Forms of Segregation: Form 1, 2A, 2B, 3A, 3B, 4A, 4B

Forms of segregation have great importance in electrical switchboard designs. Definitions of them should be known by all electrical professionals. When I was working as a project engineer in a panel manufacturer, their definitions of them helped me a lot in my project designs.

Let me explain them to you too.

The form of segregation is a regulation used for separating an assembly to facilitate access to a part of the assembly whilst other parts may remain energized and in service.

The form of segregation provides protection against contact with live parts belonging to adjacent functional units. It also provides protection against the passage of solid foreign bodies from one unit of an assembly to an adjacent unit.

The type of form will be determined according to the qualification of those involved, the protection required, and the required level of maintainability.

The use of forms enables the panel to be divided into closed protected spaces to achieve four objectives:

• Protection against direct contact with dangerous parts of neighboring functional units (the degree of protection must be at least IP xxB)
• Protection against the entry of solid objects. The degree of protection must be at least IP2x (degree IP 2x covers IP xxB). These two requirements assume that the assembly is equipped with faceplates.
• Limitation of the effects of the spread of electric arcs.
• Facilitation of panel maintenance operations.

Standard EN 60439-1 defines the internal separation of assemblies into 7 types of form (1, 2a, 2b, 3a, 3b,  4a, and 4b)

Forms of segregation

Form 1

Form 1 does not require any separation between the components inside the enclosure.

Forms 2a and 2b

Form 2a is the simplest for protecting against accidental contact with the busbars, which are considered to be the most dangerous components. Form 2b includes additional separation to make it safe to work on outgoing lines.

Forms 3a and 3b

In form 3a, each device is isolated in a compartment that protects it from the effects of incidents that may occur on another device. Form 3b combines the advantages of Form 3a and Form 2b, separating the output terminals and the busbars. Form 3a does not cover completely form 2b. The difference relates to the terminals for external conductors separated from the busbar in Form 2b and necessary in Form 3a.

Forms 4a and 4b

The requirements of form level 4a further increase the safety of working on outgoing lines by isolating the output terminals in the same compartment as the device. Form 4b provides maximum safety by separating all the functions from one another. Form 4a does not cover completely Form 3b since solutions for connecting external conductors are different. They are associated with the functional unit in Form 4a while being deported in Form 3b.