In the process of selection, use and maintenance of circuit breakers, "breaking capacity" is a core and easily confused key parameter—especially the three letter combinations Icu, Ics, and Icw. Although they seem similar, they correspond to different safety performances and application scenarios of circuit breakers. Many purchasers and buyers often confuse the definitions and differences of the three when selecting products, resulting in the selected circuit breakers failing to match the actual power distribution needs. At best, this causes equipment failures; at worst, it leads to potential safety hazards such as electrical fires and personal injuries.
Starting from the basic definitions, this article will break down the core differences between the three parameters, explain their selection logic and practical significance in detail, help you completely clarify the relationship between them, avoid mistakes in selection.
I. Basic Concept Foundation: What is the "Breaking Capacity" of a Circuit Breaker?
Before interpreting Icu, Ics, and Icw, we first clarify a basic concept—breaking capacity. Popularly speaking, breaking capacity refers to the maximum current value that a circuit breaker can safely interrupt a fault current (mainly short-circuit current) under specified conditions (such as voltage level, short-circuit type, ambient temperature, etc.) without being damaged itself and being able to maintain normal operation (or meet the specified subsequent service requirements).
Simply put, breaking capacity is the "hard power" of a circuit breaker to "withstand fault current and safely interrupt faults". The three parameters Icu, Ics, and Icw define the breaking capacity of the circuit breaker in different fault scenarios from different dimensions. They complement each other and jointly determine the safety level and application scope of the circuit breaker.
II. Detailed Analysis: Definitions and Core Functions of Icu, Ics, and Icw Respectively
(I) Icu: Ultimate Interrupting Capacity
Definition: Icu is the "ultimate bearing capacity" of a circuit breaker, referring to the maximum short-circuit current value that the circuit breaker can interrupt once under specified test conditions. The key here is "once"—after interrupting this current, the circuit breaker is allowed to be damaged, unable to be reused, or even unable to be closed again. It only requires that no danger such as explosion or fire occurs during the interruption process.
Core Function: Icu is the "safety bottom line" of the circuit breaker, used to measure the "impact resistance" of the circuit breaker under the most serious short-circuit faults (such as three-phase short circuit). It determines whether the circuit breaker can quickly interrupt the current and terminate the spread of faults in extreme fault scenarios, preventing faults from expanding into electrical fires or equipment damage.
Popular Analogy: Icu can be understood as the "maximum bearing limit" of the circuit breaker, just like the maximum weight a person can lift—lifting it once may require all efforts, after which the arms will be sore and unable to lift again, but as long as it can be lifted, the "extreme task" is completed. For a circuit breaker, as long as it can interrupt the current corresponding to Icu, even if it is damaged itself, it meets the assessment standard of Icu.
Supplementary Note: Icu is a core parameter that must be marked by the circuit breaker manufacturer when it leaves the factory. The Icu values of circuit breakers of different models and voltage levels vary greatly (for example, the Icu of low-voltage circuit breakers may range from tens of kiloamperes to hundreds of kiloamperes). The larger the Icu value, the higher the requirements for the structural design and material technology of the circuit breaker, and the higher the cost.
(II) Ics: Service Interrupting Capacity
Definition: Ics is the "practical breaking capacity" of a circuit breaker, referring to the maximum short-circuit current value that the circuit breaker can safely interrupt under specified test conditions, and can still maintain normal working capacity and be closed for reuse after interruption. Unlike Icu, Ics requires that the circuit breaker is not damaged and its performance is not affected after interrupting the current, and can be put back into operation in the power distribution system.
Core Function: Ics is the core embodiment of the "practicality" of the circuit breaker, used to measure the "sustainable working capacity" of the circuit breaker under common short-circuit faults. In actual power distribution scenarios, short-circuit faults may occur occasionally. If the circuit breaker is damaged after interrupting the fault current, it needs to be replaced frequently, which will increase maintenance costs and affect power supply continuity. Ics ensures that the circuit breaker can still be used normally after interrupting the fault.
Popular Analogy: If Icu is "lifting with all one's strength once", then Ics is "being able to lift and lift again"—for example, a person can easily lift 50 kilograms, and after lifting, the arms are not sore and can continue to lift. This 50 kilograms is his "practical lifting capacity". For a circuit breaker, the current corresponding to Ics is the maximum current that it can safely interrupt and still work normally afterwards.
Supplementary Note: Ics is always less than or equal to Icu (because Ics has higher requirements for the circuit breaker, which not only needs to interrupt the current, but also ensure subsequent reuse). Usually, the value of Ics is 60%~100% of Icu (the ratio varies for circuit breakers of different brands and models). For example, if the Icu of a certain circuit breaker is 100kA and Ics is 80kA, it means that it can safely interrupt 80kA short-circuit current and continue to be used. If it encounters an extreme short-circuit current of 100kA, it will be damaged after interruption and cannot be used again.
(III) Icw: Short-Time Withstand Current
Definition: Icw is the "short-time overload resistance" of a circuit breaker, referring to the maximum short-circuit current value that the circuit breaker can withstand within a specified time (usually 1 second, 2 seconds or 3 seconds, specifically specified by standards). During the period of withstanding this current, the structure of the circuit breaker is not damaged, the insulation is not broken down, and it can maintain its own mechanical and electrical performance.
Core Function: Icw is mainly used to cooperate with the "selective protection" of the power distribution system, preventing the circuit breaker from malfunctioning instantly when a short-circuit current occurs, and ensuring the coordination of the action between the upper-level and lower-level circuit breakers. Simply put, when a short-circuit fault occurs, the short-circuit current will increase instantaneously. Icw ensures that the circuit breaker can "withstand" this current for a short time, leaving time for the upper-level or lower-level circuit breakers to act, realizing "selective tripping" and avoiding large-area power outages.
Popular Analogy: Icw is like a person being able to "withstand heavy objects for a short time"—for example, carrying a 50-kilogram heavy object for 10 seconds. Although it cannot be carried for a long time, it will not be crushed in a short time. For a circuit breaker, the current corresponding to Icw is the maximum current that it can withstand within a specified time without being damaged, and can cooperate with other circuit breakers to achieve selective protection.
Supplementary Note:
①The core difference between Icw and Icu, Ics is that Icu and Ics emphasize "interrupting current", while Icw emphasizes "withstanding current" (not interrupting, only withstanding).
②The value of Icw is usually related to Icu, generally a certain proportion of Icu, and the longer the withstand time, the smaller the Icw value (for example, for the same circuit breaker, the 1-second withstand current may be 80kA, and the 3-second withstand current may be 63kA).
III. Key Distinction: Summary of Core Differences Between Icu, Ics, and Icw
To more clearly compare the differences between the three, we summarize and sort them out from four dimensions: "core requirements, state after interruption, core functions, and numerical relationship" to avoid confusion:
1. Different Core Requirements
● Icu: Only requires "interrupting the maximum short-circuit current once", does not require subsequent reuse, and focuses on "interrupting faults and avoiding the spread of dangers";
● Ics: Requires "the circuit breaker can still work normally and be closed again after interrupting the short-circuit current", focusing on "practicality and sustainability";
● Icw: Requires "withstanding short-circuit current within a specified time", does not interrupt the current, and focuses on "cooperating with selective protection and avoiding malfunction".
2. Different States After Interruption
● Icu: After interrupting the current, the circuit breaker is allowed to be damaged and cannot be reused (such as contact burnout, mechanism damage);
● Ics: After interrupting the current, the circuit breaker is not damaged, its performance is normal, and it can be put back into use;
● Icw: Does not interrupt the current, only withstands the current. After the withstand period (if the fault is not eliminated, it needs to be interrupted by other circuit breakers), the circuit breaker can still work normally.
3. Different Core Functions
● Icu: Ensures safety under extreme fault scenarios and is the "safety bottom line" of the circuit breaker;
● Ics: Ensures power supply continuity under common fault scenarios, reduces maintenance costs, and is the "practical core" of the circuit breaker;
● Icw: Ensures selective protection of the power distribution system, avoids large-area power outages, and is the "coordination core" of the circuit breaker.
4. Different Numerical Relationships
Priority: Icu ≥ Ics (Ics is a subset of Icu, which has higher requirements for circuit breaker performance);
Icw and Icu: Usually, Icw is a certain proportion of Icu (such as 60%~80%), and the longer the withstand time, the smaller the Icw value;
There is no fixed "which is larger or smaller" among the three (except Icu ≥ Ics), which needs to be determined according to the circuit breaker model and standards. The core is to match the power distribution scenario requirements.
IV. Significance of Selection: Why Must We Distinguish Between Icu, Ics, and Icw?
Distinguishing these three parameters directly determines the scientificity, safety and economy of circuit breaker selection—if selected correctly, it can ensure the stable operation of the power distribution system and avoid potential safety hazards; if selected incorrectly, it will either increase costs, cause faults, or even lead to safety accidents. The specific significance of selection can be divided into the following 3 points:
1. Ensure the Safety of the Power Distribution System and Avoid Potential Safety Hazards
If Icu is ignored during selection and a circuit breaker with Icu less than the actual possible short-circuit current is selected, when an extreme short-circuit fault occurs, the circuit breaker cannot interrupt the fault current, which will cause contact burnout, circuit breaker explosion, and even electrical fires and personal injuries; if Icw is ignored, the circuit breaker cannot withstand the short-time short-circuit current, and will malfunction when a fault occurs, destroying selective protection and leading to large-area power outages.
⚡️For example: The short-circuit current of an industrial workshop may reach 80kA. If a circuit breaker with Icu=63kA is selected, when an extreme short circuit occurs, the circuit breaker cannot interrupt the current, will be directly damaged, and cause a safety accident; if a circuit breaker with Icu=100kA, Ics=80kA, and Icw=63kA (1 second) is selected, it can meet the requirements of extreme fault interruption, continuous use in common faults, and cooperation with selective protection.
2. Improve Power Supply Continuity and Reduce Maintenance Costs
The selection of Ics directly affects power supply continuity and maintenance costs. If a circuit breaker with too small Ics is selected, the circuit breaker will be damaged every time the short-circuit current is interrupted, requiring frequent replacement, which not only increases procurement costs, but also causes power supply interruptions and affects production and life; if a circuit breaker with appropriate Ics is selected, it can still be used normally after interrupting the fault current, reducing the replacement frequency, lowering maintenance costs, and ensuring stable power supply.
⚡️For example: In the power distribution system of commercial buildings, short-circuit faults occur occasionally. If a circuit breaker with Ics=63kA (matching the actual common short-circuit current of 50kA) is selected, it can still be used normally after each fault interruption without frequent replacement; if a circuit breaker with Ics=40kA is selected, it will be damaged after interrupting 50kA current and needs to be replaced frequently, increasing maintenance costs.
3. Match Scenario Requirements and Achieve Economical Selection
The higher the values of Icu, Ics, and Icw, the higher the cost of the circuit breaker. Blindly pursuing high parameters will cause cost waste; if the parameters are too low, they cannot meet the scenario requirements and will cause potential safety hazards. Therefore, distinguishing the differences between the three and selecting according to the actual scenario can achieve "both safety and economy".
⚡️For example: In the residential power distribution system, the short-circuit current is usually small (generally not exceeding 31.5kA). A circuit breaker with Icu=40kA, Ics=25kA, and Icw=20kA can meet the requirements. There is no need to select a high-parameter circuit breaker with Icu=100kA to avoid cost waste; while in scenarios such as industrial workshops and substations, the short-circuit current is large, and circuit breakers with high Icu, high Ics, and high Icw need to be selected to ensure safety.
V. Practical Selection Notes
Combined with the previous analysis, we add 3 practical selection points to help everyone quickly select in actual work and avoid mistakes:
● First determine the "maximum short-circuit current" of the actual scenario: Before selection, calculate or test on site to determine the maximum short-circuit current that may occur in the power distribution system, ensuring that the Icu of the circuit breaker is greater than or equal to this current, which is the premise of selection;
● Select Ics according to "power supply continuity requirements": If the scenario has high requirements for power supply continuity (such as industrial production, hospitals, data centers), select a circuit breaker with Ics close to Icu (such as Ics=Icu or Ics=80%Icu) to ensure that it can be used continuously after interrupting the fault; if the scenario has low requirements for power supply continuity (such as ordinary residences), a circuit breaker with relatively small Ics can be selected to reduce costs;
● Select Icw in combination with "selective protection": According to the coordination requirements of the upper and lower circuit breakers in the power distribution system, determine the value and withstand time of Icw, ensuring that the circuit breaker can withstand the short-circuit current within the specified time, cooperate with the upper and lower circuit breakers to achieve selective tripping, and avoid large-area power outages.
VI. Summary
As the core parameters of the breaking capacity of circuit breakers, the core difference between Icu, Ics, and Icw lies in "whether reuse is required after interruption" and "whether to interrupt current or withstand current":
Icu is the ultimate interrupting capacity (damaged after interruption), Ics is the service interrupting capacity (can be reused after interruption), and Icw is the short-time withstand current (does not interrupt, only withstands).
Distinguishing the definitions and differences of the three can not only help everyone select scientifically and avoid potential safety hazards, but also achieve a balance between economy and safety—for all buyers, mastering this knowledge can help them select circuit breakers suitable for their own scenarios more accurately.
And the most important is if you have any questions about the matching of Icu, Ics, and Icw during the circuit breaker selection process, please leave a message for consultation. We will provide professional selection suggestions based on your specific scenario.
Post time: Feb-27-2026
