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Switchboard Forms of Segregation Explained

Do you feel out of your depth when people start talking about forms of segregation, such as Form 3B or Form 4aih?



To put it simply, these are the methods for internally separating the functional units and busbars of an Assembly, as specified by AS/NZS 61439.


The 4 main categories outlined by the standard are:

Form 1

Form 2

Form 3

Form 4

The complexity of the forms increases with the numbers.


Key definitions


Assembly

Refers to a switchboard which contains low-voltage control and switchgear.


Functional unit

Part of an Assembly comprising all the electrical and mechanical elements including switching devices that contribute to the fulfilment of the same function e.g. a motor starter.


Busbar

Refers to a low-impedance conductor to which several electric circuits can be separately connected. The term "busbar" does not presuppose the geometrical shape, size, or dimensions of the conductor.


Symbols Used













Form 1

A switchboard with no internal separation. Busbars, terminals for outgoing cables and all functional units are installed on a single central section. Form 1 segregation is suitable for smaller, lower power switchboards.












Form 2

The busbars and functional units are separated and broken down into another 2 categories – Form 2a and Form 2b

  • Form 2a – External cabling terminals are not separated from the busbar.












  • Form 2b – External cabling terminals are separated from the busbar.












There are several advantages to segregating functional units and busbars, not least of which is safety. This model allows circuit breakers to be reset when the switchboard is live because the operator is not exposed to a live busbar.


Form 3

Things start to get more complicated from here. As with Form 2, busbars and functional units are segregated. Functional units are also separated from each other in cubicles, and terminals are then separated from functional units, but they are not segregated from other functional units’ terminals.


This is also broken down into 2 categories:

  • Form 3a – External cabling terminals are not segregated from busbars.











  • Form 3b – External cabling terminals are separated from busbars.












Form 3b is frequently required for MCCs. It basically means that busbars and each motor starter are installed within their own cubicles. The advantages include safety, ease of maintenance and reliability because it’s possible to isolate and perform maintenance on each starter without having to power down the whole switchboard. Serious faults within a starter are also more likely to be contained within a cubicle meaning adjacent starters are unaffected and can operate normally.


All these advantages come at a cost as a Form 3 board is significantly bigger and more expensive than a Form 1 or 2 board.


Form 3 segregation is typically used for mining projects and larger operations that have a substantial number of loads, motors and critical processes. They are utilsed when safety, reliability and limited downtime are crucial.


Form 4

This is the highest form rating, as specified by AS/NZS 61439.1. With Form 4, busbars are separated from functional units and external cabling terminals. Functional units are further separated from each other, and from the external cabling terminals for each functional unit.

As with the other forms, this is divided into 2 sub-categories:

  • Form 4a – External cabling terminals are within the same cubicle as the corresponding functional unit.












  • Form 4b – The external cabling terminals are not in the same cubicle as the corresponding functional unit, and they are separated from the terminals of other functional units.












The major difference between Forms 3 and 4 is the separation of the terminals of each functional unit the terminals of other units. The main advantage of this model is the ability to safely connect and disconnect outgoing cables while the rest of the switchboard remains in operation.


The high cost of this rating means that Form 4 is usually only used in hospitals or for critical industrial processes.


Segregating the I and H

Metallic or non-metallic barriers are typically used to segregate functional units, terminals and busbars and achieve the form ratings outlined above. This means equipment is housed in separate cubicles within a switchboard. Standards, however, provide for other methods to be used for segregation which are easier and cheaper, such as utilising the integral compartments of a functional unit and insulating terminals or busbars.


Using Integral Housing

If a functional unit’s housing complies with protection standards IP2X (a finger-sized object cannot come into contact with any live parts), it can be used to separate functional units, not including busbars. This method is used only for segregating functional units from one another and therefore it applies only to Forms 3 and 4.


This method is identified by adding a ‘h’ to the form rating. For example, Form 3bh.


Insulating busbars or terminals

Insulation can be utilised to separate busbars from both functional units and terminals. It is also possible to shroud terminals for segregation from the functional units of other terminals.


The use of this method of is identified through the inclusion of ‘i’ to the form rating. It is used to create 2bi, 3bi and 4bi.

Combining insulation and housing

Component housings and insulation can be used together for internal segregation. The combined use of these methods is identified through the inclusion of ‘i’ and ‘h’ to the forms. This method is used to achieve forms 3bih, 4aih and 4bih.


As noted, the use of “I” and “h” is a cheaper method to achieve form ratings. It should also be considered inferior to the use of physical barriers to achieve form ratings.

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