Forms of separation (Form 1 to 4b) of low-voltage switchgear assemblies — what the separation form actually regulates

The guide on IEC 61439 versus NEN 1010 briefly mentions the forms of separation (Form 1–4) as one of the aspects IEC 61439 verifies for a switchgear assembly. This article goes deeper into what that separation actually means, and why the distinction between the sub-variants (2a/2b, 3a/3b, 4a/4b) matters in practice when assessing or specifying an assembly.

The principle: internal separation between busbars, functional units and terminals

IEC 61439-2 (the product standard for low-voltage switchgear and controlgear assemblies) defines four forms of internal separation ("Form of separation") within an assembly, indicating how far the busbars, the functional units (for example an individual circuit breaker with its associated wiring) and the terminals for external cables are shielded from each other by fixed, not-easily-removable partitions.

The four main forms

Form 1 — no internal separation: busbars, terminals and all functional units are located in one common space without fixed partitions.
Form 2 — the busbars are separated from the functional units.
- 2a: the terminals for external conductors are not separately separated from the busbars.
- 2b: the terminals are separately separated from the busbars, providing a higher level of safety.
Form 3 — the busbars are separated from the functional units, and the functional units are separated from each other.
- 3a: the terminals are not separately separated from the busbars.
- 3b: the terminals are separately separated from the busbars.
Form 4 — the busbars are separated from the functional units, the functional units are separated from each other, and the terminals of each unit are also separated from those of the other units.
- 4a: the terminals of a unit are located in the same compartment as that functional unit itself.
- 4b: the terminals of a unit are located in their own, separate compartment, separated both from the functional unit and from the terminals of other units.

Why this is mainly about working safely on one unit

The practical purpose of a higher separation form isn't primarily to prevent a fault in itself, but to limit the risk for a person working on one specific functional unit (for example replacing a circuit breaker) while the rest of the assembly — the busbars and the other units — remains live. The higher the form, the lower the chance that this person can inadvertently touch a live part of another unit or of the busbars when opening one compartment. Form 4b offers the most extensive protection in this respect: even the external cabling of an adjacent unit stays shielded when one specific compartment is opened.

Note: the exact construction requirements (material thickness, IP rating of the partitions, test method) per form are set out in the full text of IEC 61439-2; this article covers the underlying classification principle, not the complete verification procedure.

A property of the assembly, not of the installation

Like the other IEC 61439 requirements (temperature rise, short-circuit withstand, IP rating), the separation form is a property that the manufacturer or assembler of the switchgear assembly itself specifies and demonstrates — not a requirement that follows directly from NEN 1010. NEN 1010 does determine, through its general requirements for maintainability and safe working, whether a chosen form is appropriate for how the installation will actually be maintained or extended in practice (see also the guide on IEC 61439 versus NEN 1010 for this distinction between product standard and installation standard).

Practical relevance

When specifying a new switchgear assembly — for example for an industrial or horticultural installation where individual outgoing circuits are regularly worked on while the rest must stay in service — the chosen separation form directly determines how safely such maintenance can be carried out without having to de-energise the entire assembly. During a NEN 3140 inspection of an existing assembly, it is useful to know the specified form from the rating plate or test report, to assess what level of protection a technician has when opening an individual compartment.

Common mistakes

Confusing a higher separation form with a higher protection rating (IP classification) — both are separate properties of the assembly: IP describes protection against dust and moisture from outside, the separation form describes the internal separation between components.
Choosing Form 4 without weighing the extra cost and space against the actual maintenance need — for an assembly that is rarely opened while live, a lower form may well suffice.
Ignoring the 2a/2b or 3a/3b sub-variant when comparing two quotes — two assemblies with "the same form" (for example both "Form 3") can still offer a materially different level of protection depending on the a/b sub-variant.
Assuming the separation form also says something about the short-circuit withstand or temperature rise of the assembly — these are separate, independently verified properties within IEC 61439-2.