A switchgear building is usually drawn as a small steel shed with a big door, and it is priced accordingly. Then the electrical design arrives, and the building turns out to be a set of compartments with a cable void underneath and a fire separation through the middle.
The steel is the enclosure. Everything that makes the building work — separation, cable access, ventilation, egress — is inside it, and those are the decisions that should be made first.
1. The plan is a separation drawing before it is a floor plan
Switchgear is arranged in sections, and the building has to respect the sections rather than the other way round.
Two questions decide the layout. How much equipment sits in one fire compartment, and how does a person get out of a compartment that is full of smoke? Both have answers that come from the electrical philosophy, and both constrain the steel grid: a compartment boundary that lands in the middle of a bay is a wall that needs a column either side of it.
The practical consequence is that the structural grid should follow the compartment grid. A column in the middle of a switchboard line is a cable route that has to divert around it, and cable routes that divert are cable routes that fill up faster.
2. The cable void: the floor is a layer, not a slab
Most switchgear buildings have a cable void under the equipment floor, and it is the part that gets designed last and causes the most trouble.
The void needs depth for bending radii, trays that cross each other, and the entry of cables from outside. It needs to be reachable for pulling and for later additions, which means hatches in the equipment floor and a route out. And it needs to be fire-stopped where it passes a compartment boundary, because a void is a horizontal duct that connects every room in the building.
Structurally, the void turns the floor into a suspended slab with openings, and openings are what a slab likes least. The equipment floor also carries point loads from switchgear, which arrive at small feet rather than distributed — so the floor has to be designed for those, and the void's walls have to be braced for the loads the floor transfers into them.
3. Doors: the opening that changes the frame
A switchgear building has one large door for equipment and one or more person doors, and the large door is where the structure suffers.
On a small building, the gable end that carries the equipment door is often the end that was providing the lateral stability. Removing most of it leaves a portal, and a portal on a short building with a heavy roof behaves differently from a braced wall: it deflects more, and the deflection shows at the door.
The door also has to be positioned so that equipment can be moved in on the route it will actually travel — a straight run from the truck to the foundation, without a turn that a transformer cannot make. That is a site logistics question that becomes a structural one, because it decides which wall the opening is in.
4. Ventilation and heat: the equipment decides the openings
Switchgear and transformers produce heat continuously, and the building has to remove it without letting water in.
Two configurations dominate. Natural ventilation with low inlet and high outlet louvres, which works when the heat load is modest and the building can be laid out with a clear path between them. And mechanical extract, which works when the heat load is high or the building is subdivided, but needs power, maintenance and a fallback.
Either way the louvres are structural openings: they interrupt the cladding, they are where wind pressure enters, and they are where rain arrives during a storm with the wind behind it. A louvre detailed as a hole with a grille in it will leak; a louvre detailed with a sill, a drip and a blade that turns the weather out will not.
Mechanical extract also brings an obligation that outlives the handover. A fan that stops in a hot climate turns the room into an oven within an hour, and the equipment inside does not wait for a repair. Where the extract matters that much, the fallback has to be designed in — a second fan, a high-level relief opening, or a natural path that works when the power is off.
5. Earthing, trays and the things that must not touch
An electrical building carries a set of metallic systems that have to be bonded and, in some cases, kept apart.
Cable trays, earthing conductors, structural steel and the reinforcement in the floor all participate, and the design has to say which are connected and which are separated. That is an electrical decision with a structural consequence: a steel frame that is used as part of the earthing path needs its connections to be reliable for the life of the building, which means bolted connections designed for it rather than incidental contact.
Where the building is extended later, this matters twice — a new bay that is not bonded to the same system creates a potential difference across a doorway, which is both a hazard and a commissioning problem.
6. What I want before drawings
- The electrical single-line layout, and how many fire compartments it produces.
- The depth of cable void, the largest cable, and where cables enter.
- The largest item of equipment that will ever be moved in, and its route from the road.
- The heat load, and whether ventilation will be natural or mechanical.
- Whether the steel frame is part of the earthing system.
- The wind speed to be used, taken from the applicable code at the project site rather than a round number, plus the louvre and door detail — a small building with large openings has a high proportion of its surface given to those openings, and the fixings around them take the load first.
A switchgear building is small, expensive per square metre, and decided almost entirely by things that are not steel. Draw the compartments and the cable void first, and the steel will follow without argument.