# IEC 61439 — Low-Voltage Switchgear and Controlgear Assemblies

> IEC 61439 covers the design verification and routine testing of low-voltage switchgear and controlgear assemblies — motor control centres (MCCs), distribution boards, power distribution units (PDUs) and control panels. It replaced IEC 60439 and introduced mandatory design verification with defined test-based, calculation-based or comparison-based methods.

- Source : https://industryhub.site/en/standards/iec-61439
- Body : IEC
- Family : Electrical Equipment
- Status : current
- First published : 2009
- Latest revision : Various (per part, 2011–2021)
- Edition : 2
- Access : Paid (IEC webstore — price per part)
- Language : English

## Document structure

- **IEC 61439-1** — General rules : Defines the framework — terminology, service conditions, constructional requirements, design verification methods (test, calculation, comparison to reference design) and routine verification. The master part applicable to all assemblies.
- **IEC 61439-2** — Power switchgear and controlgear assemblies (PSCGA) : Applies to distribution boards and MCCs in power distribution. The most widely used part in industrial plant electrical engineering — covers busbars, compartments, form of separation, IP ratings and short-circuit withstand.
- **IEC 61439-3** — Distribution boards intended to be operated by ordinary persons (DBO) : Final distribution boards in buildings and light commercial premises. Equivalent to the former IEC 60439-3.
- **IEC 61439-4** — Particular requirements for assemblies for construction sites (ACS) : Portable and semi-fixed assemblies for use on building sites — weatherproof, impact resistant, with RCD (residual current device) protection.
- **IEC 61439-5** — Assemblies for power distribution in public networks : Secondary distribution cabinets and enclosures for public utility power networks — ring main units, compact secondary substations.
- **IEC 61439-6** — Busbar trunking systems (BTS) : Requirements for busbar trunking — rising busbars in multi-storey buildings and horizontal power distribution in industrial plants and data centres.

## Key concepts

- **Design verification** : IEC 61439-1 requires that every assembly design is verified — either by prototype testing (most rigorous), by calculation (e.g., thermal and short-circuit), or by comparison with a reference design already tested. Design verification is the responsibility of the Original Equipment Manufacturer (OEM), not the panel builder using a system.
- **Routine verification** : Tests carried out on every completed assembly before delivery — includes wiring check, dielectric test, insulation resistance, protective circuit continuity. Not the same as design verification. Routine tests are mandatory for every unit.
- **Form of separation** : Defines how busbars and functional units are physically separated within an assembly. Form 1 = no separation; Form 2 = busbars separated from functional units; Form 3 = functional units separated from each other; Form 4 = terminals separated from functional units. Form 3b and 4b are standard for industrial MCCs.
- **Rated short-time withstand current (Icw) (Icw)** : The RMS current an assembly can carry for a defined duration (typically 1 second) without damage. Must be ≥ the prospective short-circuit current at the busbar — the critical parameter for coordinating panel rating with upstream protection.
- **Rated peak withstand current (Ipk) (Ipk)** : The peak (instantaneous) current the assembly can withstand. Related to Icw by a factor n (typically 1.5–2.2 depending on power factor). Must be verified against the asymmetric fault current at the panel supply point.
- **Internal arc classification (IAC) (IAC)** : Optional classification for assemblies designed to contain and direct the pressure wave from an internal arc fault. Relevant for high-power MCCs and primary switchboards where operator safety during fault is critical (IEC 62271-200 covers MV equivalent).

## Overview

IEC 61439 is the **standard for assembled low-voltage switchgear and controlgear** — the motor control centres, distribution boards and switchboards that form the electrical backbone of every industrial plant. It replaced IEC 60439 and tightened the rules around design verification, distinguishing clearly between:

- **Original Equipment Manufacturer (OEM)**: designs and verifies the assembly system (e.g., ABB MNS, Schneider Okken)
- **Panel builder**: assembles a specific installation using a verified system

## Design Verification vs Routine Verification

| Aspect | Design verification | Routine verification |
|--------|--------------------|--------------------|
| When | Once per design / system | Every unit before delivery |
| Who | OEM (system supplier) | Panel builder / manufacturer |
| How | Test, calculation or comparison | Wiring check, dielectric, insulation |
| Purpose | Prove the design is safe | Prove this unit is correctly built |

Panel builders using OEM-verified systems can inherit the design verification — they do not need to retest every bespoke panel build, provided they follow the OEM's verified design rules.

## Form of Separation — MCC Selection

| Form | Separation provided | Typical use |
|------|--------------------|-----------:|
| Form 1 | None | Light commercial boards |
| Form 2b | Busbars from units + terminals | Secondary distribution |
| Form 3b | Units from each other | Process plant MCCs |
| Form 4b | Terminals from units | High-availability MCCs |

**Form 4b** is the standard for process plant and offshore MCCs where maintenance access to one feeder must not expose live terminals of adjacent feeders.

## Key Design Checks

- Verify **Icw ≥ Isc** at panel busbars (coordination with upstream protection)
- Check **thermal current rating** of busbars at rated load + ambient temperature
- Confirm **IP rating** matches the installation environment (IEC 60529)
- For ATEX locations: the assembly itself must comply with IEC 60079-0 Ex e or Ex d
- For arc flash mitigation: specify IAC rating and verify with arc flash study

## Applicable industries

process, manufacturing, energy, oil-and-gas, water-treatment, mining, data-centres

## References & further reading

- [IEC 61439 series — IEC webstore](https://webstore.iec.ch/searchform&q=61439)
- [IEC 61439-2 — Power switchgear and controlgear assemblies](https://webstore.iec.ch/publication/5455)