Form of separation decides which live parts a technician can be near when one feeder is opened and the rest of the board stays energised. This guide shows what each form separates, how to choose one and what it does to panel size and cost.
Internal separation inside an LV switchboard protects people from adjacent live parts and helps keep faults inside one compartment. Learn every form, the type subdivisions, how to specify the right one and how it changes the board.
What Are IEC 61439 Forms of Separation?
IEC 61439 forms of separation are the standard ways of dividing a low voltage switchboard internally, using barriers or partitions between the busbars, the functional units and the terminals for outgoing cables. They run from Form 1 with no internal separation up to Form 4b, where every functional unit and every set of terminals sits in its own compartment.
- Form 1 has no internal separation at all
- Form 2 separates busbars from functional units
- Form 3 also separates functional units from each other
- Form 4 also separates the outgoing terminals of each unit from each other
- Letter a or b tells you whether terminals are separated from the busbars or have their own compartment
The forms are defined in IEC 61439 2, the part that covers power switchgear and controlgear assemblies such as a PCC panel or a motor control centre. The form is not a default value, so the user has to state it in the specification and agree it with the panel builder.
Separation is built with metal or insulating barriers and partitions, shrouds, or insulated coverings on the busbars. According to the BEAMA guide, any barrier used for separation must give at least IP2X, which also covers protection against finger contact, so check the meaning of each digit in our IP rating explained article.
The Three Parts That Get Separated
Main and distribution busbars that carry power along the board to every section.
The breaker, fuse switch or starter that controls one circuit, with its auxiliaries.
Where the outgoing cable of each circuit is landed and glanded.
The BEAMA guide states that separation does not improve the electrical performance of an assembly. Its job is to protect against contact with adjacent live parts and to stop solid foreign bodies passing from one unit to the next.
IEC 61439 Forms of Separation Table: Form 1 to Form 4b
| Form | Busbars vs Units | Unit vs Unit | Terminals vs Busbars | Terminal vs Terminal |
|---|---|---|---|---|
| Form 1 | No | No | No | No |
| Form 2a | Yes | No | No | No |
| Form 2b | Yes | No | Yes | No |
| Form 3a | Yes | Yes | No | No |
| Form 3b | Yes | Yes | Yes | No |
| Form 4a | Yes | Yes | Yes, in unit compartment | Yes |
| Form 4b | Yes | Yes | Yes, own compartment | Yes |
Each row of iec 61439 forms of separation adds one more barrier. In Form 4a the terminals sit inside the same compartment as their own functional unit, while in Form 4b they sit in a separate compartment, often in a cable chamber at the side of the section.
The real difference is what stays live while you work. In Form 1 the whole assembly has to be isolated upstream, while in Form 4b a competent person can work on one feeder and its cable with the neighbouring circuits in service, subject to a risk assessment.
Form 1, Form 2, Form 3 and Form 4 in Detail
Form 3b is common in industrial distribution boards, while Form 4b suits large main boards and data centres where a full outage costs far more than the extra steel.
When a datasheet only says Form 4, ask whether it is 4a or 4b and which type. Two Form 4 boards from different makers can look very different inside, and the cable chamber width alone can change your room layout.
Form 4b Type 6 and the BEAMA Types 1 to 7
IEC 61439 2 stops at the letters a and b, but the UK BEAMA guide written to BS EN 61439 2 adds type numbers that say how the separation is built. These types sit on top of the iec 61439 forms of separation, not in place of them.
| Type | Applies To | How Separation Is Made |
|---|---|---|
| Type 1 | Form 2, 3 and 4a | Insulated coverings on busbars, such as sleeving or coating |
| Type 2 | Form 2, 3 and 4a | Metal or insulating partitions and barriers for busbars |
| Type 3 | Form 4a | Partitions for busbars plus integral glanding for each circuit |
| Type 4 | Form 4b | Insulated busbars, terminals in separate compartments |
| Type 5 | Form 4b | Busbar partitions, terminals separated by insulated coverings |
| Type 6 | Form 4b | Partitions and barriers for busbars and terminals |
| Type 7 | Form 4b | Partitions for busbars and extended terminals, with integral glanding |
BEAMA notes that Form 4 Type 3 or Type 7 will almost certainly increase the size of the assembly. Barriers can also restrict airflow, so the panel heat load calculation and the temperature rise verification must be done on the separated design, not on a Form 1 version of it.
Why Forms of Separation Matter for Safety and Maintenance
Internal separation also lowers the chance that an internal arc spreads, but it is not the same as an arc tested board. Arc fault performance is verified separately under IEC TR 61641, so read our guide on what is arc flash and do not treat a high form as arc proof.
BEAMA is clear that specifying a form does not by itself make live working safe. The safe route is still isolation where reasonable, with an isolation and permit to work system and suitable PPE.
How to Select the Right Form of Separation
| Application | Typical Form | Reason |
|---|---|---|
| Small office or shop distribution board | Form 1 or 2a | Low fault level, short outages are acceptable |
| Industrial sub distribution board | Form 2b or 3b | Feeder work without exposing busbars |
| Motor control centre | Form 3b or 4a | Starters change often, cables stay landed |
| Main LV switchboard of a plant | Form 4a or 4b | Outage of the whole board is costly |
| Hospital, data centre, continuous process | Form 4b | Maximum continuity and safe cable work |
These are typical choices, not rules in the standard. BEAMA lists the questions that really drive the choice, such as which tasks will be done live, which tools are used, the skill of the people and whether withdrawable units are needed for continuity of the distribution system.
Fault level matters too, since high prospective currents raise the stakes of any exposure. Check the busbar sizing and the breaker ratings at the same time, as described in our guide on how to select the right MCCB rating.
- Work on one feeder while others run
- Better protection against touching live busbars
- Fault and foreign object less likely to spread
- Cleaner cable chambers for termination
- Bigger footprint and more sections
- More steel, barriers and labour
- Restricted airflow and possible derating
- Longer delivery and harder modifications later
Cost Impact and Panel Size
BEAMA states that higher forms generally mean larger, more complex and more expensive assemblies, and it advises specifying the lowest level that meets the real need. A good way to see the size effect is to estimate how many vertical sections the outgoing feeders need.
Feeders = 24, module height = 200 mm
Usable height per section = 1800 mm
Modules per section = 1800 ÷ 200 = 9
Sections = 24 ÷ 9 = 2.67, round up to 3
Length = 3 × 600 + 800 incomer = 2600 mm
Use real catalogue values from your panel maker, because module heights differ between makers. Comparing two iec 61439 forms of separation this way shows the extra room length you pay for.
BEAMA members generally arrange fuse links so that competent people can replace them without isolating the whole assembly.
Before a factory acceptance test, walk through our electrical safety audit checklist to confirm barriers, labels and covers match the agreed form. A missing shroud found at site is far cheaper to fix before the board is energised.
Specification Checklist for IEC 61439 Assemblies
- Form of separation stated with letter, for example Form 3b, not just Form 3
- Type of construction agreed if a BEAMA type is required
- Barrier IP degree stated, at least IP2X
- Terminal location and cable entry agreed for each feeder
- Withdrawable or fixed units decided
- Temperature rise verified with barriers fitted
- Arc fault test under IEC TR 61641 stated separately if needed
- Neutral and earth bar access reviewed for the chosen form
The protective circuit inside the board is part of the same review. Our article on equipotential bonding shows why every door, barrier and gland plate should be bonded, which is also explained in this guide to electrical earthing basics.
At the FAT, open one feeder door at a time and look for any gap where a test finger could reach the busbar or a neighbouring terminal. Record the result per section, since barriers are often left out on the last section built.
The BEAMA guide below is the most quoted reference on iec 61439 forms of separation and their types, so keep it with your specification.
The short video below walks through internal separation forms in low voltage switchgear with drawings.
IEC 61439 Forms of Separation FAQ
The iec 61439 forms of separation are Form 1, Form 2a, Form 2b, Form 3a, Form 3b, Form 4a and Form 4b, defined in IEC 61439 2. Each higher form adds barriers between the busbars, the functional units and the outgoing terminals.
Form 1 has no internal separation, while Form 4b separates everything into its own compartment. The letter tells you where the outgoing terminals are placed compared with the busbars.
In Form 3b each functional unit has its own compartment and the terminals are separated from the busbars. The terminals of different circuits can still share one cable zone.
Form 4b goes one step further and separates the terminals of each circuit from each other. This lets a technician terminate one cable without being close to the live terminals of the next feeder.
Form 4b Type 6 is a construction defined in the BEAMA guide to BS EN 61439 2 rather than in the IEC standard itself. It uses partitions and barriers for the busbars and for the terminals.
The terminals sit in separate compartments outside the functional unit, usually in a cable chamber. Many main switchboards for critical buildings are specified as Form 4b Type 6 for this reason.
No single form is best for every board, because a higher form costs more and takes more space. BEAMA advises choosing the lowest form that meets the real needs of the site.
A small shop board may work well as Form 1 or Form 2a. A plant main board, hospital or data centre usually justifies Form 4a or Form 4b for continuity and safer cable work.
A higher form reduces the chance of touching adjacent live parts, but it does not make live work automatically safe. BEAMA states that specifying a form is not a guarantee for live working.
Isolation should be used wherever it is reasonable, and any remaining live work needs a risk assessment. Competent staff, a permit system, suitable tools and PPE are still required.
The BEAMA guide states that barriers used for separation must give at least IP2X, verified to BS EN 60529. This degree also covers protection against finger contact with hazardous parts.
Holes in barriers for cables or ventilation must still pass the same test. If tools or loose parts could enter energised units during expected work, discuss extra protection with the panel maker.
Related Articles
- Motor Control Center MCC Explained
- PCC Panel Power Control Centre
- Busbar Sizing Calculation Guide
- Panel Heat Load Calculation
- What Is Arc Flash
External References
- BEAMA: Guide to Forms of Separation to BS EN 61439 2
- Cleantech Controls: Forms of Segregation in Switchboards
- Wikipedia: Switchgear
What We Learn Today
- IEC 61439 forms of separation run from Form 1 with no barriers to Form 4b, where every unit and every set of outgoing terminals is separated.
- The letter a or b shows where terminals sit, and the BEAMA types 1 to 7 describe whether insulated coverings or metal partitions are used.
- Choose the lowest form that meets real maintenance and continuity needs, because higher forms add size, cost and heat, and never replace safe isolation.