Switchboards April 2026 · 6 min read
Form of segregation is one of the most misunderstood parts of switchboard specification in New Zealand. Getting it wrong affects safety, maintenance access, and compliance with AS/NZS 61439. This guide explains each form clearly and helps you specify the right one for your application.
Form of segregation defines how the internal parts of a switchboard are physically separated from each other. It is a formal requirement under AS/NZS 61439 and directly affects how safely a switchboard can be worked on, how faults propagate, and how much flexibility there is during installation and maintenance. Clive Wilson Switchboards designs and manufactures switchboards to the correct form for each application.
Imagine a fault on one circuit in a switchboard. Depending on the form of segregation, that fault may remain contained to one section, or it may affect the entire board. Similarly, a maintenance technician working on a de-energised section of a switchboard may be exposed to live busbars or terminals in an adjacent section unless physical barriers are present.
Form of segregation directly controls these risks. Higher forms provide greater isolation between components, allowing safer maintenance and reducing the consequences of a fault. They also cost more to design and manufacture, so the right balance is essential.
| Form | Busbar Separation | Functional Unit Separation | Terminal Separation |
|---|---|---|---|
| Form 1 | None | None | None |
| Form 2a | Busbars separated from functional units | No | Terminals not separated from busbars |
| Form 2b | Busbars separated from functional units | No | Terminals separated from busbars |
| Form 3a | Busbars separated from functional units | Yes — units from each other and from busbars | Terminals not separated from functional units |
| Form 3b | Busbars separated from functional units | Yes | Terminals separated from live parts |
| Form 4a | Busbars separated from functional units | Yes — each unit fully separated | Terminals not in separate enclosures |
| Form 4b | Busbars separated from functional units | Yes — each unit fully separated | Terminals in separate protected enclosures |
Form 1 switchboards have no internal segregation. All components share an open enclosure. This is acceptable only for small residential or light commercial applications with low fault levels and infrequent access requirements. It is rarely specified for industrial or commercial projects in New Zealand today.
Form 2 separates the busbars from the functional units (circuit breakers, contactors, etc.). This means a fault on a functional unit is less likely to directly affect the busbars, and vice versa. Form 2b also separates terminals from the busbars, giving technicians a safer working environment when accessing terminals on de-energised circuits. Form 2 is common in light commercial and small industrial applications.
Form 3 is the most commonly specified form for NZ industrial and commercial switchboards. Busbars and functional units are fully separated from each other. In Form 3b, terminals are also separated from live parts, which is important for safe working during partial energisation. This form allows one section of a switchboard to be de-energised and worked on while adjacent sections remain live, subject to appropriate isolation and permit-to-work procedures.
Form 4 provides the highest level of segregation. Each functional unit is in its own compartment, separated from all adjacent units. A fault in one compartment cannot propagate to others. In Form 4b, terminals are also in separate protected enclosures, allowing terminal connections to be worked on without exposure to live functional units.
Form 4b is specified for mission-critical applications: hospitals, data centres, process control systems, and any site where a busbar fault or component failure must be strictly contained. It carries a significant cost premium over Form 3b but provides substantially better fault containment and maintenance access.
| Application | Recommended Form | Reason |
|---|---|---|
| Residential / light commercial | Form 1 or 2a | Low fault level, infrequent access |
| General commercial | Form 2b or 3a | Terminal safety, moderate access frequency |
| Industrial manufacturing | Form 3b | Partial live working, fault containment |
| Food processing / dairy | Form 3b or 4a | Frequent access, safety-critical environment |
| Hospitals / data centres | Form 4b | Maximum fault containment, live working |
| Motor control centres (MCCs) | Form 3b or 4b | Individual motor circuit isolation |
AS/NZS 61439 requires that the specified form of segregation be verified either by type testing (using a tested and certified design) or by design verification for a bespoke assembly. The switchboard manufacturer is responsible for demonstrating compliance. At Clive Wilson Switchboards, we use tested assemblies on the Simotrol platform and certified designs where type-tested forms are required.
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Talk to us about form of segregation and switchboard design for your NZ project
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