The primary characteristics of a circuit breaker include: Rated Voltage (Ue); Rated Current (In); the tripping current setting ranges for overload protection (Ir or Irth) and short-circuit protection (Im); and Rated Short-Circuit Breaking Current (Icu for industrial circuit breakers; Icn for domestic 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 represents the maximum current value that a circuit breaker-equipped with specific overcurrent tripping relays-can continuously withstand indefinitely at the ambient temperature specified by the manufacturer, without the temperature of its current-carrying components exceeding their prescribed limits.
Short-Circuit Tripping Current Setting (Im): The short-circuit tripping relay (whether instantaneous or with a short time delay) is designed to rapidly trip the circuit breaker when high fault current values occur; Im represents this tripping threshold.
Rated Short-Circuit Breaking Capacity (Icu or Icn): The circuit breaker's rated short-circuit breaking current is the maximum (prospective) current value that the breaker can interrupt without sustaining damage. The current values provided in standards represent the RMS (Root Mean Square) value of the AC component of the fault current; when calculating these standard values, the DC transient component (which invariably occurs during a worst-case short-circuit scenario) is assumed to be zero. The ratings for industrial circuit breakers (Icu) and domestic circuit breakers (Icn) are typically expressed in terms of kA RMS values.
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) provides the following definitions for a circuit breaker's rated ultimate short-circuit breaking capacity and rated service short-circuit breaking capacity:
1. Rated ultimate short-circuit breaking capacity of a circuit breaker: The breaking capacity determined under the conditions specified in the prescribed test procedures, *excluding* the circuit breaker's ability to subsequently continue carrying its rated current.
2. Rated service short-circuit breaking capacity of a circuit breaker: The breaking capacity determined under the conditions specified in the prescribed test procedures, *including* the circuit breaker's ability to subsequently continue carrying its rated current.
3. The test procedure for rated ultimate short-circuit breaking capacity is O-t-CO.
The specific test sequence is as follows: The line current is adjusted to the anticipated short-circuit current value (e.g., 380 V, 50 kA). With the test button unpressed and the circuit breaker under test in the closed position, the test button is actuated; the circuit breaker then carries the 50 kA short-circuit current and immediately opens (designated as "O"). Following this operation, the circuit breaker must remain intact and capable of re-closing. "t" represents the time interval (typically 3 minutes), during which the circuit remains in a "hot standby" state; the circuit breaker then performs a closing operation (designated as "C") followed immediately by an opening operation ("O"). (This closing test serves to assess the circuit breaker's electrodynamic and thermal stability under peak current conditions.) This combined sequence is designated as "CO." If the circuit breaker successfully completes the breaking operation, its ultimate short-circuit breaking capacity is deemed qualified.
4. The test procedure for the circuit breaker's rated service short-circuit breaking capacity (Icn) is O-t-CO-t-CO. This procedure differs from that for the ultimate short-circuit breaking capacity by the addition of one extra "CO" sequence. Upon completion of the test, if the circuit breaker successfully performs the full breaking operation and extinguishes the arc, its rated service short-circuit breaking capacity is deemed qualified. Therefore, it can be observed that the *rated ultimate short-circuit breaking capacity* (Icu) refers to the ability of a low-voltage circuit breaker to continue operating normally-and to perform a second interruption-after having successfully cleared the maximum three-phase short-circuit current occurring at its output terminals; however, the circuit breaker does not guarantee its ability to perform subsequent normal closing and breaking operations. Conversely, the *rated service short-circuit breaking capacity* (Ics) refers to the ability of a circuit breaker to perform multiple normal interruptions when the maximum three-phase short-circuit current occurs at its output terminals.
The IEC 947-2 standard, *Low-voltage switchgear and controlgear - Part 2: Circuit-breakers*, stipulates that for Type A circuit breakers (defined as those featuring only long-delay overload protection and instantaneous short-circuit protection), the Ics value may be 25%, 50%, 75%, or 100% of the Icu value. For Type B circuit breakers (defined as those featuring three-stage protection: long-delay overload, short-delay short-circuit, and instantaneous short-circuit protection), the Ics value may be 50%, 75%, or 100% of the Icu value. Consequently, it is evident that the rated service short-circuit breaking capacity represents a breaking current value that is lower than the rated ultimate short-circuit breaking capacity.
Generally speaking, circuit breakers equipped with the three-stage protection functions-long-delay overload, short-delay short-circuit, and instantaneous short-circuit-are capable of achieving selective protection; as such, they are widely adopted as the primary protection switches for most main feeder lines (including the output terminals of transformers). Conversely, circuit breakers lacking the short-delay short-circuit function (i.e., those featuring only two-stage protection: long-delay overload and instantaneous short-circuit) cannot provide selective protection and are therefore suitable only for use in branch circuits.
The IEC 92 standard, *Electrical installations in ships*, notes that for circuit breakers featuring three-stage protection, greater emphasis is placed on their *rated service short-circuit breaking capacity* (Ics) value; conversely, for circuit breakers utilized in branch circuits, it is essential to ensure that they possess a sufficient *rated ultimate short-circuit breaking capacity* (Icu) value.
Regardless of the specific type, all circuit breakers possess both the Icu and Ics as key technical parameters. However, for circuit breakers deployed in branch circuits, satisfying the requirements for the *rated ultimate short-circuit breaking capacity* (Icu) alone is generally deemed sufficient. A common misconception in practice is a tendency to select a device with an unnecessarily high rating-opting for "larger" rather than "precisely appropriate"-under the mistaken belief that a higher rating inherently provides a greater margin of safety. However, selecting a capacity that is excessively high results in unnecessary waste (for circuit breakers of the same type, the "H" series-high-breaking capacity-costs 1.3 to 1.8 times more than the "S" series-standard capacity). Therefore, for circuit breakers installed on branch circuits, there is no need to blindly pursue the highest possible rated service short-circuit breaking capacity. Conversely, for circuit breakers used on main feeder lines, it is essential to satisfy not only the requirements for rated ultimate short-circuit breaking capacity but also those for rated service short-circuit breaking capacity; relying solely on the rated ultimate short-circuit breaking capacity (Icu) as the criterion for determining breaking capability adequacy creates potential safety hazards for the user.
A circuit breaker is a fundamental type of low-voltage electrical apparatus. It incorporates protection functions against overload, short circuit, and undervoltage, thereby possessing the capability to safeguard both the electrical circuit and the power source.
Its primary technical parameters are rated voltage and rated current. Depending on the specific application, circuit breakers offer a diverse range of functions; they are available in numerous varieties and specifications, each characterized by a distinct set of technical parameters.
Circuit Breaker Free Tripping: This refers to the capability of a circuit breaker to reliably and completely interrupt the circuit at any point during the closing operation, should a protective mechanism activate and energize the tripping circuit. Circuit breakers equipped with this "free tripping" feature ensure that, in the event a short-circuit fault occurs during the closing process, the device can rapidly interrupt the current flow, thereby preventing the incident from escalating in scope.
