verview of Circuit Breakers
A Circuit Breaker is a key device in the power system used for protecting and controlling circuits. It can close, carry and break current under normal or fault conditions. Its core functions include overload protection, short-circuit protection, undervoltage protection, etc. It is equivalent to a combination of fuses and over/under-voltage thermal relays, but it has higher reliability and reusability.

Main characteristic parameters
Rated voltage (Ue) : The highest voltage at which a circuit breaker operates normally, such as 220V, 380V, etc. 37

Rated current (In) : The maximum current value that can be safely carried over a long period of time, which must be greater than the operating current of the circuit by 35%.

Breaking capacity (Icu/Ics) : The ultimate short-circuit breaking capacity (Icu) refers to the ability to break the maximum short-circuit current at one time. Operating breaking capacity (Ics) refers to the current threshold that can still be used after breaking. Generally, frame circuit breakers require Ics≥50%Icu, and molded case circuit breakers require Ics≥25% ICU.

Short-time withstand current (Icw) : The ability of a circuit breaker to withstand short-circuit current within a specified period of time without damage.

Ii. Classification of Circuit Breakers
1. By voltage level
High-voltage circuit breakers: Used in systems of 3kV and above. Common arc-extinguishing media include sulfur hexafluoride (SF6), vacuum, oil, etc. 4

Low-voltage circuit breakers are classified into three types: frame type (ACB), molded case type (MCCB), and miniature type (MCB). 57.

2. By structure and application
Frame type circuit breaker (ACB
Rated current: 200A to 6300A, equipped with four-stage protection (long delay, short delay, instantaneous, and ground fault), it is mostly used for the protection of main switches in distribution systems or large-capacity equipment.

Molded case circuit breaker (MCCB
Compact structure, rated current 10A to 1600A, suitable for branch circuit protection. The electronic MCCB supports selective protection, and some models have the regional interlock function 57.

Miniature Circuit Breaker (MCB
It is used in terminal circuits below 125A (such as household and commercial ones), available in 1P to 4P specifications, and supports overload, short circuit and leakage protection.

3. Press the arc extinguishing technology
Vacuum circuit breaker: Fast arc extinguishing, long service life, suitable for frequent operation scenarios 4.

SF6 circuit breaker: It has excellent insulation and arc-extinguishing performance and is mostly used in high-voltage systems. The purity of the gas needs to be regularly tested.

Iii. Principles for Circuit Breaker Selection
Match circuit parameters
Rated voltage ≥ line voltage, rated current ≥ maximum load current, breaking capacity ≥ expected short-circuit current 57.

Load type adaptation
Motor protection needs to take into account the starting current (the instantaneous trip setting value is 1.35 to 1.7 times the starting current). The lighting circuit takes six times the load current of 78.

Selective coordination
The upper and lower circuit breakers need to meet the time difference (such as short-delay action difference ≥0.1s) and current difference (the action current of the upper level ≥ 1.2 times that of the lower level) to avoid over-level tripping.

Environmental adaptability
For high-altitude, humid or high-temperature environments, special models should be selected and the rated current adjusted (capacity reduction is required when the temperature exceeds 40℃). 13.

Iv. Circuit Breaker Testing and Maintenance
Key test items
Static/dynamic contact resistance: Detect contact loss 12.

Mechanical characteristic analysis: Opening and closing time, speed and simultaneity 14.

Insulation performance: Withstand voltage test, vacuum degree detection (for vacuum circuit breakers) 14.

Protection function verification: Calibration of overload and short-circuit tripping action values 8.

Key points of maintenance
Regular inspection: Gas pressure (SF6 circuit breaker), contact ablation, mechanism lubrication 48.

Preventive tests: Carried out in accordance with standards such as GB/T 1984 and GB 14048, once every 1 to 3 years.

Fault handling: In case of oil shortage, overheating or explosion, emergency isolation is required and problems with contact or arc extinguishing system should be investigated. 4.

V. Analysis of Common Problems
The difference between a circuit breaker and a disconnector
The disconnector (QS) is only used to isolate the power supply and has no arc-extinguishing capability. The circuit breaker (QF) can cut off the fault current of 12.

The significance of Icu and Ics
Icu reflects the ultimate breaking capacity, and Ics reflects the reliability of continuous operation. The main lines focus on Ics, while the branch lines focus on Icu8.

Selection of current-limiting circuit breakers
Match the thermal stress of the cable through the current-limiting curve, and give priority to the models with fast breaking speed (such as vacuum circuit breakers) 78.

The leakage protection malfunctioned
Mostly due to the decline in line insulation or poor grounding, it is necessary to detect the leakage current and adjust the action threshold (commonly 30mA to 300mA)