Arc-resistant switchgear (IEC 62271-200 Annex AA) — construction that contains an internal arc, not just detects it

The arc-flash relay guide covers how fast optical/current detection (ANSI 50AF) trips a breaker within milliseconds of an internal arc starting, and the incident-energy guide covers how the resulting incident energy is calculated to select the correct PPE. Both of those articles are about detecting and clearing an arc as fast as possible. This article covers a different, complementary property: whether the switchgear enclosure itself, by its construction, physically contains an internal arc fault so that a person standing outside it is not endangered — qualified through the type test defined in IEC 62271-200 Annex AA.

What the internal arc classification (IAC) type test actually verifies

An IAC type test deliberately initiates an internal arc fault, at a specified test current and for a specified test duration, inside a representative sample of the switchgear with all doors and covers correctly closed and latched — exactly as it would be found in normal service. The test verifies whether the enclosure survives that internal arc without endangering a person standing at a defined position outside it, rather than merely verifying that the switchgear clears the fault electrically. Not every metal-clad or metal-enclosed switchgear is automatically arc-resistant: the property must be demonstrated by this specific type test and stated on the equipment's rating plate as an IAC classification, not assumed from the general construction quality or compartmentalisation of the switchgear.

Accessibility classes A and B, and side classification F/L/R

The IAC classification records both who the enclosure is qualified to protect and which sides of it are qualified:

Class A — accessible to authorised, instructed personnel only.
Class B — accessible to the general public (ordinary, uninstructed persons), which implies a stricter evaluation.
Side letters F (front), L (lateral/side) and R (rear) indicate which faces of the switchgear were included in the qualified test — an enclosure installed against a wall, for example, may not need an R-side qualification, while a free-standing installation accessible from all sides typically requires F, L and R together.

A typical rating plate marking looks like "IAC AFLR 25 kA 1s" — class A, qualified on front/lateral/rear sides, for an arc current of 25 kA lasting up to 1 second.

The pass criteria the test evaluates

Although the exact evaluation details are set out in the full standard text, the type test generally assesses criteria along these lines:

Correctly secured doors and covers must not open.
No parts of the enclosure may be ejected in a way that could endanger a person at the qualified position.
No burn-through holes may form in accessible parts of the enclosure up to a defined height.
Indicators placed at a defined distance from the enclosure, representing where a person could be standing, must not ignite — confirming that no dangerous thermal effect reaches that position.
Earthed metal parts must remain effectively connected to earth throughout the test, so that no new shock hazard is created.

Why this is a passive property, tied to a specific tested duration

An arc-resistant enclosure works by construction: reinforced doors and latches, adequately dimensioned internal volume, and pressure-relief flaps or exhaust ducts that channel the hot gas and plasma from an internal arc away from the operator's position (often upward through the roof, or via a cable basement) — none of which depends on how fast the protection relay trips. This is precisely why the classification carries a specific tested current and duration: the IAC rating is only valid for that combination. If the actual protection scheme in a given installation clears the fault slower than the tested duration (for example, due to a backup-protection time delay after a failed primary trip — see the breaker failure protection guide), the enclosure's containment is no longer guaranteed to hold for the full actual fault duration, even though its IAC rating looks adequate on paper.

Practical relevance

When assessing switchgear for personnel safety, first establish whether it actually carries an IAC classification matching both the expected fault current and the actual protection clearing time of the specific installation (not just the manufacturer's generic catalogue rating), and whether the F/L/R sides tested match how the switchgear is actually installed and accessed. An arc-resistant rating protects a person standing at the qualified position with the doors closed during normal operation; it does not remove the need for an incident-energy calculation and appropriate PPE for any task that requires opening the enclosure itself, such as racking a breaker or performing internal maintenance.

Common mistakes

Assuming any metal-clad or metal-enclosed switchgear is automatically arc-resistant — the property must be demonstrated by the IAC type test and stated on the rating plate; it is not implied by general build quality or internal compartmentalisation alone.
Ignoring the F/L/R side classification — an enclosure tested only for its front side does not give the same guarantee on its rear or sides; a free-standing installation needs the sides that are actually accessible to be covered.
Comparing the IAC rated duration to the primary protection clearing time only — if backup protection (breaker failure, 50BF) takes longer to clear a fault than the tested duration, the enclosure's containment margin for that specific scenario is reduced.
Treating an IAC rating as a substitute for arc-flash PPE and incident-energy assessment — the classification protects a person outside the closed enclosure, not someone who must open it to perform work.