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IEC 61641:2008 — Low-voltage Switchgear Assemblies — Internal Arc Tests
IEC 61641:2008 (Consolidated with Corrigendum 1:2009) defines a type-test method for evaluating the behavior of low-voltage switchgear and controlgear assemblies (switchgear assemblies) under internal arc fault conditions. With the global push toward safer electrical distribution, internal arc testing has become a critical qualification for MV and LV switchgear. This article provides a focused engineering walkthrough of the standard, test setup, and design implications.
Why It Matters
An internal arc fault releases enormous thermal energy (up to 20,000 °C), vaporizing copper and producing a pressure wave that can rupture enclosures. IEC 61641 provides a reproducible method to verify that the assembly design can contain this event and protect personnel.
An internal arc fault releases enormous thermal energy (up to 20,000 °C), vaporizing copper and producing a pressure wave that can rupture enclosures. IEC 61641 provides a reproducible method to verify that the assembly design can contain this event and protect personnel.
1. Scope and Applicability
IEC 61641 applies to low-voltage switchgear assemblies as defined in IEC 61439-1 (previously IEC 60439-1). It covers assemblies rated up to 1,000 V AC or 1,500 V DC. The standard does not mandate internal arc testing for all assemblies — it is an optional type test applied when specified by the manufacturer or user.
The key objectives of the test are:
- Verify that the enclosure can withstand the mechanical and thermal effects of an internal arc.
- Ensure that doors, covers, and vents do not open dangerously.
- Confirm that hot gases and particles are directed away from personnel access areas.
- Assess the effectiveness of arc quenching or containment devices.
Designer’s Note
Unlike arc fault protection devices (AFDDs) that attempt to extinguish an arc, IEC 61641 tests the passive containment capability of the enclosure. Both active protection and passive containment are needed for a comprehensive safety strategy.
Unlike arc fault protection devices (AFDDs) that attempt to extinguish an arc, IEC 61641 tests the passive containment capability of the enclosure. Both active protection and passive containment are needed for a comprehensive safety strategy.
2. Internal Arc Test Procedure—Step by Step
2.1 Test Setup
The assembly is placed on a grounded metal floor, surrounded by vertical indicator walls (typically cotton gauze or polyester fabric) at defined distances — 300 mm for front and 200 mm for sides/rear. These indicator materials detect ignition from hot exhaust gases.
2.2 Arc Initiation
A copper wire (typically 0.5 mm to 2.5 mm diameter) is used to initiate a three-phase or phase-to-earth arc at the test location. The short-circuit current is set to the declared internal arc rating (e.g., 50 kA for 100 ms). Arc ignition points are selected at locations deemed most vulnerable by the manufacturer, such as:
- Behind the main incoming terminals
- Inside cable connection compartments
- Near busbar supports
- At the rear of withdrawable units
2.3 Acceptance Criteria
| Criterion | Requirement |
|---|---|
| Doors and covers | Do not open; no ejection of fasteners |
| Envelope integrity | No holes or tears larger than 5 mm |
| Indicator ignition | No ignition of vertical indicators |
| Protective circuit continuity | Earth conductor remains intact |
| Accessibility | No parts become accessible to IPXXB probe |
All criteria must be met for a pass. Partial fulfillment is documented but does not constitute a “type-tested” internal arc assembly.
3. Engineering Design Insights for Arc Containment
Experienced switchgear designers know that passing an internal arc test requires deliberate mechanical design from the outset, not just a “test-and-fix” approach. Here are key design strategies derived from IEC 61641 requirements:
- Pressure relief flaps: Properly sized pressure relief vents (typically 1–2 % of enclosure volume) reduce peak overpressure by 40–60 %. They must open outward and be restrained by cables to prevent projectile hazards.
- Busbar bracing: Internal arc forces on busbars can exceed 300 N/m per kA². Reinforced supports at 300–500 mm intervals are essential for withstanding electromagnetic forces during the fault.
- Seam welding: Enclosure seams must be continuously welded or sealed with fire-rated mastic. Spot-welded seams can peel open under arc pressure.
- Door gaskets: Silicone foam gaskets with a minimum 10 mm compression provide both IP rating and arc-gas sealing. Intumescent gaskets add an extra layer of protection.
- Arc chute placement: Internal arc-containment chambers (arc chutes) at busbar ends can absorb and cool the plasma jet, reducing the risk of indicator ignition.
Real-World Application
A major European switchgear manufacturer reduced internal arc test failure rate from 35 % to under 5 % by implementing computational fluid dynamics (CFD) simulation of arc pressure distribution during the design phase. The investment in simulation paid for itself in reduced prototype iterations.
A major European switchgear manufacturer reduced internal arc test failure rate from 35 % to under 5 % by implementing computational fluid dynamics (CFD) simulation of arc pressure distribution during the design phase. The investment in simulation paid for itself in reduced prototype iterations.
4. Relationship with Other Standards
IEC 61641 is closely related to the IEC 61439 series (low-voltage switchgear assemblies) and the IEEE C37.20.7 standard (medium-voltage arc-resistant switchgear). While IEEE C37.20.7 covers MV switchgear up to 38 kV with an accessibility Type 1/2 classification, IEC 61641 addresses LV assemblies with simpler pass/fail criteria. The testing philosophy — arc initiation by fusible wire, indicator walls, and pressure containment — is shared across both standards.
Common Pitfall
A frequent mistake is conflating “arc-fault withstand” (IEC 61641) with “arc-fault quenching” (IEC 61439-2 Annex DD or arcing current mitigation devices). The former tests the enclosure; the latter tests active protection circuitry. They address complementary but different failure modes.
A frequent mistake is conflating “arc-fault withstand” (IEC 61641) with “arc-fault quenching” (IEC 61439-2 Annex DD or arcing current mitigation devices). The former tests the enclosure; the latter tests active protection circuitry. They address complementary but different failure modes.
5. Frequently Asked Questions
Q1: Is internal arc testing mandatory for LV switchgear?
No. IEC 61641 is an optional type test. However, many utilities and industrial users now require internal arc certification as a contractual condition, particularly for installations in public-access areas such as building service entrances and data centers.
Q2: What is the difference between IAC (Internal Arc Classification) in IEC 61439-2 and IEC 61641?
IEC 61439-2 Annex BB introduces IAC ratings (A, B, C) with accessibility categories. IEC 61641 provides the test method that underpins those ratings. A manufacturer typically uses IEC 61641 testing to claim IAC classification under IEC 61439-2.
Q3: Can a tested assembly configuration be modified without retesting?
The standard requires that any modification potentially affecting arc containment — such as changing busbar spacing, enclosure dimensions, vent location, or door thickness — will require a new test. Minor changes like replacing identical components from a different supplier typically do not require retesting, but this must be justified by engineering analysis.
Q4: How does the test current value affect the result?
The internal arc rating (Iarc) declared by the manufacturer must be tested at that specific current and duration. Testing at 50 kA for 100 ms does not validate performance at 65 kA or 200 ms. Design conservatism (safety margin) is strongly recommended — a 20 % margin on both current and duration is typical.
