically from another project.
IAC Rating and Short-Circuit Rating Are Not the Same Thing
Engineers should distinguish between the switchgear's electrical short-circuit rating and its Internal Arc Classification.
Short-circuit ratings address the ability of the main circuit and switching equipment to withstand or interrupt defined fault duties.
IAC addresses the behaviour of the switchgear assembly during an internal arc test under defined accessibility and installation conditions.
The two values may appear similar because both can be expressed in kA and seconds, but they describe different performance requirements.
This distinction should be clear in tender documents so that the procurement team does not assume that a switchgear short-time withstand rating automatically means the equipment has the required IAC classification.
Internal Arc Classification Is a Type-Tested Characteristic
IAC should be supported by appropriate type-test evidence for the switchgear design being proposed.
IEC 62271-200 covers type testing for applicable metal-enclosed switchgear, and Leistung's AMS switchgear, for example, is listed as factory assembled and type-tested to IEC 62271-200. The AMS product page states an internal arc fault type-tested rating of A-FLR up to 40 kA/1 s.
This is important during technical bid evaluation.
An EPC contractor should not simply accept a statement saying:
“Switchgear complies with IEC 62271-200.”
Instead, the vendor should clearly identify the actual IAC classification applicable to the offered arrangement and provide relevant supporting test documentation.
IAC Ratings in Leistung MV Switchgear
Different switchgear architectures can have different IAC capabilities.
For example, Leistung Energie lists its AMS air-insulated metal-clad switchgear as internal arc fault type-tested A-FLR up to 40 kA/1 s, depending on the configuration.
The Enerswit+ secondary switchgear range is listed as internal arc fault type-tested A-FLR up to 25 kA/1 s, with individual electrical ratings depending on the voltage configuration.
Leistung's Airing SF₆-free MV switchgear is also listed with an internal arc fault type-tested rating of A-FLR 25 kA/1 s.
These examples show why the required IAC rating should be checked against the particular switchgear model and project duty rather than treated as a generic characteristic of all MV switchgear.
Personnel Safety Depends on More Than the Panel
An important engineering point is that IAC should not be evaluated in isolation from the installation.
The physical switchroom can influence how an internal arc event is managed.
Engineers need to consider:
Switchgear position
Wall clearance
Rear access
Ceiling height
Arc exhaust direction
Pressure-relief flaps
Arc ducts or plenums
Cable trenches
E-house dimensions
Doors and escape routes
Personnel operating positions
This is particularly important in compact substations, mining e-houses, battery energy storage projects and renewable energy collector substations.
Leistung's renewable MV switchgear specification guidance recommends reviewing internal arc performance together with the physical switchroom or enclosure arrangement, including arc-gas exhaust and pressure-relief paths.
Arc Containment and Pressure Relief
When an internal arc develops, the switchgear enclosure must manage rapidly developing pressure and hot gases according to its tested design.
Depending on the switchgear architecture, the system may use:
Pressure-relief flaps
Top exhaust
Rear exhaust
Internal plenums
External arc ducts
Dedicated pressure-relief channels
The appropriate arrangement depends on the product and installation.
The important procurement point is that an EPC specification should not prescribe an IAC rating without also establishing how the fault gases are expected to leave the switchgear and where they will be discharged.
A technically compliant switchgear panel installed in a room that does not accommodate its intended pressure-relief arrangement can create an installation problem.
AFL or AFLR: Which Should Engineers Specify?
There is no universal answer.
The selection should be based on which sides of the equipment are accessible to personnel.
AFL May Be Appropriate When
Switchgear is installed against a wall
Rear access is physically prevented
Operation and maintenance are performed from the front
Lateral areas remain accessible
The manufacturer-approved installation matches this arrangement
AFLR May Be Appropriate When
The equipment is free-standing
Rear access is available
Maintenance personnel may work behind the board
The switchroom has circulation space around the lineup
The project safety philosophy requires classified protection on all accessible sides
Do not specify AFLR only because it appears to be a “higher specification.”
The classification should match the intended installation and personnel accessibility.
Likewise, do not accept AFL simply because it is cheaper if the final installation allows rear access.
IAC A and IAC B: Understand the Difference
Accessibility Type A is intended for areas restricted to authorised personnel, while Type B addresses unrestricted accessibility, including the general public.
This distinction is important for projects such as:
Utility substations
Industrial plants
Mining sites
Shopping centres
Transport infrastructure
Public infrastructure
Distribution kiosks
A restricted electrical switchroom and switchgear located in an area accessible to members of the public do not present the same accessibility condition.
The specification should therefore define the site's actual access philosophy rather than automatically copying an IAC designation from a previous project.
IAC Is Not the Same as an Arc Flash Study
Internal Arc Classification and a site-specific arc-flash risk assessment should be treated as related but different engineering considerations.
IAC describes tested switchgear behaviour under defined internal arc conditions.
An arc-flash assessment addresses the electrical system and work environment, including factors such as fault current, protection operating time, equipment configuration and worker exposure.
Therefore, specifying IAC-rated switchgear should not be treated as an automatic replacement for the project's wider electrical safety assessment.
For an EPC project, the strongest approach is to combine correctly classified equipment with appropriate protection design, operating procedures, remote operation where required and the site's broader electrical safety strategy.
Protection Clearing Time Matters
The duration used for the internal arc classification should be considered together with the protection system.
A project using IAC 31.5 kA 1 s, for example, is specifying a particular tested internal arc duration.
But the actual power system should still be designed to detect and clear faults as rapidly and selectively as practical within the protection philosophy.
Modern switchgear projects may incorporate:
Numerical protection relays
Arc detection
Optical sensors
Busbar protection
Breaker failure protection
Fast tripping logic
Remote circuit breaker operation
IAC provides a passive containment-related safety layer, while fast fault clearing can reduce the duration for which fault energy is released. The two concepts should be coordinated rather than treated as alternatives.
Closed-Door Operation Can Reduce Exposure
Switchgear specification can also address how personnel interact with the equipment.
Depending on project requirements and available switchgear features, engineers may consider:
Closed-door breaker operation
Closed-door circuit breaker racking
Motorised racking
Remote open/close control
Remote earthing-switch operation
Remote switching stations
Leistung's AMS switchgear, for example, lists motorised circuit-breaker racking and earthing-switch operation among its available safety-oriented features.
These features can form part of a broader risk-reduction strategy where personnel exposure during switching operations is a project concern.
IAC for E-Houses and Containerised Substations
E-houses are increasingly used for mining, renewable energy, BESS, utility and industrial projects because they allow electrical equipment to be assembled and tested before delivery to site.
However, installing IAC switchgear inside an e-house creates an important interface between two engineered systems.
The project team should review:
Switchgear arc exhaust
E-house wall and roof arrangement
Ceiling clearance
Pressure-relief ducting
Personnel access
Escape paths
HVAC openings
Cable basement or trench
External discharge location
Do not assume that a switchgear type test automatically proves the complete e-house arrangement.
The switchgear supplier, e-house supplier and EPC electrical engineer should coordinate the final installation conditions.
Internal Arc Classification for Renewable Energy and BESS
IAC is particularly relevant for renewable energy and Battery Energy Storage System substations because these projects often use compact electrical rooms and prefabricated buildings.
Typical installations include:
Solar collector substations
Wind farm substations
BESS collector systems
Mining microgrids
Grid connection substations
Renewable e-houses
Leistung's existing renewable switchgear guidance specifically includes internal arc classification, pressure relief and enclosure compatibility among the parameters that should be defined before ordering MV equipment.
Space-saving should therefore never be considered independently from personnel access and arc-management requirements.
What Engineers Should Include in an MV Switchgear Specification
Instead of writing:
“Switchgear shall be arc-resistant.”
use a structured technical requirement.
A suitable specification schedule should define the following:
Specification Item | What to Define |
Applicable standard | IEC 62271-200, project-required edition |
Internal Arc Classification | IAC required |
Accessibility type | A, B or project-specific |
Accessible sides | F, FL or FLR |
Internal arc current | e.g. project fault-study requirement |
Arc duration | e.g. 0.5 s or 1 s as engineered |
Installation arrangement | Wall-mounted, free-standing, e-house |
Pressure relief | Top, rear, plenum or ducted |
Arc exhaust destination | Safe area defined by project |
Rear access | Permitted or prevented |
Type-test evidence | Required with technical submission |
Panel configuration | Must correspond to tested design |
Arc detection | If required |
Remote operation | If required |
Documentation | Test reports, drawings and installation instructions |
Leistung's existing MV switchgear specification checklist similarly recommends that buyers define IAC, current, duration, accessibility type, arc exhaust arrangements, remote operation and type-test documentation rather than relying on generic “arc-proof” terminology.
What EPC Contractors Should Check During Bid Evaluation
When comparing switchgear suppliers, check more than the words “IEC compliant.”
Ask:
What is the exact IAC designation?
What fault current was tested?
What duration was tested?
Which sides are classified?
Is accessibility Type A or Type B?
Does the type-tested lineup correspond to the offered construction?
How are hot gases exhausted?
Is an external arc duct required?
What minimum wall and ceiling clearances apply?
Can the rear remain accessible?
Does the offered panel arrangement change the IAC configuration?
Is the relevant type-test documentation available?
This helps prevent a common procurement problem where suppliers quote electrically similar switchgear but with different safety classifications and installation requirements.
Common IAC Specification Mistakes
Several mistakes regularly appear in MV specifications.
Writing Only “Arc Proof”
This does not define accessibility, current, duration or tested sides.
Writing AFLR Without Current or Duration
The IAC designation is incomplete for procurement purposes unless the project also defines the arc fault current and duration.
Ignoring Rear Accessibility
If personnel can stand behind the equipment, the rear-side classification should be reviewed.
Assuming Short-Circuit Rating Equals IAC Rating
They describe different performance requirements.
Ignoring the Switchroom
Arc ducts, pressure relief, ceiling height and wall spacing may affect installation.
Not Requesting Type-Test Evidence
The vendor should be able to substantiate the classification being offered.
Copying Another Project's IAC Requirement
The required accessibility, fault level and installation layout may be different.
Treating IAC as the Entire Arc-Safety Strategy
Protection clearing time, operating procedures, remote operation and site-specific safety studies remain important.
Conclusion
Internal arc classification switchgear should be treated as a defined engineering requirement, not as a marketing label.
For engineers and EPC contractors, IAC provides a structured way to specify how MV switchgear has been tested for an internal arc condition in relation to personnel accessibility, accessible sides, arc current and fault duration.
The key is to look beyond a simple AFL or AFLR designation.
When these requirements are coordinated early, project teams can evaluate switchgear suppliers more accurately and design substations that better align equipment performance with personnel safety and practical site conditions.
Talk to Leistung Energie About IAC-Rated MV Switchgear
Planning a utility, mining, renewable energy, BESS, infrastructure or industrial MV project?
Leistung Energie Australia provides medium voltage switchgear solutions with internal arc type-tested options for a range of primary and secondary distribution applications. Leistung's portfolio includes AMS air-insulated switchgear, Enerswit+ secondary switchgear and Airing SF₆-free switchgear with published internal arc classifications for specific configurations.
Contact Leistung Energie to discuss your required voltage, fault level, Internal Arc Classification, accessibility arrangement, protection philosophy, switchroom layout and lifecycle support requirements before finalising your MV switchgear specification.
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