IEC 61439 Panels
Blog/IEC 61439 Internal Separation for MCC Design with Siemens

IEC 61439 Internal Separation for MCC Design with Siemens

Deep dive on IEC 61439 internal separation, compartmentalization and IAC implications for MCC layouts using Siemens breakers and busbar systems.

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Key Takeaways

  • IEC 61439 defines four internal separation Forms (1–4) that control busbar, functional unit and terminal compartmentalization to reduce personnel risk and arc propagation in MCCs.
  • For Motor Control Centers (MCCs) using Siemens breakers and busbar systems, Form 3B or Form 4 (4A/4B) is the industry-preferred approach for safe withdrawability and live-working mitigation.
  • Internal Arc (IAC) performance must be verified at the assembly level (IEC 61439-2:2020 and IEC/TR 61641 guidance); separation, barriers, and pressure relief design dictate IAC results more than individual breakers.
  • Practical design factors: metallic or insulated barriers, IP2X finger protection, minimum clearances per IEC 61439-1 Clause 8.2, and Siemens product clearances/withdrawable options must be coordinated into the final assembly.
  • Patrion can design and supply IEC 61439-compliant MCCs with Siemens components, tested or verified to project-specific IAC and thermal/short-circuit requirements.

Introduction

Motor Control Centers (MCCs) combine multiple motor feeders, starters, protection and control devices into a single assembly. For projects specifying Siemens circuit-breakers and busbar systems, correctly applying IEC 61439 internal separation rules is essential to meet safety, maintainability and Internal Arc (IAC) objectives. This article explains the Forms of separation, their practical implementation in MCC layouts with Siemens gear, and the implications for IAC testing and verification.

Relevant standards: IEC 61439-2:2020 (power assemblies) with IEC/TR 61439-0 (MCC checklists) and IEC/TR 61641 (internal arc test guidance). Manufacturer clearance and withdrawal data (Siemens SIRIUS / SENTRON / 3VA/3VL families) must be referenced during design.

Forms of Internal Separation — What They Mean for MCCs

IEC 61439 describes separation "Forms" that define how busbars, functional units (breakers, starters, contactors) and terminals are arranged and separated.

Summary of Forms

  • Form 1: No internal separation; all busbars, devices and terminals share the same space. Basic IP2X covers required.
  • Form 2: Functional units are separated from busbars; terminals may remain shared.
  • Form 3: Busbars separated from functional units and units separated from each other.
    • 3A: Units share a common terminal area.
    • 3B: Each unit has separated terminals.
  • Form 4: Adds separate terminal compartment or fully segregated terminals.
    • 4A: Terminals in a separate chamber.
    • 4B: Terminals integral but isolated inside the unit.

For high-availability, high-risk industries (oil & gas, petrochemicals), Form 3B or Form 4 is the norm because they allow withdrawals and cable work with minimized exposure to live busbars.

Practical Implementation with Siemens Components

Siemens offers withdrawable and fixed breakers in SIRIUS, SENTRON and 3VA/3VL product lines. These products support MCC architectures with Form 3/4 separation when installed in an appropriately partitioned enclosure.

Key design actions

  • Use metallic barriers (min. 2 mm steel typical) or molded insulating housings to separate units and busbars in accordance with IP2X/IPXXB rules.
  • Maintain required air and creepage distances per IEC 61439-1 Clause 8.2; verify Siemens product spacing data (e.g., phase clearances in manufacturer datasheets).
  • Provide withdrawable racking and mechanical interlocks for safe access and to enforce de-energized states where required.
  • Use dedicated terminal chambers (Form 4A) or per-unit terminal isolation (Form 3B/4B) to allow cable connection work without removing busbar covers.
  • Ensure ventilation and thermal spacing for rated ambient conditions and diversity factors (use IEC TR 61439-0 checklist for diversity / temperature rise calculations).

Internal Arc (IAC) Implications

IEC 61439 includes verification clauses for short-circuit and temperature rise; internal arc withstand verification follows IEC/TR 61641 guidance. IAC is assessed at assembly level — you cannot rely solely on individual breaker ratings.

  • Separation reduces arc initiation probability and limits arc propagation. Clause 2.5.0.5.2 of IEC 61439-2 emphasizes separation to reduce arc fault risk.
  • A Form 3B or Form 4A/4B layout increases the likelihood of passing IAC tests because compartmentalization limits pressure and element movement.
  • IAC testing (e.g., 50–100 kA for 1 s depending on project) requires provision for pressure relief, proper door retention, and clear diagnostic windows or indicators.
  • Project teams must decide whether to perform full-scale IAC tests or to use verified vendor configurations with documented IAC performance.

Comparison: Forms, Maintenance, and IAC Considerations

Feature Form 1 Form 2 Form 3B Form 4A / 4B
Busbar separation No Yes Yes Yes
Unit-to-unit separation No No Yes Yes
Per-unit terminal separation No No Yes (3B) Yes (4A/4B)
Withdrawable unit safe handling Low Medium High Very High
IAC performance (assembly-level) Poor Moderate Good Best
Typical industry use Simple switchboards General LV panels Most MCCs Critical petrochemical / mines

Checklist for MCC Layout Using Siemens Breakers

  • Confirm target Form (3B or 4 preferred) early in spec stage.
  • Reference IEC 61439-2:2020 and IEC/TR 61439-0 for checklist items (temperature rise, short-circuit, clearances).
  • Coordinate Siemens breaker mechanical drawings with barrier locations and withdrawable frames.
  • Design terminal chambers for cable bending/rating needs and gland plate segregation (Form 4A).
  • Plan for IAC route: pressure relief direction, door retention strength, and inspection windows if IAC testing is required.
  • Verify IP ratings (IP2X for internal barriers; consider IPXXD for high ingress protection needs).
  • Document the assembly verification plan: which clauses will be verified by calculation, which by type test, and which by inspection.

Manufacturer and Standards References

Designers should consult the following authoritative references:

(Also useful: IEC/TR training and explainer content and manufacturer application notes linked above.)

Industry Use Cases

  • Petrochemical / Oil & Gas: Form 4A/4B for terminal isolation and Form 3B minimum for unit separation; IAC testing commonly specified to 50–100 kA. See motor control center.
  • Industrial Manufacturing: Form 3B with withdrawable starters for rapid maintenance; coordinate with drives per IEC 61800 (motor control center).
  • Utilities & Mining: Rugged enclosures with Form 4 design for cable safety and pressure relief (motor control center).

Design Example — Siemens Withdrawable MCC Module

A practical Siemens-based module for a Form 3B MCC:

  • 3VA/3VL main breaker in a withdrawable frame, isolated from busbar by fixed metallic barrier.
  • Per-unit terminal cover with IP2X gland interface; cable chamber separated by a dedicated partition.
  • Interlock that prevents racking unless draw-out is in “service” position; door with pressure relief vents aligned top/rear.
  • Thermal calculations use diversity factors per IEC TR 61439-0 to size conductor and temperature-rise verification.

This configuration supports safe withdrawability and simplifies replacement without exposing the busbar.

Key Verification Steps Before Site Handover

  • Temperature-rise verification (Clause 10.10) with installed Siemens devices and full cable runs.
  • Short-circuit verification (Clause 10.9) including prospective fault current and fault level coordination.
  • Dielectric and insulation testing (Clause 10.8).
  • IAC verification documented per IEC/TR 61641 if specified by client or local code.

Related Resources on this Site

Next Steps

Patrion engineers can develop an MCC layout that meets IEC 61439-2:2020 internal separation requirements and integrates Siemens breakers and busbar systems into Form 3B or Form 4 assemblies. We provide design verification, factory testing or witnessed site testing, and full documentation packages for compliance.

Contact Patrion for project-specific proposals ([email protected]). Relevant panel offerings:

References

If you need a tailored layout, short-circuit/IAC evaluation, or a quotation for an IEC 61439-compliant Siemens MCC, Patrion can help — email [email protected] and include project fault levels, required Form of separation, and preferred Siemens product families.

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