What Does IEC 61439 Require for Low Voltage Switchgear Design?
IEC 61439 establishes comprehensive design rules for low voltage switchgear assemblies up to 1000V AC or 1500V DC, mandating verification of temperature rise limits, short-circuit withstand strength, dielectric properties, and protection against electric shock through testing, calculation, or design comparison with reference assemblies. The standard eliminates the distinction between Type-Tested Assemblies (TTA) and Partially Type-Tested Assemblies (PTTA), requiring all assemblies to meet the same safety and performance benchmarks regardless of verification method.
Βασικά συμπεράσματα
- IEC 61439-1:2020 serves as the general rules standard applicable to all low voltage switchgear and controlgear assemblies up to 1000V AC or 1500V DC
- Three verification methods are accepted: testing, calculation, and comparison with a reference design—offering flexibility while maintaining safety rigor
- Όρια αύξησης θερμοκρασίας must not exceed 105K for bare copper busbars and 70K for terminals under rated current conditions multiplied by the Rated Diversity Factor (RDF)
- Short-circuit withstand strength verification is mandatory for all assemblies, either through testing, calculation, or comparison with a tested reference design
- Clear responsibility separation exists between the Original Manufacturer (system design) and the Assembly Manufacturer (final conformity) under the standard’s framework
- Rated Diversity Factor (RDF) enables realistic current loading assumptions—typically 0.8-1.0 depending on outgoing circuit count and application type
- Εσωτερικές μορφές διαχωρισμού (Form 1 through Form 4b) define arc fault containment and accessibility levels critical for personnel safety
Understanding the IEC 61439 Standard Series
The IEC 61439 standard series, which replaced IEC 60439 in 2009, represents a fundamental shift in how low voltage switchgear assemblies are designed, verified, and certified. Unlike the previous standard that created a two-tier system of Type-Tested Assemblies (TTA) and Partially Type-Tested Assemblies (PTTA), IEC 61439 establishes uniform requirements for all assemblies regardless of verification method.
The standard is organized into multiple parts:
- IEC 61439-1: General Rules — Defines fundamental requirements applicable to all assembly types including construction, performance, and verification requirements
- IEC 61439-2: Power Switchgear Assemblies — Covers power distribution systems, motor control centers, and switchboards
- IEC 61439-3: Distribution Boards — Addresses assemblies intended for operation by ordinary persons (DBO)
- IEC 61439-6: Busbar Trunking Systems — Specifies requirements for busbar trunking, tap-off units, and associated components
This modular structure allows manufacturers to apply the general rules in combination with product-specific requirements relevant to their application. For B2B manufacturers like VIOX Electric, understanding which parts apply to specific product lines is essential for compliance and market access.
Critical Design Requirements Under IEC 61439
Temperature Rise Limits and Thermal Management
Temperature rise verification is among the most critical aspects of IEC 61439 compliance. Excessive heat degrades insulation, accelerates aging, and creates fire hazards. The standard establishes specific temperature rise limits that must not be exceeded under rated current conditions.
IEC 61439-1 Table 6: Maximum Temperature Rise Limits
| Στοιχείο | Temperature Rise Limit (K) | Σημειώσεις |
|---|---|---|
| Bare copper busbars | 105 | Higher limits for silver-plated or nickel-plated surfaces |
| Busbars with Tinned joints | 90 | Limited by solder joint integrity |
| Terminals for external insulated cables | 70 | Based on cable insulation rating (PVC/PE) |
| Terminals for external XLPE cables | 90 | Higher temperature capability of XLPE insulation |
| Manual operating means (metal) | 25 | Safety-critical touchable surfaces |
| Manual operating means (insulating) | 35 | Lower limit for insulating materials |
| Enclosure external surfaces | 30 | Safety consideration for adjacent materials |
The temperature rise verification accounts for the Rated Diversity Factor (RDF), which recognizes that not all circuits operate at full load simultaneously. RDF values range from 1.0 for incoming supply circuits down to 0.4 for distribution boards with many outgoing circuits. This factor multiplies the rated current for temperature rise calculations, enabling more realistic and economical designs without compromising safety.
For thermal management, engineers must consider:
- Natural convection through ventilation openings positioned to utilize the chimney effect
- Forced air cooling for high-density assemblies exceeding 6300A
- Heat dissipation from διακόπτες κυκλώματος and other components based on IEC 60947 power loss data
- Ambient temperature derating when installations exceed the standard 35°C reference
Short-Circuit Withstand Strength Verification
IEC 61439 mandates that all assemblies must withstand the mechanical and thermal stresses of short-circuit currents. The assembly’s short-circuit withstand current rating (Icw) represents the maximum current the assembly can safely carry for a specified duration (typically 1 second) without damage.
Verification Options:
- Δοκιμές — Full short-circuit test on the actual assembly or representative sample
- Υπολογισμός — Analytical verification using recognized engineering methods with safety margins
- Comparison with Reference Design — Comparison against a tested reference design with equal or greater parameters
The short-circuit verification must consider:
- Peak current withstand (related to Icw through the factor “n” typically 1.5-2.1 depending on power factor)
- Thermal stress (I²t) through the protective device’s clearing characteristics
- Electromagnetic forces between conductors, particularly for ράγες μεταφοράς without adequate bracing
- Coordination with protective devices to ensure the assembly is protected under fault conditions
For copper busbar systems, spacing and support requirements are critical. IEC 61439 permits design rule verification of busbar short-circuit withstand strength through calculation or comparison with tested reference designs, provided all criteria including conductor dimensions, spacing, and support arrangements meet or exceed the reference.
Dielectric Properties and Clearances
Insulation coordination ensures assemblies withstand operational voltages, temporary overvoltages, and transient overvoltages. IEC 61439 specifies:
Minimum Clearances and Creepage Distances:
| Rated Insulation Voltage (V) | Minimum Clearance in Air (mm) | Minimum Creepage Distance (mm) — Pollution Degree 3 |
|---|---|---|
| ≤ 300 | 5.5 | 8.0 |
| 300-600 | 8.0 | 12.0 |
| 600-1000 | 14.0 | 20.0 |
The standard requires assemblies to withstand:
- Power-frequency withstand voltage tests (typically 2kV AC for 1 second for 400V systems)
- Impulse withstand voltage tests (8kV for 400V systems in overvoltage category III)
- Verification that clearances are maintained during assembly and throughout service life
Designers must account for altitude derating—clearances must increase by approximately 1% per 100m above 2000m. This is particularly important for switchgear destined for high-altitude installations.
Forms of Internal Separation: Arc Fault Containment
IEC 61439 defines Forms of Internal Separation that specify the degree of segregation between busbars, functional units, and terminals. These forms range from Form 1 (no separation) to Form 4b (separation of busbars, functional units, and terminals including interconnections between units).
| Form | Busbar Separation | Functional Unit Separation | Terminal Separation | Εφαρμογή |
|---|---|---|---|---|
| Form 1 | Κανένας | Κανένας | Κανένας | Simple distribution, minimal safety requirements |
| Form 2a | Ναι | Κανένας | Κανένας | Basic busbar isolation |
| Form 2b | Ναι | Κανένας | Ναι | Terminal access separation |
| Form 3a | Ναι | Yes, no terminals | Κανένας | Κέντρα ελέγχου κινητήρων with limited segregation |
| Form 3b | Ναι | Yes, no terminals | Ναι | Standard industrial switchgear |
| Form 4a | Ναι | Yes, including terminals | Yes (same compartment) | High-integrity separation |
| Form 4b | Ναι | Yes, including terminals | Yes (separate compartments) | Maximum safety, critical applications |
Higher form numbers provide greater arc fault containment and personnel protection but increase cost and complexity. Form 4b, for example, requires separate compartments for each functional unit’s terminals, significantly impacting enclosure design and heat dissipation.
The selection of separation form involves balancing:
- Safety requirements (personnel access, arc fault containment)
- Maintenance needs (accessibility for servicing individual units)
- Thermal management (segregation can impede airflow)
- Cost constraints (higher forms require more material and complex construction)
- Application criticality (data centers, hospitals typically specify Form 4)
Verification Methods: Testing, Calculation, and Design Rules
IEC 61439 provides three verification pathways, recognizing that full testing of every assembly variant is impractical:
Verification by Testing
The traditional approach where the actual assembly undergoes laboratory testing. Required for:
- Temperature rise (unless design rules apply)
- Short-circuit withstand (unless calculation or design rules apply)
- Dielectric properties
- Μηχανική λειτουργία
- Degree of protection (IP rating verification)
Verification by Calculation
Analytical methods permitted for certain characteristics:
- Temperature rise using thermal modeling with validated data
- Short-circuit withstand strength using electromagnetic force calculations
- Creepage and clearance verification through dimensional analysis
Calculations must use recognized engineering methods with appropriate safety margins. The standard requires conservative assumptions—device ratings must be derated by 20% when used in calculations unless specific component data is available.
Verification by Design Rules
Comparison with tested reference designs:
- Permitted for short-circuit withstand when busbar cross-sections, materials, and support spacing meet or exceed the reference
- Annex N of IEC 61439-1 provides specific design rule parameters for busbar systems
- The reference design must have been tested to the same or higher stress levels
- All parameters must be equal or superior to the reference—no interpolation permitted
This approach is particularly valuable for busbar trunking systems and standardized switchgear ranges where multiple configurations share common construction principles.
Responsibility Framework: Original Manufacturer vs. Assembly Manufacturer
IEC 61439 clearly delineates responsibilities between two key entities:
Original Manufacturer (System Manufacturer):
- Designs the switchgear assembly system
- Establishes design rules and verification methods
- Provides tested reference designs
- Specifies components, materials, and construction methods
- Issues system documentation and compliance guidance
Assembly Manufacturer (Panel Builder):
- Constructs the final switchgear assembly
- Verifies compliance with the standard using methods provided by the Original Manufacturer
- Performs routine verification (routine tests on every assembly)
- Assumes responsibility for the finished assembly placed on the market
- Maintains technical documentation and Declaration of Conformity
This framework ensures that while system design expertise resides with the Original Manufacturer, accountability for the finished product rests with the Assembly Manufacturer. For procurement professionals, understanding this distinction is essential when evaluating supplier claims of compliance.
Practical Implementation: Design Checklist for Engineers
Pre-Design Phase
- Define application requirements — Voltage, current, fault level, environmental conditions
- Select appropriate IEC 61439 part — -2 for power switchgear, -3 for distribution boards, -6 for busbar trunking
- Determine Rated Diversity Factor — Based on load characteristics and circuit count
- Establish required Form of Separation — Based on safety requirements and application criticality
- Identify applicable derating factors — Temperature, altitude, harmonics, installation conditions
Design Phase
- Calculate busbar sizing — Based on rated current, RDF, temperature rise limits, and busbar material
- Verify short-circuit withstand — Test, calculate, or compare with reference design
- Determine clearances and creepage — Based on rated insulation voltage and pollution degree
- Design thermal management — Natural ventilation, forced cooling, or air conditioning
- Select enclosure protection rating — IP rating based on environment, IK rating for mechanical impact
- Plan internal separation — Form 1 through 4b based on safety requirements
Verification Phase
- Conduct design verification — Testing, calculation, or design rules as applicable
- Perform routine tests — Dielectric, wiring, continuity, and mechanical operation on every assembly
- Compile technical documentation — Drawings, specifications, test reports, risk assessment
- Issue Declaration of Conformity — CE marking documentation for EU market access
Common Design Pitfalls and How to Avoid Them
Pitfall 1: Ignoring the Rated Diversity Factor
Τεύχος: Designing all busbars for simultaneous full-load operation leads to oversized, expensive systems.
Λύση: Apply appropriate RDF values—0.9-1.0 for incoming circuits, 0.8 for power distribution, 0.6-0.7 for distribution boards with many circuits.
Pitfall 2: Inadequate Thermal Management
Τεύχος: Reliance on theoretical calculations without accounting for installation conditions (enclosed rooms, solar gain, adjacent heat sources).
Λύση: Perform thermal modeling with realistic boundary conditions; specify forced ventilation for high-density assemblies; allow adequate clearance around enclosures.
Pitfall 3: Short-Circuit Rating Mismatch
Τεύχος: Assembly Icw rating exceeds protective device breaking capacity, or insufficient bracing for electrodynamic forces.
Λύση: Ensure διακόπτης κυκλώματος breaking capacity equals or exceeds assembly withstand rating; verify busbar support spacing meets design rule requirements.
Pitfall 4: Neglecting Clearance Verification
Τεύχος: Assuming standard clearances without accounting for installation tolerances, material swelling, or conductor movement under fault conditions.
Λύση: Design with margin—specify clearances 20% greater than minimum requirements; verify with physical inspection during prototype assembly.
Pitfall 5: Form of Separation Incompatibility
Τεύχος: Specifying high separation forms (Form 4) without considering the thermal impact of compartmentalization.
Λύση: Evaluate thermal management requirements early; specify ventilation or cooling for Form 3 and 4 assemblies; consider electrical panel ventilation strategies.
Σύντομη ενότητα FAQ
Q: What is the difference between IEC 61439 and the old IEC 60439 standard?
A: IEC 61439 replaced IEC 60439 in 2009 and eliminates the distinction between Type-Tested Assemblies (TTA) and Partially Type-Tested Assemblies (PTTA). Under IEC 61439, all assemblies must meet the same safety requirements regardless of verification method (testing, calculation, or design rules). The new standard also introduces clearer responsibility separation between Original Manufacturers and Assembly Manufacturers, and establishes the Rated Diversity Factor (RDF) concept for realistic load calculations.
Q: Can I use IEC 61439 for DC switchgear design?
A: Yes, IEC 61439-1:2020 explicitly includes requirements for DC applications up to 1500V DC. However, DC introduces unique challenges including continuous arcing during faults (no natural current zero-crossing), higher temperature rise due to lack of skin effect redistribution, and different creepage distance requirements. For DC applications, pay particular attention to Αυτόματος διακόπτης DC selection, arc chute design, and polarity considerations.
Q: How do I determine the correct Rated Diversity Factor (RDF) for my switchgear assembly?
A: RDF depends on the number of outgoing circuits and application type. IEC 61439-1 provides reference values: 1.0 for incoming supply circuits; 0.9 for 2-3 outgoing circuits; 0.8 for 4-5 circuits; 0.7 for 6-9 circuits; and 0.6 for 10+ circuits. Distribution boards (DBOs) per IEC 61439-3 use different criteria based on connected load diversity. Always document the basis for your RDF selection in the technical file.
Q: Is third-party certification required for IEC 61439 compliance?
A: No, IEC 61439 does not mandate third-party certification. The standard operates on self-certification by the Assembly Manufacturer, who assumes responsibility for conformity. However, many specifications (particularly in oil & gas, data centers, and critical infrastructure) require third-party verification through bodies like UL, IECEx, or notified bodies for CE marking. While not mandatory, third-party certification provides independent validation of compliance claims.
Q: What routine tests must be performed on every IEC 61439 assembly?
A: Every assembly must undergo routine testing before dispatch: insulation testing (dielectric withstand at 1kV AC or 1.5kV DC for 1 second); continuity of protective circuits (maximum 0.05Ω between enclosure and earth terminal); inspection of wiring and component installation; and mechanical operation verification (switches, διακόπτες κυκλώματος, interlocks). Test results must be recorded and retained in the technical file.
Q: How does IEC 61439 address arc flash hazards?
A: While IEC 61439 does not specifically mandate arc fault containment testing (refer to IEC TR 61641 for that), the Forms of Internal Separation (Form 2b through 4b) provide degrees of arc fault containment. Form 4b offers the highest protection with complete compartmentalization. For applications requiring verified arc fault containment (such as oil & gas), specify compliance with both IEC 61439 and IEC TR 61641, which provides test methods for internal arc classification (IAC).
Conclusion: Engineering Excellence Through Standards Compliance
IEC 61439 represents a mature, comprehensive framework for low voltage switchgear design that balances safety rigor with engineering practicality. By providing multiple verification pathways—testing, calculation, and design rules—the standard accommodates the diverse needs of custom panel builders and mass manufacturers alike while maintaining consistent safety benchmarks.
For electrical engineers and procurement professionals, understanding IEC 61439 is not merely about compliance checkbox-ticking. The standard’s requirements for temperature management, short-circuit withstand, and internal separation directly impact equipment reliability, service life, and personnel safety. Proper application of the Rated Diversity Factor can yield significant cost savings without compromising performance, while correct specification of Forms of Separation ensures appropriate protection for the application environment.
As switchgear assemblies become increasingly sophisticated—integrating smart monitoring, προστασία από υπερτάσεις, and renewable energy interfaces—the foundational requirements of IEC 61439 remain essential. The standard’s design verification framework, responsibility delineation, and performance benchmarks provide the technical foundation upon which modern electrical distribution systems are built.
For B2B manufacturers like VIOX Electric, compliance with IEC 61439 is both a market access requirement and a competitive differentiator. Assemblies designed and verified to this standard demonstrate engineering rigor, safety commitment, and global market readiness—qualities that procurement professionals prioritize when selecting partners for critical infrastructure projects.
Technical Reference: This guide is based on IEC 61439-1:2020 “Low-voltage switchgear and controlgear assemblies — Part 1: General rules” and associated product-specific parts. For complete compliance requirements, always consult the full standard text and applicable national deviations. As a B2B manufacturer of electrical protection equipment, VIOX Electric provides IEC 61439 compliant components and technical support for switchgear assembly manufacturers worldwide.
