Form of segregation defines how the live parts of a switchboard are separated from each other. It directly drives maintainability, safety and downtime. A board with the wrong form for the application either costs more than it should or makes the operations team take the whole assembly out of service to work on a single circuit.
Quick answer
Forms 1 through 4b describe increasing levels of internal separation in a low-voltage switchboard under AS/NZS 61439. Form 1 has no separation. Form 4 separates busbars, functional units and terminals from each other. Form 4a houses each unit’s terminals in the same compartment as the unit; Form 4b houses them in a separate compartment. Form 4a is the CWS Simotrol standard for NZ industrial MSBs and MCCs.
In this article
This guide covers what form of segregation means in a switchboard, the four levels defined under AS/NZS 61439, when each one suits an application, and how CWS approaches Form selection on NZ projects. Clive Wilson Switchboards has been building LV switchboards for industrial and commercial NZ sites for over 55 years.
A switchboard with no internal separation is a row of breakers sharing the same enclosure. Pull one breaker and you expose live busbars. Maintain one feeder and the rest of the board has to be isolated too.
Form of internal separation introduces physical barriers between busbars, between functional units and between the terminals belonging to each unit. The higher the form, the more of the assembly stays safely de-energised when a single circuit is being worked on.
That has three direct consequences: safety for the maintenance team, uptime for the operations team, and incremental cost for the assembly. The right form for a project balances those three.
AS/NZS 61439 defines four forms of internal separation, with sub-levels at Form 4 (4a and 4b) for terminal location. Each form adds a physical barrier on top of the level below.
No physical barriers between busbars, functional units or terminals. Everything inside the enclosure is at risk when the door opens. Suits small commercial boards on low-duty applications.
A barrier separates the busbars from the functional units. The busbars stay protected when working on individual units, but the functional units are not separated from each other. Suits commercial distribution boards where the duty is low and downtime tolerance is high.
Adds barriers between functional units, so one unit can be worked on without exposing the next. Terminals stay with their unit. Suits mid-tier industrial boards where some circuit isolation is needed but Form 4 cost is not justified.
Adds barriers around the terminals as well as the units. A single functional unit can be fully isolated, including its terminals, while the rest of the assembly stays in service.
The right form for a board depends on three project drivers: safety case, downtime tolerance, and cost envelope.
Typical NZ Form selections we see across the boards we build:
These are general patterns, not absolutes. The right form for any specific project comes from the engineer of record, the operations team, and the asset owner’s downtime tolerance.
Form of internal separation is one of the few specs that a finished switchboard cannot be upgraded on. The form declared on the rating plate is the form it stays at for its service life. Get it right at design stage.
Form of internal separation is one of the verification items in the AS/NZS 61439 type-test envelope. A Form 4a Simotrol assembly has been type-tested as a Form 4a system, with the specific barrier arrangements, gland plate cut-outs and busbar bracing that the type-test pack defines.
Substituting a different barrier arrangement, changing the gland plate layout outside the type-test envelope, or fitting components the type-test did not verify all break the Form rating. This is why we build to the platform envelopes (Simotrol, Logstrup, Schneider Prisma Plus G) rather than substituting components project by project.
For more on the AS/NZS 61439 verification framework and how it feeds into fault rating, see our switchboard fault rating guide.
Form 4a houses each functional unit’s terminals in the same compartment as the unit itself. Form 4b houses them in a separate compartment from the unit. Form 4b carries a higher build cost and is specified where the highest level of segregation is required. Form 4a is the standard segregation on CWS Simotrol MSBs and MCCs for NZ industrial sites.
In practice, no. Form is tied to the AS/NZS 61439 type-test envelope of the original assembly. Retrofit barriers do not change the type-tested form rating of the board. If a project requires Form 4, that needs to be specified up front.
Indirectly. Higher forms add more internal mass and physical bracing, which contributes to the assembly’s ability to contain a short-circuit. But fault rating (Icw and Ipk) is a separate type-test outcome and is declared independently on the rating plate.
Form 4a on the CWS Simotrol platform. It gives the maintenance team the ability to isolate a single starter module while the rest of the MCC stays in service, without the build-cost premium of Form 4b’s separate-compartment terminals.
On the rating plate. Form of internal separation is declared alongside rated current, Icw, Ipk, rated insulation voltage and IP rating. If a rating plate is missing or the form is not stated, the assembly cannot be relied on to carry the form rating informally.
Reviewed by Chris Wilson, Co-Director, Clive Wilson Switchboards. Registered electrician, 15+ years in LV switchboards. Updated May 2026.
