Basic Knowledge of 12kV Ring Main Units (RMUs)

12kV ring main units primarily include: circuit breaker cabinets, load switch cabinets, combined load switch-fuse cabinets, and disconnector (isolator) cabinets. Different cabinet types serve distinct functions and are applied in various scenarios.

Circuit Breaker Cabinet

Its core function is to protect electrical circuits. It can automatically interrupt the circuit in the event of faults such as short circuits or overloads, preventing equipment damage and safety incidents. It is capable of breaking normal load current and short-circuit current, and features protection functions against overload, short circuit, under-voltage, etc. Commonly used in critical protection points of distribution systems, it is widely applied in power plants, substations, and industrial/mining enterprises, making it one of the most important protective devices in power systems.

Load Switch Cabinet

Used for controlling the connection and disconnection of circuits, it can operate under load but cannot interrupt short-circuit currents. Equipped with a simple arc-extinguishing device, it can break load currents and certain overload currents. Often used in conjunction with fuses, it is employed for controlling small-capacity transformers, motors, and other equipment. It is frequently utilized for interconnection in RMU incoming/outgoing lines and for controlling incoming lines of small-capacity distribution transformers.

Combined Load Switch-Fuse Cabinet

Used for controlling the connection and disconnection of circuits, it can operate under load and utilizes fuses to interrupt overload currents. The load switch-fuse combination appliance essentially employs a load switch for control and fuses for protection. Its components include a load switch and current-limiting fuses. Primarily used in 10kV high-voltage distribution systems to control and protect infrequently operated power supply equipment—mainly 10/0.4 kV transformers—it is commonly installed upstream of 10kV user transformers.

Disconnector (Isolator) Cabinet (PT Cabinet)

Primarily used for isolating power sources to provide a safe disconnection point for equipment maintenance, ensuring personnel and equipment safety. Often used in conjunction with circuit breakers, it can be configured as upper or lower isolation. Lacking an arc-extinguishing device, it is strictly prohibited to operate under load and is only suitable for switching under no-load or extremely light-load conditions. Commonly applied in high-voltage systems for isolating busbar PT equipment during maintenance or for isolating Switchgear during repairs.

Different cabinet types have distinct functions: circuit breakers emphasize high-current protection, disconnectors are mainly used for isolation during maintenance, load switches are for on-load control, and combined load switch-fuse cabinets provide overload protection via fuses. The prices of these cabinets also vary: a circuit breaker cabinet is approximately twice the price of a load switch cabinet, while a combined load switch-fuse cabinet is about 1.3 times the price. When used in combination, these cabinets collectively ensure the economical, rational, and safe operation of the power system.