The main characteristics of a circuit breaker include: rated voltage Ue; rated current In; trip current setting range for overload protection (Ir or Irth) and short-circuit protection (Im); rated short-circuit breaking current (Icu for industrial circuit breakers; Icn for household circuit breakers), etc.
Rated operating voltage (Ue): This is the voltage at which the circuit breaker operates under normal (uninterrupted) conditions.
Rated current (In): This is the maximum current that a circuit breaker equipped with a dedicated overcurrent trip relay can withstand indefinitely under the ambient temperature specified by the manufacturer, without exceeding the temperature limits specified for the current-carrying components.
Short-circuit relay trip current setting (Im): The short-circuit trip relay (instantaneous or short-delay) is used to quickly trip the circuit breaker when a high fault current value occurs; its tripping limit is Im.
Rated short-circuit breaking capacity (Icu or Icn): The rated short-circuit breaking current of the circuit breaker is the highest (expected) current value that the circuit breaker can interrupt without being damaged. The current values provided in the standard are the root mean square (RMS) values of the AC component of the fault current. When calculating the standard values, the DC transient component (which always occurs under worst-case short-circuit conditions) is assumed to be zero. Industrial circuit breaker ratings (Icu) and household circuit breaker ratings (Icn) are typically given in kA RMS value form.
Short-circuit breaking capacity (Ics): The rated breaking capacity of a circuit breaker is divided into rated ultimate short-circuit breaking capacity and rated operational short-circuit breaking capacity. The national standard "Low-voltage switchgear and controlgear - Low-voltage circuit breakers" (GB14048.2-94) defines the rated ultimate short-circuit breaking capacity and rated operational short-circuit breaking capacity of circuit breakers as follows:
Rated ultimate short-circuit breaking capacity of a circuit breaker: The breaking capacity under the conditions specified in the prescribed test procedure, excluding the circuit breaker's ability to continue carrying its rated current.
Rated operational short-circuit breaking capacity of a circuit breaker: The breaking capacity under the conditions specified in the prescribed test procedure, including the circuit breaker's ability to continue carrying its rated current.
The test procedure for rated ultimate short-circuit breaking capacity is O-t-CO.
The specific test is as follows: Adjust the line current to the expected short-circuit current value (e.g., 380V, 50kA), with the test button closed and the circuit breaker in the closed position. Press the test button, and the circuit breaker should immediately open (open, abbreviated as O) after passing a 50kA short-circuit current. The circuit breaker should function properly and be able to be reclosed. t represents the interval time, typically 3 minutes. During this time, the line remains in hot standby mode. The circuit breaker then performs another closing (C) and immediately following it, an opening (O) test. (The closing test assesses the circuit breaker's electrical and thermal stability under peak current.) This procedure is called CO. If the circuit breaker can completely break the circuit, its ultimate short-circuit breaking capacity is qualified.
The test procedure for the rated operating short-circuit breaking capacity (Icn) of the circuit breaker is O-t-CO-t-CO. This procedure includes one more CO test than the Icn test procedure. If, after the test, the circuit breaker can completely break the circuit and extinguish the arc, its rated operating short-circuit breaking capacity is considered qualified.
Therefore, it can be seen that the rated ultimate short-circuit breaking capacity (Icn) refers to the ability of a low-voltage circuit breaker to operate normally and break the same short-circuit current again after breaking the maximum three-phase short-circuit current at its outgoing terminals. Whether it can normally connect and disconnect again afterward is not guaranteed by the circuit breaker. The rated operating short-circuit breaking capacity (Ics), on the other hand, refers to the ability of the circuit breaker to normally disconnect multiple times when the maximum three-phase short-circuit current occurs at its outgoing terminals.
IEC 947-2, "Low-voltage switchgear and controlgear - Low-voltage circuit breakers," specifies that: For Class A circuit breakers (those with only overload long-time delay and short-circuit instantaneous tripping protection), the Ics can be 25%, 50%, 75%, and 100%. For Class B circuit breakers (those with three-stage protection: overload long-time delay, short-circuit short-time delay, and short-circuit instantaneous tripping), the Ics can be 50%, 75%, and 100% of the rated ultimate short-circuit breaking capacity. Therefore, it can be seen that the rated operating short-circuit breaking capacity is a breaking current value smaller than the rated ultimate short-circuit breaking current. Generally, circuit breakers with three-stage protection functions-overload long-time delay, short-circuit short-time delay, and short-circuit instantaneous trip-can provide selective protection and are used as main protection switches in most main lines (including transformer outgoing lines). Circuit breakers without short-circuit short-time delay (only overload long-time delay and short-circuit instantaneous trip protection) cannot provide selective protection and can only be used in branch lines. IEC 92 "Marine Electrical Systems" states that circuit breakers with three-stage protection emphasize their operational short-circuit breaking capacity, while circuit breakers used in branch lines should ensure they have sufficient ultimate short-circuit breaking capacity.
While all circuit breakers possess the important technical indicators Icu and Ics, circuit breakers used in branch lines only need to meet the rated ultimate short-circuit breaking capacity. A common misconception is to err on the side of higher values, believing that higher values are safer. However, excessively high breaking capacity can lead to unnecessary waste (for the same type of circuit breaker, the H-type-high breaking capacity type is 1.3 to 1.8 times more expensive than the S-type-standard type). Therefore, it's unnecessary to solely pursue the operational short-circuit breaking capacity of circuit breakers on branch lines. For circuit breakers used on main lines, they must meet both the rated ultimate short-circuit breaking capacity and the rated operational short-circuit breaking capacity requirements. Judging a circuit breaker's breaking capacity solely by its rated ultimate short-circuit breaking capacity (Icu) will create safety hazards for users.
A circuit breaker is a basic low-voltage electrical appliance with overload, short-circuit, and undervoltage protection functions, capable of protecting lines and power supplies.
The main technical indicators are rated voltage and rated current. Circuit breakers have different functions depending on the application, resulting in many varieties, specifications, and specific technical indicators.
Free tripping of a circuit breaker: At any moment during the closing process, if the protection action connects the trip circuit, the circuit breaker can reliably disconnect; this is called free tripping. Circuit breakers with free tripping can ensure that the circuit breaker can quickly disconnect when a short circuit fault occurs during closing, thus preventing the scope of the accident from expanding.