Form 1, 2b, 3b and 4b Internal Separation Explained
What the IEC 61439 internal separation forms mean, how they differ and the criteria for choosing the right one.
ContentsInternal separation forms classify how far the inside of a low voltage switchboard is divided between busbars, functional units and outgoing terminals. IEC 61439-2 grades that division from Form 1 up to Form 4b. The form you choose directly determines whether one section can be worked on while another stays live, how far a fault can propagate, and how large and expensive the assembly becomes.
What does separation actually solve?
Internal separation serves three concrete purposes. The first is personnel safety: removing the risk of contact with live parts in the compartment being maintained. The second is fault containment: stopping an arc or short circuit that starts in one unit from spreading to neighbouring units and to the main busbar. The third is continuity of operation: keeping the rest of the installation energised while work is carried out on a single outgoing way.
If those three purposes are not considered together, the choice of form either creates unnecessary cost or fails to deliver the flexibility the operator expected. A distribution board in a warehouse that tolerates downtime does not need the same form as the main board of a production line running around the clock.
What the forms mean
| Form | Busbar separation | Separation between units | Terminal separation |
|---|---|---|---|
| Form 1 | None | None | None |
| Form 2a | Busbars separated | None | Terminals not separated |
| Form 2b | Busbars separated | None | Terminals separated from busbars |
| Form 3a | Busbars separated | Units separated | Terminals not separated from units |
| Form 3b | Busbars separated | Units separated | Terminals separated from units |
| Form 4a | Busbars separated | Units separated | Terminals in the same compartment as their unit |
| Form 4b | Busbars separated | Units separated | Each unit terminals in a separate compartment |
Which form for which project?
The choice of form is not a matter of prestige, it follows from the operating scenario. In practice four typical cases cover most projects.
- Form 1: small installations, single user distribution, applications tolerant to downtime; the most economical solution
- Form 2b: medium sized distribution boards where removing the risk of contact with live busbars is sufficient
- Form 3b: main boards feeding several sections where outgoing circuits must be maintained individually
- Form 4b: production plants, data centres and hospitals where downtime is expensive; the highest level, allowing each circuit to be isolated on its own
Our main distribution board guide covers how the form decision interacts with short circuit withstand and busbar sizing.
What separation looks like physically
Compartments are built from sheet metal, insulating barriers or a combination of both. One point is critical: the barrier has to remain continuous where cables and busbars pass through it. In a Form 4b board, an opening left around a busbar penetration effectively reduces that board to Form 3b. This is why cable entry grommets, busbar bushings and barrier joint details are an inseparable part of the design.
Separation also changes thermal behaviour. Every additional barrier restricts air circulation and makes it harder for busbars and devices to cool. As the form level rises, the temperature rise verification has to be revisited, and if necessary cross-sections increased or forced ventilation provided.
Key takeaways: internal separation forms
- What does separation actually solve — Internal separation serves three concrete purposes.
- Which form for which project — The choice of form is not a matter of prestige, it follows from the operating scenario.
- What separation looks like physically — Compartments are built from sheet metal, insulating barriers or a combination of both.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Is Form 4b always better?
No. Form 4b offers the highest level of maintenance safety, but it increases enclosure volume, weight, cost and internal heat load. If the operating scenario does not require isolating individual circuits, Form 3b is sufficient and more efficient in most installations.
Is the form level the same as the IP rating?
No. The IP rating describes protection against ingress of solid objects and water from outside, while the form describes separation between sections inside. A board can be IP54 and Form 1, or IP31 and Form 4b.
Does the form have to be documented?
Yes. The internal separation form is stated on the assembly label and in the technical file together with the rated characteristics. That information is the basis for maintenance procedures and permit to work practices.
Related articles
- Internal Arc Withstand and Arc Protection: What IEC 61641 Adds
- IP Protection Rating Explained: How to Read an IP Code
- IK Impact Rating Explained: IK07, IK08 and IK10 Compared
- All technical articles
At Devpan we set the form level together with the customer maintenance scenario, downtime tolerance and site conditions, and we assess the chosen form alongside temperature rise and short circuit verification. Share the operating profile of your facility and we can clarify which form is genuinely required.
