The form of internal separation is the single line in a low voltage specification that moves price, footprint and delivery together. It is also the line most often copied from the last project without anyone asking what it buys.
Forms of internal separation are defined in IEC 61439-2, the part covering power switchgear and controlgear assemblies. Edition 3 of that part, published in July 2020, clarified the requirements rather than changing the scheme. The scheme itself is simple once the two questions behind it are separated out.
The forms answer two questions, not one
Question one: are the busbars separated from the functional units, and are the functional units separated from each other? Question two: what happens to the terminals for external conductors? The number answers the first question and the letter answers the second.
| Form | What is separated |
|---|---|
| Form 1 | No internal separation. One enclosed volume. |
| Form 2a | Busbars separated from the functional units. Terminals not separated from the busbars. |
| Form 2b | Busbars separated from the functional units. Terminals separated from the busbars. |
| Form 3a | Busbars separated from the functional units, and the functional units separated from one another. Terminals not separated from one another. |
| Form 3b | As 3a, with the terminals separated from the busbars but still grouped. |
| Form 4a | Full separation, with each unit’s terminals in the same compartment as its own functional unit. |
| Form 4b | Full separation, with each unit’s terminals in a separate enclosed space of their own. |
Separation may be achieved with metallic or non-metallic partitions and barriers; the standard does not require steel. Edition 3 also made explicit what good practice already did: in any form above Form 1, parts inside a functional unit compartment that remain live when that unit is switched off must be protected to at least IPXXB — finger-safe to a standard test probe.
What each form lets somebody actually do
The forms exist for maintenance access, not for reliability. A Form 1 board and a Form 4b board of the same design have the same fault rate. What changes is what can be done to one circuit while the rest of the board stays energized.
| With the board live, you can… | From |
|---|---|
| Nothing inside the enclosure | Form 1 |
| Work in the functional unit area without exposed busbars | Form 2 |
| Work on one functional unit without entering another | Form 3 |
| Terminate or re-terminate one outgoing circuit without entering any other unit or the busbar zone | Form 4 |
| Do the same without the cable gland area of the adjacent circuit being open to you | Form 4b |
What a higher form costs
Three things, and the third is the one that surprises people.
- Space. More compartments means more panel width and often more depth. On a tight switchroom this is the constraint that decides the layout, not the current rating.
- Build time and price. Partitions, glands, separate terminal chambers and the extra copper to reach them. The jump from 3b to 4b is routinely in the order of a fifth of the panel cost, and it lengthens the build.
- Thermal performance. Compartmentalization restricts convection. The same devices in the same enclosure run hotter at Form 4b than at Form 2b, which means the verified temperature rise is reached at a lower current. A higher form can therefore reduce the usable rating of the board, or force a larger enclosure to recover it. This is a real effect, it is why temperature-rise verification is tied to the arrangement, and it is almost never priced into a specification that raises the form late in design.
What good specifications do
They set a default and allow it to be argued down. The IOGP JIP33 standardized specification for low voltage switchgear, S-560, takes this approach: a default form for ordinary functional units, a higher one for air circuit-breaker units, and the data sheets free to vary it per project. It also fixes the barrier integrity rather than leaving it implied — IP 2X between horizontal busbar compartments and other compartments, and IPXXB between a functional unit compartment and the rest.
That is the pattern worth copying. State the form, state the required degree of protection at the barriers, and state whether live working is permitted on site at all. Those three lines together produce a board that matches the way the site is run. The form alone does not.
Where we stand
We build to the form the specification sets, and we will tell you when the form you have asked for is not the one your operating procedure can use — before the order, not after the layout is frozen. Where a higher form reduces the usable rating of a board, the derating is shown in the proposal rather than discovered during temperature-rise verification. Arrangement drawings and the verification basis for the form supplied are issued against a named enquiry.