What is a QGBT (main low-voltage switchboard)?
QGBT is the Portuguese acronym for Quadro Geral de Baixa Tensão — the main low-voltage switchboard. It is the principal switchboard of a factory, service building or infrastructure: it receives power from the transformer station or the grid and distributes it, with protection, to every downstream circuit and sub-board. If the electricity of a building were a tree, the QGBT would be the trunk.
What it does
A QGBT concentrates three critical functions:
- Distribute — it splits the incoming power across the building’s circuits and sub-boards;
- Protect — breakers and protections cut the current on overload or short-circuit, protecting people and equipment;
- Switch — it allows sections of the building to be energised, isolated and sectioned safely, including the main disconnect.
What is inside
A typical QGBT holds the main disconnect switch, the copper busbars that carry the current, the protections (main and outgoing breakers), metering devices and, depending on the project, equipment such as power analysers or residual-current protections. All assembled and wired inside a metal enclosure sized for the project’s current.
QGBT, sub-board and control panel — the difference
The QGBT is the main board; sub-boards sit downstream and serve specific zones or functions (a floor, a production line, lighting); control panels drive equipment and processes. In a factory it is common to have one QGBT plus several sub-boards and control panels — all custom-built to the project.
How a QGBT is custom-built
Every QGBT starts from the project’s single-line diagram or specification. The panel builder sizes the gear (protections, busbars, contactors), assembles the structure, executes the identified wiring and submits the board to continuity, insulation and functional tests. At Shortlec we build QGBT up to 6300 A to the IEC 61439 standard, with a test report and CE marking on every board.
When to modernise a QGBT
A well-maintained QGBT lasts decades, but obsolete gear, abnormal heating, frequent tripping or an expansion of the building or production are signs it is time for a retrofit or specialised maintenance — often the board can be modernised without replacing it entirely.
Frequently asked questions
What current can a QGBT carry?
It depends on the project: from a few hundred amperes in a service building to several thousand in an industrial plant. Shortlec builds QGBT up to 6300 A.
Does a QGBT need certification?
Yes. Manufacturing and testing must follow the IEC 61439 standard, and the board must be delivered with a test report and CE marking — that is what attests it was built and verified properly.