Temperature Rise Verification in IEC 61439
How temperature rise limits are verified in panel assemblies per IEC 61439-1.
Temperature Rise Verification in IEC 61439
This article explains the temperature rise verification requirements for low-voltage switchgear and controlgear assemblies under IEC 61439-1, clarifies the limits in Table 6, summarizes accepted verification methods (test, comparison, calculation), and documents practical guidance and manufacturer practices. The text cites the relevant clauses and guidance documents and provides concrete numbers, test requirements, and industry best practices for panel designers and test laboratories.
Overview and Purpose
IEC 61439-1 requires designers and manufacturers to verify that assemblies do not exceed defined temperature-rise limits at rated conditions in order to avoid damage to current-carrying parts, insulation, terminals and to ensure safe accessible surface temperatures (Clause 10.10, Table 6). Temperature-rise verification protects functional integrity, prevents accelerated ageing of insulation and soldered/tinned joints, and reduces fire and human-burn risks during normal service life. Per IEC 61439-1 the verification may be performed by type test, comparison with an adequately tested reference design, or by assessment/calculation using recognized methods such as IEC TR 60890 (Clause 10.10 and related annexes).[1][4]
Key Limits from IEC 61439-1 (Table 6)
IEC 61439-1 Table 6 defines maximum permitted temperature rises for specific parts of an assembly. These limits are given as temperature rise (K) above ambient and, where relevant, imply absolute maximum temperatures when design ambient is known (commonly 40 °C for fielded assemblies). The most frequently applied values are:
- Copper busbars and conductors: 105 K maximum temperature rise (e.g., at 40 °C ambient this equals an absolute temperature of ≤145 °C). Per IEC 61439-1 Table 6 this is the design target for bare copper parts exposed to current heating.[1]
- Bolted silvered contacts: IEC 61439-1 Table 6 allows up to 105 K, but IEC 62271-1 includes a 75 K value for some high-voltage definitions, which leads to interpretive ambiguity when low-voltage bolted silvered contacts mirror high-voltage practice. Designers must ensure that contact materials and plating tolerate the tested absolute temperature without loss of mechanical or electrical function (see discussion below).[1][2]
- Tinned contacts/soldered joints: Tinned surfaces are limited by tin creep or softening. Because the creep-softening point of tin occurs near 105 °C absolute, designers commonly apply a practical limit equivalent to 65 K rise at 40 °C ambient (absolute 105 °C) for tinned parts to avoid joint damage.[1]
- Terminals for external conductors: 70 K maximum rise for copper terminals and 65 K for aluminium terminals (Table 6). These limits protect clamping hardware, insulation sleeves, and conductor terminations that are frequently subject to mechanical manipulation during installation and maintenance.[3]
- Accessible external enclosure surfaces: To prevent burns, IEC 61439-1 limits temperature rise for accessible metal surfaces to 30 K and for accessible insulating (plastic) surfaces to 40 K above ambient.[3][5]
Always interpret the listed rise values relative to the test ambient: for parts that cite absolute behaviour (e.g., tin at 105 °C), convert the allowed rise to the testing ambient. For example, a 105 K rise at 40 °C ambient gives an absolute limit of 145 °C for bare copper, whereas a 65 K rise at 40 °C yields 105 °C for tinned parts. Documentation and the design dossier must record whether the limit applies to absolute temperature or incremental rise and how ambient was considered (IEC 61439-1 Clause 10.10 notes this distinction).[1][3]
Permitted Verification Methods (Clause 10.10)
IEC 61439-1 permits three accepted routes to demonstrate compliance with temperature-rise limits (Clause 10.10):
- Type testing (measurement): Apply rated current (InA) to the assembly or to representative parts and measure steady-state temperatures at prescribed points. The standard requires that average ambient during the test does not exceed 35 °C (practical limits and manufacturer notes often cite 40 °C as a maximum field ambient; see ABB and Legrand guidance).[3][4][5]
- Comparison with a reference design: Demonstrate thermal equivalence to a tested reference assembly by showing the same functional units, construction, cross-section and dimensions of busbars, separation and barriers, and the same outgoing circuits and loss profile. The rated diversity factor (RDF) is applied to outgoing circuits to reflect expected diversity during operation.[3][5]
- Assessment and calculation: Use accepted calculation methods (IEC TR 60890 is the recognized calculation methodology) to predict temperature rise. IEC 61439-1 limits the calculation route to assemblies with rated currents typically up to 1600 A; beyond that tests or reference designs are preferred. The guidance also encourages conservative margins (industry practice uses a 20% derating in assessed currents and losses).[3][4][5]
Practical Test Requirements and Instrumentation
When performing a type test to verify temperature rise, laboratories and manufacturers must follow disciplined procedures to ensure reproducible, representative results:
- Test current and sources: Apply the rated current (InA) and, where the assembly contains multiple parallel conductors or sub-conductors, use current sources that reliably feed all parallel paths. IEC 61439-1 and industry guidance warn that weak current sources or poor current distribution can mask hotspots and produce non-reproducible results; designers often use multiple synchronized supplies to ensure correct current sharing during tests (see manufacturer notes and test guides).[2][3]
- Ambient control: Maintain the average ambient temperature ≤35 °C during tests (standard test protocol). Some manufacturer test reports accept up to 40 °C in field conditions but document conversions accordingly (ABB and Schneider examples).[3][4][5]
- Steady-state criteria: Measure until temperatures stabilize (commonly defined as change ≤1–2 °C over one hour, or per laboratory test plan). Record the time to reach steady-state. Use thermocouples or calibrated probes attached to hotspots and representative locations such as busbar surfaces, terminal screws, insulating supports and enclosure panels.
- Measurement points: Place probes on current-carrying parts, terminal bolts, contact faces (where accessible), insulating supports, and accessible surfaces. Include temperatures on tinned/soldered regions when present to verify tin creep limits. Additionally record internal air temperature and ambient at standard reference locations.[3]
- Post-test mechanical/insulation checks: Confirm that parts show no damage by mechanical and insulation tests; perform the ball pressure test on polymeric insulation supports at 125 °C for live-part supports and at 70 °C for others with an impression limit ≤2 mm per IEC 61439-1 notes.[3]
Comparison vs Calculation: When to Use Which Method
Designers must select the verification route that best fits the assembly’s complexity and rating. The table below summarizes applicability and practical considerations.
| Verification Method | Typical Maximum Rated Current | When Appropriate | Advantages / Limitations |
|---|---|---|---|
| Type Test (Measurement) | All ratings (preferred for >1600 A) | New designs, high-current assemblies, complicated geometries, non-standard materials | Most authoritative; captures real effects (contact resistances, joint heating). Requires test lab and reproducible current sources. |
| Comparison to Reference Design | All ratings | When an identical or sufficiently similar assembly has been type-tested | Efficient for families of products; must demonstrate equivalence in losses, spacing and construction. Requires rigor in dossier evidence. |
| Calculation / Assessment (IEC TR 60890) | Generally up to 1600 A | Standard constructions, well-characterized components, when test is impractical | Fast and repeatable; depends on validated loss models and conservative margins. IEC recommends using a safety margin (industry practice uses ~20% derating). |
Design and Test Best Practices
Based on IEC clauses and manufacturer guidance, the following best practices reduce risk of non-compliance, improve reproducibility, and simplify certification:
- Apply RDF consistently: Use the rated diversity factor for outgoing circuits when comparing to a reference design or when setting currents for calculation. Remember that conductors feeding feeders must be rated for Inc ≥ Ib; document RDF usage in the technical dossier (IEC 61439-1 and -2 guidance).[3][5]
- Use robust current sources: For assemblies with parallel conductors or busbar branches, use synchronized, high-quality current supplies to avoid skewed current sharing that hides hotspots. Test reports show large variation if sources are weak or if current distribution is not representative (field observations underline this risk).[2]
- Conservative margins in calculation: When using IEC TR 60890, apply a safety margin (industry commonly applies 20% derating of current or losses) and validate calculations versus at least one representative test to confirm model fidelity (ABB and Eaton guidance recommend this approach).[3][4]
- Document material limitations: Pay special attention to tinned surfaces, plated contacts and polymeric supports. Convert rises to absolute temperatures and verify against material softening/creep limits (e.g., tin creep point ≈105 °C absolute) and the ball pressure test requirements in IEC 61439-1.[1][3]
- IP rating and internal ambient: Enclosure protection (IEC 60529 IP codes) influences internal convection and heat dissipation. Higher IP may increase internal ambient and affect calculation/test conditions; include IP specification and ventilation details in the dossier.[4]
- Post-test mechanical inspection: Perform mechanical checks (contact forces, joint torque) and insulation tests after thermal testing to confirm no deterioration occurred. Record all measurements in the type-test report.[3]
Common Ambiguities and How to Resolve Them
Several recurrent issues arise during temperature-rise verification:
- 105 K vs 75 K for silvered contacts: IEC 61439-1 allows up to 105 K for certain current-carrying parts, but IEC 62271-1 (high-voltage standard) uses 75 K in some tables. This difference requires designers to analyze contact material and functional requirements: if manufacturer data or high-voltage practice specifies a lower limit, adopt the stricter value and document rationale. When in doubt, test the exact assembly and contact type under representative conditions to eliminate ambiguity.[1][2]
- Tinned contacts and absolute temperatures: Because tinned regions may creep or degrade above ~105 °C, convert allowed rises into absolute temperatures based on the chosen test ambient and ensure design margins below the tin creep threshold.[1]
- Ambient specification: The standard’s test ambient requirement is an average ≤35 °C but industry practice and manufacturer datasheets sometimes refer to a 40 °C site ambient. Maintain clarity in the dossier: state which ambient was used for test or conversion and show the calculation converting rise limits to absolute temperatures.[3][4][5]
Manufacturer Practices and Tools
Major equipment manufacturers provide design tools, validated reference designs and technical guidance that align with IEC 61439 verification routes:
- ABB: Provides a Panel Design Configurator and the Technical Workbook "IEC 61439 in Practice" with calculation examples up to 630 A and datasets for losses, RDF and busbar temperatures. ABB recommends converting test ambient values and documents the use of IEC TR 60890 for calculations (Technical Workbook and Application Paper).[4][5]
- Siemens: Publishes design manuals and software for NXPLUS and related panels. Siemens documentation emphasizes reference-design comparison and the use of IEC TR 60890 for calculations up to 1600 A while applying RDF.[3]
- Schneider Electric, Eaton, Rittal, Legrand: Provide white papers and application notes describing temperature-rise verification strategies, recommended test procedures, and validated configurations. Legrand’s white paper covers construction and certification, and Rittal documents enclosure impacts and IP considerations.[5][6]
- DEHN and industry guides: Supply notes on integrating surge protective devices into assemblies and how their losses should be included in thermal verifications.[7]
Specification and Example Values
The following specification table provides typical limits, the corresponding absolute temperatures at a 40 °C ambient for clarity, and the originating clauses or industry notes to help authors of technical dossiers.
| Component | Temperature Rise (K) | Absolute Temp @ 40 °C Ambient (°C) | Reference / Note |
|---|---|---|---|
| Copper busbars and conductors | 105 K | 145 °C | IEC 61439-1 Table 6 (Clause 10.10) [1] |
| Bolted silvered contacts | Up to 105 K (interpretation varies; 75 K in IEC 62271) | 145 °C (or 115 °C if 75 K limit applied) | IEC 61439-1 Table 6; cross-ref IEC 62271-1 [1][2] |
| Tinned contacts / soldered joints | ~65 K (practical limit) |