Partition class describes the nature of the barrier between opened accessible compartments and hazardous live parts of the main circuit. PM and PI are precise assembly classifications; they should not be used as vague synonyms for metal-clad or insulated switchgear.

Learning objectives

Distinguish metallic and non-metallic partitions, understand the role of shutters and connect partition class to earthing, continuity and access procedures.

Core engineering principles

PM relies on metallic partitions and shutters

Metallic parts separating an opened compartment from live main-circuit parts must be connected to the earthing system as required by the design. Their continuity and mechanical operation are therefore safety-relevant.

PI permits insulating partitions in the separation

Insulating barriers can provide the required separation, but their dielectric design, contamination behaviour, mechanical integrity and ageing must remain within the validated product construction.

Partition class applies to the declared assembly condition

Removing a barrier, defeating a shutter or changing a component can alter the separation on which the classification depends. Field modifications require manufacturer assessment.

PM or PI is not an arc classification

The class does not state internal-arc current, duration or accessibility sides. It also does not replace safe isolation, proving dead and earthing for work.

Engineering application method

  1. Step 1: Locate every partition and shutter on the assembly section drawing.
  2. Step 2: Identify material, protective bonding and operating mechanism.
  3. Step 3: Verify the separation maintained when each declared compartment is open.
  4. Step 4: Check modifications and maintenance instructions against the classified construction.

Practical example

A withdrawable breaker panel can use grounded metallic shutters over fixed contacts and achieve PM separation. Another design may use an insulating barrier and be classified PI. Both still require separate evaluation of LSC, IAC and the intended maintenance task.

Common mistakes

  • Calling every metal-enclosed panel PM.
  • Assuming PI is inherently less safe.
  • Failing to bond removable metallic partitions.
  • Using partition class as proof of internal-arc containment.

Design and review checklist

  • Are partitions metallic, insulating or mixed?
  • Are metallic shutters reliably earthed?
  • What separation remains after opening the compartment?
  • Does supplied construction match the tested classification?

Standards basis and official sources

Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.

Similar Posts