When selecting RMU (Ring Main Unit) circuit breakers, key parameters such as electrical performance, mechanical characteristics, environmental adaptability, intelligent requirements and economic efficiency must be taken into account to ensure the safe, reliable and efficient operation of power grid equipment. The following are specific parameters and options to consider:
I. Electrical Performance Parameters
Rated Voltage and Current
Rated Voltage: Must match the working voltage of the power grid (e.g. 10kV, 20kV, 35kV) and consider the maximum working voltage of the system (e.g. 12.6kV for a 12kV system) to ensure that the circuit breaker operates reliably within voltage fluctuation range.
Rated current: leaves room (usually 1.2 to 1.5 times the load current) for short-term overload or future expansion needs, depending on load current selection. For example, for 400A load current, a 630A rated current circuit breaker is available.
Short circuit outage capacity
Rated Short Circuit Breaker (Icu): Maximum short-circuit current at rated voltage at which the circuit breaker can safely be interrupted. It must be greater than the maximum possible short-circuit current in the system (determined by short-circuit calculation). For example, when the system has a short-circuit circuit current of 25kA, a circuit breaker with Icu≥25kA should be selected.
Rated Short Circuit Generates Current (Icm): The peak current that a circuit breaker can withstand when closed. It is usually 2.5 times the size of an Icu and must be reliably closed and disconnected in the event of a short-circuit circuit failure.
Insulation Performance
3. Frequency Withstand Voltage: refers to the power frequency test voltage that the circuit breaker can withstand in 1 minute (e.g., 42kV for a 12kV system), reflecting the insulation strength of the circuit breaker between the ground and two phases.
Impulse Withstand Voltage: can withstand lightning overvoltage or switch overvoltage (e.g. 75kV for a 12kV system) to ensure no failure during transient overvoltage.
Protection Characteristics
Tripping curve: select the overload protection tripping characteristics (long delay, short delay, instantaneous) according to load type. For example, the motor motor loads a a D-type tripping curve (10-14 times the rated current for instantaneous protection operations) and a C-type (5-10 times the rated current) for lighting loads.
Protection Functions: whether to provide overload, short circuit, leakage, underpressure, overpressure and other protective functions, needs to be configured according to the requirements of power grid protection.
ii. Mechanical Characteristic Parameters
Type of operational mechanism
Spring operation mechanism: fast energy storage, reliable operation, suitable for frequent operation (such as ring host output circuit).
Permanent magnet mechanism: simple structure, long service life (up to 100,000 times), but high cost, suitable for very high reliability requirements.
Electromagnetic Operating Mechanism: simple structure, high energy consumption, phase-out.
Mechanical life: The maximum number of times a circuit breaker can be closed / opened (e.g. 10,000 mechanical times for vacuum circuit breaker) and needs to be selected based on frequency of operation. For example, for applications with more routine operations, products with longer mechanical life should be selected.
Opening and closing hours
Inherent Opening Time: from the time of receiving the boot to the time of contact separation (usually ≤50ms), affecting the troubleshooting speed of short circuit faults.
Total opening time: includes arc time (usually ≤100ms) and must meet system protection coordination requirements.
Shutdown time: affects grid synchronization and must match system requirements.
III. Environmental Adaptability Parameters
Temperature range: circuit breaker must adapt to temperature variations in the installation environment (e.g. -25°C to +40°C). Extreme temperatures can affect the performance of insulation materials or the condition of the grease. For example, cold-resistant models should be selected in extremely cold regions.
Protection rating: The protection rating of the containment structure (e.g. IP43, IP65) must match the installation environment. External hosts typically require IP65 (dust and water resistance), while indoor requirements can be reduced appropriately.
Corrosion Resistance: In corrosive environments such as coastal areas or chemical plants, stainless steel enclosures or anticorrosion coatings should be selected to prevent rust and poor contact of metal parts. Altitude Adaptability: At high altitudes (e.g. above 2000 m), the effects of reduced air density on insulation and arc performance must be considered. High altitude circuit breaker or reducer should be selected.
IV. INTRODUCTION Intelligent and Communication Parameters
Intelligent Monitoring Functions: Do circuit breaker have real-time monitoring of parameters such as current, voltage, temperature, and state monitoring of contact wear and SF6 gas pressure,if applicable, to support predictive maintenance?
Communication Protocols: support standard communication protocols such as IEC 61850, DNP3 and Modbus to facilitate integration with higher-level dispatching systems or energy management systems for remote monitoring and automated control.
Data Interfaces: Fibre optic, Ethernet or wireless communication interfaces (e.g., 4G/5G) must be configured to meet communication needs in different scenarios. Wireless communications, for example, can be used in remote areas to reduce wiring costs.
V. Economic and maintenance parameters
Lifecycle Cost: In addition to initial procurement costs, it is important to assess operating and maintenance costs (e.g., spare parts prices, repair cycles), energy consumption (such as operating (e.g., operating mechanism power consumption) and loss of power supply (such as the impact of failure rate on reliability of power supply reliability). Modular Design
Priority is given to modular circuit breakers to quickly replace faulty units and reduce power outage time. vacuum circuit breakers with solid-state sealing rods, for example, improve reliability and simplify maintenance.
Spare Parts Availability
Select products from mainstream manufacturers to ensure timely supply of parts and avoid maintenance difficulties due to spare parts discontinuation.
