IEC 61439 vs IEC 60439: what changed for LV switchboards

By Eng. Mannan Maqsood, Electrical EngineerPublished 5 November 20266 min read

Many specifications in the Gulf still ask for switchboards that are "type tested to IEC 60439-1" or "TTA". IEC 60439 was withdrawn years ago. The international transition period ended in November 2014, and every LV assembly built since then should be declared to the IEC 61439 series instead. The words have changed, but more importantly the responsibilities have changed. If you review panel submittals or witness factory tests, you need to know what to ask for under the new standard.

The short version

• TTA and PTTA no longer exist. IEC 60439 split assemblies into type-tested (TTA) and partially type-tested (PTTA). IEC 61439 has one route: every assembly must be design verified and every unit must be routine verified.
• Two manufacturers, two jobs. The original manufacturer (the system owner, for example the switchgear brand) carries out design verification. The assembly manufacturer (the panel builder) is responsible for the finished board and its routine verification.
• Verification can be by test, by comparison with a tested reference design, or by assessment, depending on the characteristic. Testing is no longer the only acceptable evidence, but the method used must be stated.
• New ratings appear on the nameplate and in the submittal, including the rated current of the assembly (InA), the rated current of each circuit (InC) and the rated diversity factor (RDF).

The structure of the IEC 61439 series

IEC 60439 was one general part plus a few product parts. IEC 61439 is a family. Part 1 holds the general rules and is never used alone. It is always read with the product part that applies to the board in front of you:

• IEC 61439-2: power switchgear and controlgear assemblies (PSC assemblies). This covers main switchboards, MDBs, SMDBs and MCC panels.
• IEC 61439-3: distribution boards intended to be operated by ordinary persons (DBO), such as final distribution boards in apartments and offices.
• IEC 61439-4: assemblies for construction sites.
• IEC 61439-5: assemblies for power distribution in public networks.
• IEC 61439-6: busbar trunking systems (busways and busbar risers).
• IEC 61439-7: assemblies for specific applications such as marinas, camping sites, market squares and EV charging stations.

Australia and New Zealand adopt the same series as AS/NZS 61439, which replaced AS/NZS 3439. The UK and Europe use BS EN 61439. The technical content is aligned, so a design verification done to IEC 61439 is normally recognised across these regions, but always check the edition your specification quotes.

Design verification: what it covers

Design verification proves that the design of the assembly system meets the standard. It is done once for a system, not for every panel. IEC 61439-1 lists the characteristics that must be verified, including:

• strength of materials and parts (corrosion, UV resistance for outdoor enclosures, lifting, mechanical impact)
• degree of protection (IP code)
• clearances and creepage distances
• protection against electric shock and the integrity of protective circuits, including the short-circuit strength of the protective conductor
• incorporation of switching devices and components
• internal electrical circuits and connections
• terminals for external conductors
• dielectric properties (power-frequency and impulse withstand)
• temperature-rise limits
• short-circuit withstand strength
• electromagnetic compatibility
• mechanical operation

Two items cause most of the arguments on projects: temperature rise and short-circuit withstand. For temperature rise, the standard allows testing, derivation from a tested design, or calculation. Calculation is limited to lower ratings and specific conditions (the IEC 60890 method for multi-compartment boards up to 1600 A, and simpler rules for small single-compartment boards). A 4000 A main switchboard therefore needs test evidence or a valid comparison with a tested reference design. For short-circuit withstand, the evidence must cover the actual busbar arrangement, supports and protective devices, at the rated short-time withstand current (Icw) and peak (Ipk) declared for the board.

Routine verification: done on every board

Routine verification is what the panel builder must do on each assembly before it leaves the factory. It is what you witness at a FAT. It includes checks of the degree of protection, clearances and creepage, protection against electric shock and the continuity of protective circuits, the correct incorporation of components, the tightness of internal connections, the terminals for external conductors, mechanical operation, a dielectric test, and a check of the wiring and functional operation. The panel builder should hand over a routine verification report signed for that serial number. If a board arrives on site without one, it is not complete under the standard, whatever the brand on the door.

Rated diversity factor: the rating nobody reads

Under IEC 60439, a board with ten 250 A outgoing MCCBs often looked as if it could supply all ten at full current. IEC 61439 makes the manufacturer state a rated diversity factor (RDF). It is the fraction of their rated current that outgoing circuits can carry continuously and at the same time without overheating the assembly. A typical default in IEC 61439-2 for a board with several outgoing circuits is well below 1.0. If your load schedule shows several feeders running close to their breaker rating together, for example chillers or data centre loads, ask for an RDF of 1.0 for those circuits and make sure the temperature-rise evidence supports it.

Forms of separation and internal arc

Forms of internal separation (Form 1 to Form 4b, with the Type 1 to Type 7 variants used in BS EN practice) are unchanged in concept. What changed is that the form is now part of the declared design and must be backed by the design verification of that system. Internal arc protection is a different subject. It is assessed with IEC TR 61641, which sits outside IEC 61439. If your project specification asks for arc containment, ask for a separate IEC TR 61641 test report for the same system, the same form and the same arc current and duration.

What to ask for in a switchboard submittal

1. A declaration of conformity to IEC 61439-1 and the correct product part (usually -2 for main boards, -3 for final DBs), with the edition.
2. Design verification reports from the original manufacturer for the exact system offered, covering at least temperature rise and short-circuit withstand at the ratings on your drawings.
3. Confirmation of the assembly manufacturer and whether the panel builder is licensed by the original manufacturer for that system.
4. The ratings that apply: Ue, Ui, Uimp, InA, InC for each circuit, RDF, Icw and duration, Ipk, IP code, form of separation and pollution degree.
5. For high-ambient sites, the ambient temperature used in the temperature-rise evidence. IEC tests are based on an average ambient of 35 °C. A plant room at 45 °C or 50 °C needs derating or confirmed evidence at that temperature.
6. The routine verification test sheet template, so you know what will be witnessed at FAT.

Common mistakes on projects

• Accepting a generic brand certificate. A certificate for a 6300 A system with copper bars does not cover a 2500 A board built with a different bar size and support spacing unless the comparison rules allow it. Check that the tested configuration covers what is offered.
• Mixing brands inside one assembly without evidence. Fitting another brand's ACB into a tested system changes the heat and the short-circuit path. It needs its own verification.
• Ignoring the ambient. The board passes the temperature-rise test in the factory and then trips or discolours in an LV room with poor ventilation.
• Specifying "TTA to IEC 60439". Update your specification text. It signals an outdated document and makes it harder to reject a weak submittal.

How to practise this

The LV Switchgear Verification & Assembly Lab walks through design and routine verification items and builds a FAT certificate. The LV Switchgear Design Lab lets you change the incomer, busbars and form of separation and see the effect on heat and fault level. Use them together with the checklist in the Switchgear & DB Checklist Suite before your next panel review.

Disclaimer. This article is general technical information for education only. It is not professional engineering, design, legal or safety advice for any specific project, and it does not replace the judgement of a qualified, registered engineer who has reviewed the actual installation. Worked examples and figures are illustrative and do not describe any identified project, client or site. Codes, authority requirements and product data change, so confirm every value against the edition that applies to your project, the authority having jurisdiction and the manufacturer's data. Any testing or work on live systems must be carried out only by competent, authorised people under the approved procedure. The views are the author's own and do not represent any employer, client or authority. To the extent permitted by law, the author, the editor and MEPVirtual accept no liability for any loss, damage or injury arising from the use of, or reliance on, this content. See the full disclaimer or report a correction.