LV panels may look simple on a single-line diagram—a box, a busbar, and a row of breakers. In reality, they are complex equipment that must meet strict electrical, thermal, and mechanical requirements. Engineers must verify these requirements before installation. This post explains how to specify and verify an LV panel and introduces the key standards that govern it. These fundamentals form the basis for the rest of this series on LV Main/Sub-Main Boards and Fixed MCCs.
A Complete Guide to IEC 61439 & the Standards Every Engineer Should Know
Why an LV Panel Is More Than “a Box with Breakers”
An LV panel—whether it is a Main LV Board fed from the auxiliary transformer of a 380kV substation, a Sub-Main Distribution Board, or a Fixed Motor Control Centre—is what IEC 61439 calls an assembly. It combines switchgear and controlgear devices, an enclosure, internal connections, and structural parts into one complete unit.
For many years, engineers used terms such as type-tested assembly (TTA) and partially type-tested assembly (PTTA). The current IEC 61439 edition no longer uses this approach. Instead, it requires engineers to perform design verification for the relevant characteristics of the assembly. Engineers can verify the design through testing, comparison with an already verified reference design, or engineering assessment through calculation. This part of the standard has significant practical value, but engineers often misunderstand it.
Your specification tells the panel builder exactly what to verify. It also defines the required values and the applicable verification method. A clear specification helps you avoid unnecessary testing costs. It also helps you identify any gaps before the FAT or, worse, after the panel reaches site.
The Standards That Govern Every LV Panel Spec
At the center of almost every modern LV switchgear specification sits one family of standards:
IEC 61439 — the master standard (replaces the older IEC 60439)
- IEC 61439-1 — General rules. Defines what a PTTA/TTA actually is, how its ratings are verified, temperature rise limits, dielectric properties, short-circuit withstand, and routine testing requirements.
- IEC 61439-2 — Power switchgear and controlgear assemblies. This is the part number that applies directly to Main LV Boards, Sub-Main Boards, and MCCs — and it’s where the Form of Separation table lives (more on that in Part 2 of this series).
- IEC 61439-3 — Distribution boards for use by ordinary persons. Smaller, less relevant for main industrial boards.
- IEC 61439-6 — Busbar trunking systems, relevant if your LV board feeds a rising busway riser.
The component standards underneath it
IEC 61439 governs the assembly, but every device inside it is qualified against its own standard — and your spec needs to call these out too:
- IEC 60947 series — the switchgear/controlgear device family: -1 (general), -2 (circuit breakers), -3 (switch-disconnectors), -4-1 (contactors & motor starters), -5-1 (control circuit devices).
- IEC 60529 — degrees of protection (the IP code).
- IEC 62262 — mechanical impact protection (the IK code).
- IEC 60204-1 — electrical safety of machinery, relevant if the MCC directly feeds machine loads.
- IEC/TR 61641 — internal arc testing guidance (covered fully in Part 5 of this series).
- IEC 60364 series — electrical installation design, protection coordination, and selectivity.
On projects with a US/NEMA influence, you may also see ANSI/UL 845 (motor control centres) and UL 891 (switchboards) referenced instead of, or alongside, the IEC series — worth confirming early, since the two frameworks aren’t interchangeable.
Understanding Design Verification: Testing, Comparison, or Assessment
TThis part of IEC 61439-1 clearly shows how the standard has evolved. The standard does not require physical testing for every characteristic. Instead, Table 1 — Design Verifications lists the characteristics that require verification. It also identifies the relevant clause and the permitted verification method for each characteristic.
The standard allows three verification methods:
Testing
Engineers confirm the characteristic through a physical test on a sample.
Comparison with a reference design
Engineers compare the design with an already verified reference design.
Assessment
Engineers verify the characteristic through engineering calculations or a documented technical assessment without physical testing.
For some characteristics, IEC 61439-1 allows only one verification method. For others, the manufacturer can select the method that best suits the design. The following table presents the complete list of characteristics covered by Table 1 and the applicable verification methods for each.
| # | Characteristic | Clause | Testing | Comparison | Assessment |
|---|---|---|---|---|---|
| 1 | Strength of materials and parts — resistance to corrosion | 10.2.2 | Yes | No | No |
| 1 | — thermal stability | 10.2.3.1 | Yes | No | No |
| 1 | — resistance to abnormal heat and fire from internal electrical effects | 10.2.3.2 | Yes | No | Yes |
| 1 | — resistance to UV radiation | 10.2.4 | Yes | No | Yes |
| 1 | — lifting | 10.5.5 | Yes | No | No |
| 1 | — mechanical impact | 10.2.6 | Yes | No | No |
| 1 | — marking | 10.2.7 | Yes | No | No |
| 1 | — mechanical operation | 10.2.8 | Yes | Yes | No |
| 2 | Degree of protection of enclosures | 10.3 | Yes | No | Yes |
| 3 | Clearances | 10.4 | Yes | No | No |
| 4 | Creepage distances | 10.4 | Yes | No | No |
| 5 | Protection against electric shock — continuity of the protective circuit | 10.5.2 | Yes | No | No |
| 5 | — short-circuit withstand strength of the protective circuit | 10.5.3 | Yes | Yes | No |
| 6 | Incorporation of switching devices and components | 10.6 | No | No | Yes |
| 7 | Internal electrical circuits and connections | 10.7 | No | No | Yes |
| 8 | Terminals for external conductors | 10.8 | No | No | Yes |
| 9 | Dielectric properties — power-frequency withstand voltage | 10.9.2 | Yes | No | No |
| 9 | — impulse withstand voltage | 10.9.3 | Yes | No | Yes |
| 10 | Temperature-rise limits | 10.1 | Yes | Yes | Yes |
| 11 | Short-circuit withstand strength | 10.11 | Yes | Yes | No |
| 12 | Electromagnetic compatibility (EMC) | 10.12 | Yes | No | Yes |
What the IEC 61439-1 Verification Table Tells Us
A few important points stand out when we look at the table as a whole.
- Some safety-critical characteristics, such as clearances, creepage distances, and protective circuit continuity, require testing only. The standard does not allow an alternative verification method for these characteristics.
- Other characteristics, such as the incorporation of switching devices, internal wiring, and terminal design (Items 6, 7, and 8), require assessment only. The manufacturer verifies these through documented engineering checks. These characteristics mainly depend on correct design and workmanship, so physical testing adds little value.
- Two characteristics usually receive the most attention: temperature-rise performance (Item 10) and short-circuit withstand strength (Item 11). These allow greater flexibility in verification. The manufacturer can use testing or comparison with a verified reference design. For temperature rise, the manufacturer can also use an engineering assessment.
This flexibility makes the actual panel design very important. Cable arrangement, ventilation, enclosure design, and heat dissipation all affect the final performance. When a manufacturer uses a reference design or calculation, the actual installation must match the conditions and assumptions behind that verification.
For an engineer reviewing a vendor’s documentation, this table is a useful checklist in itself: for every characteristic, you can ask “was this verified, and by which method?” — and expect a clear answer backed by a test report, a reference-design comparison, or a calculation.
KSA Saudi Energy-Specific Requirements for LV Panels
On Saudi projects, IEC 61439 is often only the starting point. SEC, for example, issues its own material specifications for auxiliary AC/DC panels. SEC-MSS-131-02 references IEC 61439-1 and IEC 61439-2 along with several other IEC standards and SEC specifications. It also adds project-specific design and construction requirements.
These requirements cover many details, including Form 3b separation for AC main distribution panels and Form 2b for DC panels. The specification also defines minimum short-time withstand ratings, enclosure requirements, cable entry, ventilation, dust protection, and heat-loss considerations.
SEC also sets specific requirements for circuit breakers, busbars, wiring, grounding, testing, and documentation. For example, the main AC panel must have a short-time withstand rating of at least 25 kA for 1 second, unless the project requirements specify a higher value.
The specification also requires design and routine tests in accordance with IEC 61439. SEC must review the test results.
Therefore, when preparing a tender for an SEC project, do not rely on a generic IEC 61439 specification alone. Always check the applicable SEC MSS, project technical specification, data schedule, and PTS requirements. These documents can add requirements that directly affect the panel design, pricing, testing, and compliance.
What a Complete LV Panel Specification Should Fix
A specification that will actually hold up through FAT and site handover needs to define, in order:
- Governing standard and edition — standards get revised, and editions change things like rated diversity factor tables. Always state the year.
- System data — voltage, frequency, prospective fault level, earthing system (TN-S, TN-C-S, IT).
- Rated currents — assembly rating (InA), per-circuit rating (Inc), and diversity factor if not 1.0.
- Short-circuit withstand — rated short-time withstand current (Icw) and duration, and rated peak withstand current (Ipk).
- Form of Separation — driven by your operation & maintenance philosophy (Part 2 of this series covers this in depth).
- Degree of protection — IP rating (ingress) and IK rating (impact).
- Enclosure construction — material, finish, mounting type, indoor or outdoor duty.
- Cable entry arrangement — top or bottom entry, gland plate design, cable sizes and quantities per way (this has a direct, often underestimated, effect on the panel’s thermal rating — covered in Part 3).
- Internal arc requirement — IAC classification if required (Part 5).
- Ambient and derating conditions — design ambient temperature, altitude, enclosure temperature rise limits.
- Component and device schedule — preferred makes, types, ratings, and protection coordination.
- Verification method — for each key characteristic, whether it will be confirmed by testing, comparison with a reference design, or assessment (see the table above).
- Testing regime — routine tests (performed on 100% of assemblies), witnessed FAT, and site acceptance tests.
Bringing It All Together
A well-structured LV panel specification reads through this same sequence: scope and applicable standards first, then service conditions, electrical system parameters, assembly ratings, enclosure requirements, internal arrangement, component schedule, internal arc requirement, design verification method, and finally nameplate and labelling requirements per IEC 61439-1 (manufacturer, type designation, standard reference, rated values, IP/IK, weight, and arc classification where applicable).
Fixing these thirteen items early — before detailed design begins — is what prevents the two most common problems with LV switchgear: discovering during acceptance testing that the assembly wasn’t actually verified for the conditions it will operate in, and finding on site that the real cable arrangement doesn’t match what the panel’s temperature-rise verification assumed.
Up next in this series: LV Panel Construction & Form of Separation Explained — Form 1 to Form 4b, where we break down exactly how segregation between busbars, functional units, and terminals affects both safety and cost, and why Form 4b has become the default for Main LV Boards and MCCs on substation auxiliary systems.