Form of separation describes the extent to which the functional units inside an electrical switchboard are separated from one another. Under the IEC 61439 standard it is classified from Form 1 to Form 4. Barriers placed between compartments prevent a fault in one area from spreading to neighboring areas and stop personnel from touching live parts during maintenance and intervention.
What Is Form of Separation?
In low voltage distribution switchboards, the busbar system, incoming and outgoing functional units and connection terminals are housed in the same enclosure. Form separation means dividing these elements into independent compartments using metal or insulating barriers. The higher the degree of separation, the lower the likelihood of a fault spreading, the lower the risk to technical staff working on the switchboard, and the better the operating sections of the facility are protected.Fault Containment
The effect of a short circuit or arc event remains limited to the compartment where it occurs.Personnel Safety
The risk of contact with live busbars is eliminated and working safety increases.Maintenance Without Shutdown
While work is carried out on one outgoing feeder, the other supply circuits can remain in service.Easy Extension
There is no need to shut down the entire system when adding a new functional unit.Form Types
IEC 61439 places the internal separation levels of distribution switchboards within a common framework. The higher the form number, the deeper the separation between compartments; the a and b subclasses indicate the type of barrier used to achieve it.Form 1
The most basic configuration. All components inside the switchboard share a single common space with no barriers or partitions between them. It is chosen for simple, low-cost applications.Form 2
Supply circuits are separated from switching and control circuits, so the busbar area and the functional units become independent of each other.- Form 2a: Components are separated only by a physical barrier.
- Form 2b: An insulating separator is used in addition to the physical barrier.
Form 3
Each circuit or functional unit is placed in its own compartment. This provides significantly higher isolation inside the switchboard and is mostly used in complex distribution systems.- Form 3a: Circuits are separated only by physical barriers.
- Form 3b: Circuits are separated by both physical and insulating barriers.
Form 4
The most comprehensive configuration. Components are placed in fully independent compartments, achieving the highest possible level of isolation and protection.- Form 4a: Each terminal is located in its own separate compartment.
- Form 4b: In addition to the terminals, each power and control circuit has its own isolated compartment.
Form Types Comparison Table
| Form | Separation | Subclasses | Key Benefit |
|---|---|---|---|
| Form 1 | No separation, one common space | – | Simple construction, lowest cost |
| Form 2 | Busbars separated from functional units | 2a · 2b | Safe working in the busbar area |
| Form 3 | Each functional unit in its own compartment | 3a · 3b | Faults contained at circuit level |
| Form 4 | Units and terminals in fully separate compartments | 4a · 4b | Highest isolation and continuity of service |
The level of separation is determined by manufacturing tolerances even before the project.
In Forms 2, 3 and 4, the barriers, mounting plates and covers that provide internal separation are produced by sheet metal fabrication; any deviation in bending dimensions directly affects how well a barrier fits into the enclosure.If a barrier does not fit properly, the expected level of separation is not achieved on site, even if the project specifies Form 4. That is why sheet metal parts are dimensionally checked before assembly.Cable entry areas, ventilation cut-outs and barrier openings are planned at the same fabrication stage.Barriers, covers and mounting plates are entered into the cutting list together with the enclosure dimensions from the project; because the parts are numbered, there is no confusion during assembly about which part belongs to which compartment.Summary
- Form 1: No separation; all components are in one common space.
- Form 2: Busbars are separated from switching and control circuits (2a / 2b).
- Form 3: Each functional unit is in its own compartment (3a / 3b).
- Form 4: Units and terminals are in fully separate compartments (4a / 4b); the highest level of isolation.
- Subclass a refers to a physical barrier only, subclass b to a physical plus insulating barrier.
Frequently Asked Questions
What is form of separation?
Form of separation is the degree to which the busbar system, functional units and connection terminals inside a low voltage distribution switchboard are separated from one another by metal or insulating barriers. IEC 61439 classifies this separation from Form 1 to Form 4.
How many form types does IEC 61439 define?
The standard defines four main form types: Form 1, Form 2, Form 3 and Form 4. Forms 2, 3 and 4 are further divided into two subclasses, a and b, giving a total of seven different separation levels in practice.
What is the difference between Form 2a and Form 2b?
In Form 2a, components are separated only by a physical barrier. In Form 2b, an insulating separator is used in addition to the physical barrier, providing a higher level of protection.
What is the main difference between Form 3 and Form 4?
In Form 3, each functional unit has its own compartment, but the terminals may share a common space. In Form 4, the terminals are also placed in separate compartments; in Form 4b, each power and control circuit also has its own isolated compartment.
Which form type should be chosen?
The choice depends on the facility’s need for continuity of service, maintenance frequency and safety expectations. Form 4 is commonly chosen for critical facilities that cannot tolerate interruptions, while Form 2 or Form 3 is widely used in standard distribution applications.
Does a higher form type increase cost?
Yes. As the level of separation increases, more barriers, partitions and labor are needed, so the switchboard costs more. On the other hand, downtime, maintenance risk and production losses decrease, so the total cost of ownership is often lower.
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