The primary characteristics of a circuit breaker include: rated voltage (Ue); rated current (In); trip current setting ranges for overload protection (Ir or Irth) and short-circuit protection (Im); and rated short-circuit breaking capacity (Icu for industrial circuit breakers; Icn for household circuit breakers), among others.
Rated operational voltage (Ue): This is the voltage at which the circuit breaker operates under normal (continuous) conditions.
Rated current (In): This is the maximum current value that a circuit breaker equipped with a dedicated overcurrent release relay can carry indefinitely at the ambient temperature specified by the manufacturer, without the current-carrying components exceeding their specified temperature limits.
Short-circuit release trip current setting (Im): The short-circuit release relay (instantaneous or short-time delay) is designed to trip the circuit breaker rapidly when high fault currents occur; Im represents the tripping threshold.
Rated short-circuit breaking capacity (Icu or Icn): The rated short-circuit breaking capacity is the maximum (prospective) current value that the circuit breaker can interrupt without sustaining damage. The current values specified in standards represent the root-mean-square (RMS) value of the AC component of the fault current; the DC transient component (which always appears during a worst-case short-circuit) is assumed to be zero when calculating these standard values. Ratings for industrial circuit breakers (Icu) and household circuit breakers (Icn) are typically expressed as RMS values in kA.
Short-circuit breaking capacity (Ics): The rated breaking capacity of a circuit breaker is categorized into two types: rated ultimate short-circuit breaking capacity and rated service short-circuit breaking capacity. The national standard "Low-voltage switchgear and controlgear-Part 2: Circuit-breakers" (GB14048.2-94) defines the rated ultimate short-circuit breaking capacity and the rated service short-circuit breaking capacity of circuit breakers as follows:
1. Rated ultimate short-circuit breaking capacity: The breaking capacity under conditions specified by the prescribed test sequence, excluding the capability of the circuit breaker to continue carrying its rated current;
2. Rated service short-circuit breaking capacity: The breaking capacity under conditions specified by the prescribed test sequence, including the capability of the circuit breaker to continue carrying its rated current;
3. The test sequence for rated ultimate short-circuit breaking capacity is O-t-CO.
The specific test procedure is as follows: Adjust the circuit current to the prospective short-circuit current value (e.g., 380V, 50kA) while the test button is not yet engaged and the circuit breaker under test is in the closed position. Press the test button; the circuit breaker conducts the 50kA short-circuit current and immediately opens (abbreviated as O). The circuit breaker must remain intact and be capable of re-closing. "t" represents the interval time, typically 3 minutes, during which the circuit remains in a "hot standby" state. The circuit breaker then performs a closing operation (abbreviated as C) followed immediately by an opening operation (O) (the closing test evaluates the electromagnetic and thermal stability of the circuit breaker under peak current). This sequence constitutes the CO operation. If the circuit breaker can successfully perform the full breaking operation, its ultimate short-circuit breaking capacity is deemed compliant.
4. The test sequence for the rated service short-circuit breaking capacity (Icn) is O-t-CO-t-CO. This sequence includes one additional CO operation compared to the ultimate breaking capacity test. If the circuit breaker successfully performs the full breaking operation and extinguishes the arc throughout the test, its rated service short-circuit breaking capacity is deemed compliant. Thus, it is evident that the rated ultimate short-circuit breaking capacity ($I_{cu}$) refers to the ability of a low-voltage circuit breaker to interrupt the maximum three-phase short-circuit current at its load-side terminals and subsequently remain capable of normal operation and interrupting that same short-circuit current one more time; however, the circuit breaker does not guarantee its ability to make and break the circuit normally thereafter. In contrast, the rated service short-circuit breaking capacity ($I_{cs}$) refers to the ability of the circuit breaker to interrupt the maximum three-phase short-circuit current at its load-side terminals multiple times while maintaining normal operation.
The IEC 947-2 standard ("Low-voltage switchgear and controlgear – Part 2: Circuit-breakers") specifies that for Class A circuit breakers (those providing only long-time delay overload protection and instantaneous short-circuit protection), the $I_{cs}$ value can be 25%, 50%, 75%, or 100% of $I_{cu}$. For Class B circuit breakers (those providing three-stage protection: long-time delay overload, short-time delay short-circuit, and instantaneous short-circuit protection), the $I_{cs}$ value can be 50%, 75%, or 100% of $I_{cu}$. Consequently, the rated service short-circuit breaking capacity represents a breaking current value lower than the rated ultimate short-circuit breaking capacity.
Generally, circuit breakers equipped with three-stage protection-long-time delay overload, short-time delay short-circuit, and instantaneous short-circuit protection-are capable of selective protection; they are predominantly used as main protection switches for trunk lines (including at transformer output terminals). Circuit breakers lacking short-time delay short-circuit protection (offering only two-stage protection: long-time delay overload and instantaneous short-circuit protection) cannot provide selective protection and are suitable only for branch circuits. The IEC 92 standard ("Electrical installations in ships") indicates that for circuit breakers with three-stage protection, emphasis is placed on the rated service short-circuit breaking capacity ($I_{cs}$), whereas for circuit breakers used in branch circuits, the priority is ensuring sufficient rated ultimate short-circuit breaking capacity ($I_{cu}$).
Regardless of the type, all circuit breakers possess these two critical technical specifications: $I_{cu}$ and $I_{cs}$. However, for circuit breakers used on branch lines, meeting the requirement for rated ultimate short-circuit breaking capacity ($I_{cu}$) alone is sufficient. A common misconception is the tendency to select a higher rating rather than the appropriate one, under the assumption that a higher rating offers greater safety. However, selecting a rating that is excessively high leads to unnecessary waste (for the same type of circuit breaker, the H-type-high breaking capacity-costs 1.3 to 1.8 times more than the S-type-standard capacity). Therefore, there is no need to blindly prioritize the rated service short-circuit breaking capacity for circuit breakers installed on branch lines. Conversely, for circuit breakers used on main lines, it is essential to meet the requirements for both rated ultimate short-circuit breaking capacity and rated service short-circuit breaking capacity; relying solely on the rated ultimate short-circuit breaking capacity (Icu) to assess breaking performance creates potential safety hazards for the user.
A circuit breaker is a fundamental low-voltage electrical device that provides overload, short-circuit, and undervoltage protection, thereby safeguarding circuits and power sources.
Its primary technical specifications are rated voltage and rated current. Circuit breakers offer various functions depending on the application; they come in a wide range of types and specifications, each with its own specific technical parameters.
Circuit breaker "free-trip" mechanism: This refers to the capability of the circuit breaker to reliably open the circuit at any moment during the closing process if the protection system triggers the trip circuit. A circuit breaker equipped with a free-trip mechanism ensures rapid disconnection when a short-circuit fault occurs during the closing operation, thereby preventing the fault from escalating.
