How to choose a form of separation for LV switchboard and how to classify them

Forms of internal separation

By form of separation it is meant the type of subdivision provided within the switchboard. Separation by means of barriers or partitions (metallic or insulating) may have several functions.

And these functions:

1. Provide protection against direct contact (at least IPXXB) in the case of access to a part of the switchboard which is not live, with respect to the rest of the switchboard which remains live;
2. Reduce the risk of starting or propagating an internal arc;
3. Impede the passage of solid bodies between different parts of the switchboard (degree of protection of at least IP2X).

A partition is a separation element between two parts, while a barrier protects the operator from direct contact and from arcing effects from any interruption devices in the normal access direction.

The following table from Standard IEC 61439-1-2 highlights typical forms of separation which can be obtained using barriers or partitions:

Symbols

Where:

1. Housing
2. Internal segregation
3. Functional units including the terminals for the associated external conductors
4. Busbars, including the distribution busbars

Form 1

(no internal segregation)

Form 2

(segregation of the busbars from the functional units)

Form 2a

Terminals not separated from the busbars

Form 2b

Terminals separated from the busbars

Form 3

(separation of the busbars from the functional units + separation of the functional units from each other)

Form 3a

Terminals not separated from the busbars

Form 3b

Terminals separated from the busbars

Form 4

((separation of the busbars from the functional units + separation of the functional units from each other + separation of the terminals from each other)

Form 4a

Terminals in the same compartment as the associated functional unit

Form 4b

Terminals in the same compartment as the associated functional unit

Form 2B Segregations assembling (VIDEO)

Animation about segregation form 3b 4b (VIDEO)

Classification of LV switchgears

Different classifications of electrical switchboard exist, depending on a range of factors. Based on construction type, Standard IEC 61439-1 firstly distinguishes between open and enclosed assemblies.

Open switchboards, with or without front covering, which have the live parts accessible. These switchboards may only be used in electrical plants.

External Design

With regard to external design, switchboards are divided into the following categories:

1. Cubicle-type assembly

Used for large scale control and distribution equipment. Multi-cubicle-type assembly can be obtained by placing cubicles side by side.

2. Desk-type assembly

Used for the control of machinery or complex systems in the mechanical, iron and steel, and chemical industries.

3. Box-type assembly

Characterized by wall mounting, either mounted on a wall or flush-fitting. These switchboards are generally used for distribution at department or zone level in industrial environments and in the tertiary sector.

4. Multi-box-type assembly

Each box, generally protected and flanged, contains a functional unit which may be an automatic circuit-breaker, a starter, a socket complete with locking switch or circuit-breaker.

Intended function

With regard to the intended function, switchboards may be divided into the following types:

1. Main distribution boards

Main distribution boards are generally installed immediately downstream of MV/ LV transformers, or of generators; they are also termed power centres.

Main distribution boards comprise one or more incoming units, busbar connectors, and a relatively smaller number of output units.

2. Secondary distribution boards

Secondary distribution boards include a wide range of switchboards for the distribution of power, and are equipped with a single input unit and numerous output units.

3. Motor operation boards

Motor control boards are designed for the control and centralised protection of motors. Therefore they comprise the relative coordinated devices for operation and protection, and auxiliary control and signalling devices.

4. Control, measurement and protection boards

Control, measurement and protection boards generally consist of desks containing mainly equipment for the control, monitoring and measurement of industrial processes and systems.

5. Machine-side boards

Machine-side boards are functionally similar to the above. Their role is to provide an interface between the machine with the power supply and the operator.

6. Assemblies for construction sites (ASC)

Assemblies for construction sites may be of different sizes, from a simple plug and socket assembly to true distribution boards with enclosures of metal or insulating material. They are generally mobile or, in any case, transportable.

Reference // Electrical installation handbook – Protection, control and electrical devices by ABB