Switchboard - Forms of Internal Separation 

By Steven McFadyen

IEC 61439 'Low-voltage switchgear and controlgear assemblies', specifies standard arrangements of switchboard (call forms of internal separation).

The are labelled as Form 1, Form 2, Form 3 and Form 4. Forms 2, 3 and 4 are further broken down into Form 2a, 2b, 3a, 3b, 4a and 4b.

Each Form relates to the internal separation of the busbars, functional units and terminals, each being defined as:

Busbar - low impedance conductor to which several electric circuits can be connected

Main busbar - busbar to which one or more distribution busbar, incoming unit or outgoing unit can be connected

Distribution busbar - busbar in one section which is connected to the main busbar from which incoming or outgoing units can be connected

Functional Unit - part of the assembly comprising the electrical and mechanical elements that contribute to the fulfilment of the same function

Incoming unit - functional unit which feeds energy into the assembly

Outgoing unit - functional unit supplying energy to the outgoing circuits

Terminals - part of the assembly which provide for connection of incoming and outgoing cable and busbar

Internal separation is achieved by the use of barriers or partitions (including metallic or non-metallic), insulation of live parts or an integral housing (i.e. moulded case circuit breaker). The internal barriers should provide protection against contact to IP XXB and against the ingress of foreign bodies to at least IP 2X.

Form 1 has no internal separation between , while Form 4b has the greatest with busbars, functional units and terminals all being separated (see illustration):

Form 1 - no internal separation of the busbars, functional units and terminals from each other.

Form 2a - separation of the busbars from the functional units. Terminals are not separated from the busbars.

Form 2b - as for 2a, but with the terminals are not separated from the functional units.

Form 3a - separation of the busbars from the functional units and each functional unit from the other units. Terminals of each functional unit are not separated from each other. Terminals are not separate from the busbars.

Form 3b - as per 3a, but with the terminals separated from the busbar (and functional units)

Form 4a - separation of the busbars from the functional units and each functional unit from the other units. Separation of the terminals for a functional unit from the busbars and those of any other unit. Terminals enclosed in the same compartment as the functional unit.

Form 4b - as per 4a, but with the terminals for each functional unit enclosed in their own space.

BS EN National Annex

The United Kingdom implementation of the IEC standard (in BS EN 61439-2) further defines the separation of busbars and cables by the use of Types.  These have been widely adopted within and outside the United Kingdom.