Low-Voltage Switchboard Forms of Separation: Engineering and Safety Considerations

A low-voltage switchboard may be called “Form 2b”, “Form 3b” or “Form 4b” in a specification. The label describes how the inside of the assembly is divided by barriers or partitions, and therefore how far people are protected against contact with live parts of adjacent units, and faults kept from spreading, when one part is isolated and the rest stays live. It affects cost, size and heat dissipation as well as safety and maintainability, yet it is often chosen by habit rather than by thinking about how the switchboard will be operated.

This article answers the question: What do Forms 1 to 4 of internal separation mean under IEC 61439-2, and how should a project choose and specify the right one?

What “form of separation” means

IEC 61439-2 defines a form of internal separation as a classification of the physical separation within a PSC-assembly. Separation is achieved by barriers or partitions, which may be metallic or non-metallic, or by the integral housings of devices. The aims are:

• protection against contact with hazardous live parts in adjacent functional units, with a degree of protection of at least IPXXB; and
• protection against the passage of solid foreign objects from one unit to another, with a degree of protection of at least IP2X (which also covers IPXXB).

The standard presents the forms as typical arrangements in a table, with illustrations in an informative annex. The form of separation, and any higher degrees of protection, are matters for agreement between the assembly manufacturer and the user.

The 2020 edition of IEC 61439-2 added a requirement for forms higher than Form 1: parts in a functional-unit compartment that remain live when that unit is switched off must be protected to at least IPXXB. This covers, for example, incoming terminals of a switching device that are fed from the busbar.

The three parts that are separated

• Busbars include the main and distribution busbars that carry power to the functional units.
• Functional units are the parts of the assembly that perform one function, typically an outgoing circuit with its switching and protective device.
• Terminals for external conductors are where the outgoing or incoming cables connect.

The forms describe which of these are separated from which.

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Forms 1 to 4 explained

In Forms 3 and 4 the terminals are also separated from the functional units of other circuits. In Form 3 the terminals of different circuits share space; in Form 4 each circuit’s terminals are separated from those of other circuits.

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Form 1

All parts can be within one enclosure. To work safely inside, the assembly must normally be isolated upstream. Form 1 is common in small distribution boards where the whole board can be switched off for work.

Form 2

The busbars are separated from the functional units, and in Form 2b the terminals are also separated from the busbars. The functional units are not separated from each other. In practice, Form 2 gives limited benefit for work on one circuit, because adjacent circuits remain exposed. Where internal access is needed, the assembly will usually still be isolated.

Form 3

Each functional unit is in its own compartment, separated from the busbars and from other functional units. Form 3 can reduce exposure to adjacent live parts when one functional unit is isolated for work, such as fault finding or replacing fuse-links, while others remain in service. The unit being worked on must itself be isolated, proved dead and locked off under the site’s safe system of work. Because the terminals of different circuits are not separated from each other, connecting or disconnecting the cable of one circuit may still expose adjacent live terminals.

Form 4

Form 4 adds separation of each circuit’s terminals from those of other circuits. In Form 4a the terminals are in the same compartment as their functional unit. In Form 4b they are in a separate compartment or enclosed space. Form 4 is chosen where the cables of an isolated circuit must be connected or disconnected while adjacent circuits remain in service, for example in main switchboards and motor control centres in facilities that cannot easily shut down.

Choosing a form

IEC 61439-2 indicates that the most appropriate form should be determined by the operations to be carried out when the assembly is energised and in service, but with individual circuits de-energised. A useful approach is to list those operations and the people who will perform them:

• Operating devices from outside the assembly, such as switching, resetting and reading indications. This needs no internal access and is unaffected by the form.
• Inspection inside the compartment of an isolated unit while others remain in service.
• Fault finding and replacement of a device in one isolated functional unit.
• Connecting or disconnecting outgoing cables of one isolated circuit while others remain in service.
• Adding or modifying circuits during the life of the installation.

The more of these must be done without shutting the whole assembly down, the higher the form usually needed. If the whole switchboard can be isolated for any internal work, a lower form may be adequate.

Safety note. A form of separation protects against contact with live parts of adjacent functional units and helps limit the spread of faults between them. It does not by itself make work on or near energised equipment safe. Any work requires isolation, proving dead and lock-out/tag-out under the applicable rules, for example EN 50110-1 and national equivalents in Europe or NFPA 70E in North America, together with a risk assessment, competent persons formally nominated for the task and suitable PPE. Internal arc performance is a separate consideration (IEC TR 61641).

Other factors to weigh:

• Continuity of service. For critical facilities, the cost of shutting down the whole board may justify a higher form.
• Competence and procedures. A higher form reduces exposure to adjacent live parts, but any work on or near energised equipment remains subject to the electrical safety rules and safe systems of work described in the safety note above.
• Size, heat and cost. Barriers and partitions take space and restrict air flow. A higher form usually means a larger assembly and may affect the temperature-rise performance and rating of devices.
• Cable termination arrangements. The way cables enter and are glanded affects whether a given form is practical.
• Mixed forms. One assembly can use more than one form. For example, an MCB distribution section may be treated as one functional unit inside a Form 4 board.

What forms of separation do not cover

• Withdrawable or plug-in units. IEC 61439 treats fixed, removable and withdrawable parts separately. A Form 4 board can have fixed devices, and a withdrawable board does not imply a particular form.
• Enclosure degree of protection. The IP rating of the enclosure to IEC 60529 is a separate requirement.
• Internal arc performance. Testing of assemblies under arcing conditions due to internal fault is addressed in IEC TR 61641, not by the form.
• Short-circuit and temperature-rise performance. These are verified through the design verification process in IEC 61439-1 and -2. They should be specified independently.

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UK practice: national annex types

In the UK, BS EN IEC 61439-2 includes a national annex that defines constructional types within each form. The types describe how busbar separation is achieved, for example by insulated coverings or rigid barriers, and how cables are glanded, for example in a common cabling chamber or with integral glanding for each functional unit. These types are UK practice and are not part of the IEC standard. Specifications for other markets should describe the required arrangement directly rather than cite a UK type number.

Verification and documentation

The form of separation is part of the assembly’s design, so it should be covered by the assembly manufacturer’s design verification under IEC 61439 and confirmed in the documentation supplied with the assembly. Useful evidence for the buyer includes:

• the declared form of separation, and sub-form, for each section of the assembly;
• general arrangement drawings showing barriers, partitions and the location of cable terminals;
• confirmation of the degree of protection achieved by barriers and partitions, including any openings, which must still meet the minimum IP2X requirement;
• the routine verification records for the assembly as built.

Any later modification, such as adding a functional unit or cutting a barrier to pass a cable, can reduce the form achieved. Record such changes and have them assessed against the original design.

IEC and ANSI/IEEE practice; 50 and 60 Hz

The forms of separation concept belongs to the IEC 61439 framework. North American LV switchboards and LV switchgear are built to different national and IEEE standards (for example IEEE C37.20.1 for metal-enclosed LV power circuit breaker switchgear), which describe compartmentalisation differently. Form numbers should not be assumed to map onto those constructions. The form of separation does not depend on rated frequency, but the assembly’s rated frequency, 50 Hz or 60 Hz, must still be declared.

Specification and procurement considerations

For an LV switchboard, state:

• the required form of separation, including the sub-form (a or b), for each section if forms differ;
• the operations expected while the assembly is energised, to explain why the form was chosen;
• the means of separation if it matters to the user, for example rigid barriers rather than insulated coverings;
• the cable termination and glanding arrangements;
• whether functional units are fixed, plug-in or withdrawable;
• the enclosure degree of protection and any higher internal degree of protection;
• any internal arc performance requirement, with the test reference;
• rated voltage, frequency, rated currents, short-circuit ratings and the ambient temperature;
• the design verification evidence and routine verification required under IEC 61439.

For where the switchboard sits in the system, see LV Distribution: From Transformer to Final Circuit. For the devices inside it, see Choosing LV Circuit Breakers: MCB, MCCB and ACB. For general guidance on wording, see Writing a Clear Electrical Equipment Specification.

Common specification mistakes

• Stating “Form 4” without a or b. The sub-form determines where the terminals are and how they are separated.
• Choosing the highest form by default. Without a reason, a higher form adds cost, size and heat without benefit.
• Assuming a higher form permits live working. It reduces exposure to adjacent live parts, but the circuit being worked on must still be isolated, proved dead and locked off under the applicable safety rules.
• Confusing form with withdrawability. Specify both separately.
• Confusing form with arc containment. Internal arc performance needs its own requirement.
• Citing UK national annex types for projects outside the UK. Describe the arrangement instead.
• Ignoring the effect on rating. Barriers can affect temperature rise and therefore device ratings.

Conclusion

Forms of internal separation describe which parts of an LV assembly are separated from each other, and so how well adjacent live parts are protected when one isolated part is accessed while others remain in service. Choose the form from the operations that must be carried out with the assembly in service, then specify the sub-form, termination arrangements and related requirements explicitly. This gives a switchboard that is no larger or more expensive than it needs to be, and safe for how it will actually be used.

Related AdepaTrade solutions

AdepaTrade sources LV distribution and control equipment, including switchboards, distribution boards, motor control centres and circuit-breakers, to project specifications. Low Voltage Distribution & Control · Critical & Backup Power

Relevant standards

IEC standards are the primary reference. Confirm the applicable edition for each project; national rules on electrical safety at work also apply.