Open the door of an LV panel and the first thing you will notice is how it’s divided up inside. In some panels, the busbars, breakers, and cable terminals all sit in one open compartment. In others, every single circuit is boxed off in its own sealed section. That difference isn’t cosmetic. It’s a formally defined design choice called the Form of Separation. It’s one of the most important decisions in any LV panel design.
This post explains what Form of Separation means, walks through each form from 1 to 4b, and looks at the construction choices that go into building the enclosure itself.
This is the second post in our LV Panel Engineering Series. If you’re new here, you may want to start with Part 1: How to Specify an LV Panel, which covers the standards and the overall specification process before diving into construction details.
What “Construction” Covers in an LV Panel
Before getting into segregation, it helps to understand what the physical enclosure itself is made of and how it’s built.
Enclosure material
- Sheet steel (mild steel), powder-coated — the standard choice for indoor panels.
- Galvanized or stainless steel — used in corrosive or humid environments, such as coastal areas or desalination plants.
- GRP (glass-reinforced polyester) — a fully insulated enclosure material, used where corrosion resistance or electrical insulation of the enclosure itself is especially important.
Mounting arrangement
- Floor-standing (cubicle) panels — used for Main LV Boards and Motor Control Centres.
- Wall-mounted panels — used for smaller sub-distribution boards.
Structural design
LV panels are usually built as modular, extendable sections bolted together, so a board can be extended later without disturbing the sections already in service. This is why many Main LV Boards are specified with 20–25% spare space, future extension is planned in from day one, not added as an afterthought.
Panels also need to be designed around how they’ll be accessed. Front-access-only designs (sometimes called “dead front”) versus designs that also allow rear or side access. This choice affects how much space needs to be allowed around the panel, which is a detail that has to be coordinated with the room layout early in a project.
What Form of Separation Actually Means
Form of Separation, defined in IEC 61439-2, describes how the internal parts of a panel — the busbars, the individual functional units (like a breaker and its associated components), and the outgoing cable terminals — are separated from each other using metal barriers or partitions.
It’s important to understand what this choice does and doesn’t affect:
- It does not change the electrical rating of the panel.
- It does determine how safely someone can work on one part of the panel while the rest stays energized, and how well a fault in one section is contained from spreading to another.
In simple terms: the higher the Form, the more individual metal compartments the panel is divided into, the safer it is to maintain, and the more it costs to build.
The Forms
Following are the forms of separation
| Form | What’s separated | What it means in practice |
|---|---|---|
| Form 1 | Nothing — busbars and all functional units share one compartment | The cheapest option, but the whole panel must be switched off to work on any single circuit. Rarely used for main or sub-main boards. |
| Form 2a | Busbars separated from functional units; terminals not separated from busbars | |
| Form 2b | Busbars separated from functional units; terminals also separated from busbars | A basic level of protection — a fault on the busbar is contained away from the outgoing devices. |
| Form 3a | Busbars separated from functional units; functional units separated from each other; terminals not separated from busbars | |
| Form 3b | As 3a, but terminals are also separated from busbars (though not from each other) | A common choice for sub-main boards — you can isolate and work on one feeder without being exposed to the ones next to it. |
| Form 4a | Full separation including each unit’s terminals, but the outgoing cable terminals of different units still share a compartment | |
| Form 4b | Complete separation: busbars, each functional unit, and each unit’s own outgoing terminals are all in their own individually enclosed compartments | Allows someone to safely isolate, remove, and re-terminate one outgoing circuit while the busbar and every other feeder remain live and in service. |
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Why Form 4b Is the Common Choice for Main Boards and MCCs
The logic behind choosing a Form comes down to a simple question.
how much exposure to live parts is acceptable during routine maintenance?
A Form 1 or 2 panel is cheaper and takes up less space. However working on any circuit with Form 1 or 2 panel means shutting the whole board down. A Form 4b panel costs more and takes up more room. Every compartment needs its own partition, and often its own door. But it lets a technician safely isolate a single feeder, open just that compartment, and carry out work while the busbar and every other circuit stay live.
For a Main LV Board or an MCC feeding protection, control, or safety systems, the kind of loads found on a substation’s auxiliary supply system — that continuity of supply to unrelated feeders isn’t a nice-to-have, it’s usually a hard operational requirement. That’s why Form 4b has become the standard choice for these boards on most industrial and substation projects. The cost premium is accepted because the alternative. Taking down an entire board to work on one circuit is often simply not acceptable.
Form of Separation also connects directly to two other topics. We’ll cover in this series
- Accessibility (who is allowed to open which compartment, and what tools or keys they need), and
- Internal arc protection (Part 5)
because segregate compartments are also what limits how far an internal arc fault can spread before it’s contained or cleared.
Up next in this series: IP Rating, Accessibility & Cable Entry in LV Panels — and Why They Cause Derating, where we look at how ingress protection, accessibility, and cable termination design combine to directly affect how much current a panel can actually carry.