What is the function of a low-voltage electrical room, and what equipment does it consist of?

What is a Switchgear Room?

A switchgear room is an indoor electrical distribution facility that supplies power to low-voltage consumers. It typically includes medium-voltage incoming lines (with limited outgoing lines), distribution transformers, and low-voltage switchgear. Facilities operating at 10kV or below are categorized as either high-voltage or low-voltage switchgear rooms. A high-voltage switchgear room generally refers to a 6kV–10kV high-voltage switch compartment, while a low-voltage switchgear room usually refers to a 400V distribution room supplied by a 10kV or 35kV station service transformer.

Components of a Switchgear Room:

(1) Switching Station (Switchgear Substation)

Literally meaning an electrical facility containing only switching equipment, a switching station serves to distribute electrical power without changing the voltage level of incoming and outgoing lines. It is equipped with incoming and outgoing feeders for power redistribution, and may optionally include a distribution transformer.

(2) Outgoing Feeder Cabinet

Also known as a power distribution cabinet, this equipment distributes electrical energy from the busbar to individual outgoing circuits. It typically includes circuit breakers, current transformers (CT), potential transformers (PT), disconnect switches, and other components.

(3) Incoming Line Cabinet (Receiving Cabinet)

This cabinet receives electrical power from the grid (from incoming lines to the busbar). It is usually equipped with circuit breakers, CTs, PTs, and disconnect switches.

(4) PT Cabinet (Potential Transformer Cabinet)

Connected directly to the busbar, the PT cabinet measures busbar voltage and enables protection functions. Key components include potential transformers (PT), disconnect switches, fuses, and surge arresters.

(5) Isolator Cabinet

Used to electrically isolate two sections of busbar or separate powered equipment from the supply, providing operators with a visible disconnection point for safe maintenance and repair. Since isolator cabinets cannot interrupt load currents, the withdrawable unit must not be operated (inserted or withdrawn) when the associated circuit breaker is closed. Interlocking mechanisms between the circuit breaker auxiliary contacts and the isolator trolley are typically installed to prevent operational errors.

(6) Bus Coupler Cabinet (Bus Tie Cabinet)

Also called a bus sectioning cabinet, it connects two busbar sections (bus-to-bus). It is commonly used in single busbar sectionalized or double busbar systems to allow flexible operating modes or enable selective load shedding during faults.

(7) Capacitor Cabinet (Reactive Power Compensation Cabinet)

Used to improve the power factor of the grid—also known as reactive power compensation. Key components include banks of parallel-connected capacitors, switching control circuits, and protective devices such as fuses. Capacitor cabinets are usually installed adjacent to incoming line cabinets and can operate individually or in parallel.

After being disconnected from the grid, capacitor banks require time to discharge completely. Therefore, internal components—especially the capacitors—must not be touched directly. For a certain period after power-off (depending on the capacitor bank's capacity, e.g., 1 minute), re-energizing is prohibited to avoid overvoltage that could damage the capacitors. When automatic control is used, the switching cycles of each capacitor bank should be evenly managed to prevent premature failure of any single group.