Meaning of Circuit Breaker Parameters: Detailed Explanation of High Voltage and Low Voltage Circuit Breaker Parameters

Meanings of Low-Voltage Circuit Breaker Parameters

1. The rated current In of a circuit breaker is the maximum continuous current it can carry under specified ambient temperature conditions without abnormal heating, ensuring normal operation of the circuit breaker. It is also referred to as the tripping device's rated current.

2. Frame size rating current. It represents the size grade of the circuit breaker, indicating the maximum rated current of the circuit breaker.

3. Circuit breaker short-circuit breaking capacity:

The ultimate short-circuit breaking capacity (Icu) refers to the ability to fully interrupt and extinguish an arc under specified voltage, current, and power factor conditions, following two test procedures (o-co). It ensures no damage beyond prescribed limits (contact wear and arcing damage). After testing, the specimen must withstand a certain power frequency withstand voltage test, and the overload trip unit must operate correctly at a specified setting current. This capacity meets the conditions defined by the prescribed test procedures but does not include the breaking capacity of the circuit breaker's continuous ability to carry its rated current. It is expressed as the prospective breaking current (kA) (for AC conditions, the RMS value of the AC component is used).

Rated service (operating) short-circuit breaking capacity Ics. It refers to the ability to completely interrupt the current, extinguish the arc, and avoid exceeding specified damage under defined voltage, current, and cos Φ conditions, following the o-co-co test sequence. After testing, the specimen must not only meet the required power frequency withstand voltage and overload trip verification tests but also undergo assessments of temperature rise and operational performance (5% electrical life verification). The specified test sequence conditions must be met, including the circuit breaker's continuous current-carrying capability. This breaking capacity is expressed in terms of the prospective breaking current (kA), corresponding to a specific percentage of the rated ultimate short-circuit breaking capacity (Icu), rounded to the nearest integer. It can be represented as a percentage of Icu (e.g., Ics = 25% Icu). On the other hand, when the rated service short-circuit breaking capacity equals the rated short-time withstand current, it can be specified as the rated short-time withstand current value (kA), provided it is not less than the corresponding minimum value. If the Icu for Category A exceeds 200 kA or for Category B exceeds 100 kA, the manufacturer may declare the Ics value as 50 kA.

It is evident that the Ics rating is more stringent than the Icu rating. In other words, after passing the Ics test, the product can still be used, whereas after the Icu test, it cannot. When selecting a circuit breaker, we must ensure it can reliably interrupt fault current without causing the fault to escalate, while also maintaining power continuity after the fault. Generally, the short-circuit breaking capacity of the circuit breaker is selected based on the Ics rating.

Rated short-time withstand current (Icw). The maximum short-circuit current that a circuit breaker can withstand without characteristic changes during a short period (0.5 or 1 second). It reflects the short-circuit thermal stability performance of the circuit breaker under short-term conditions.

Due to different usage scenarios, important circuit breakers with three-stage protection focus more on their rated short-circuit breaking capacity, while those used for non important circuit breakers mainly ensure that they have sufficient ultimate short-circuit breaking capacity. My understanding of this is that after cutting off the fault current in the important circuit, the circuit breaker should be able to continue supplying power, carry the rated current for a period of time, and be replaced at an appropriate time. And for an important circuit, after the breaking and re opening of the maximum short-circuit current, its mission has been completed and it can be replaced with a new one after power outage (the impact of power outage is relatively small). However, whether it is a frame type or a molded case circuit breaker, it must have the two important technical indicators of Icu and Ics. However, the performance of Ics values is slightly different between the two types of circuit breakers. The minimum allowable Ics for molded case circuit breakers can be 25 [%] Icu, while the minimum allowable Ics for frame type circuit breakers is 50 [%] Icu. Circuit breakers with Ics=Icu are relatively rare. Molded case circuit breakers using rotating double break (point) technology have excellent current limiting performance and a large margin of breaking capacity, which can achieve Ics=Icu, but at a high price.

In practice, the selection should be based on the usage situation. When someone selects a circuit breaker based on the expected short-circuit current of the calculated line, the rated operating short-circuit breaking capacity of the circuit breaker is used to determine that a certain circuit breaker (whose ultimate short-circuit capacity is greater than the expected short-circuit current of the line, but whose operating short-circuit breaking capacity is lower than the calculated current) is unqualified. This is a misunderstanding.

4. Overload and short-circuit protection characteristics of circuit breakers. In the distribution system protected by circuit breakers, when a fault occurs, the circuit breaker closest to the fault point can selectively operate according to the specified protection characteristics to cut off the fault, while other levels of circuit breakers do not operate, thereby limiting the power outage caused by the fault to the minimum range, so that other fault free power supply circuits can still maintain normal power supply. This is what is required for circuit breaker protection. There are four types of protective release devices equipped with circuit breakers: ① long delay thermal overload protection with inverse time characteristics, ② short circuit short delay protection with a certain time limit, ③ short circuit instantaneous protection, and ④ grounding protection. In daily use, according to the usage intention and technical and economic comparison, four types of protection can be selected, or long delay, instantaneous or short delay three types of protection can be selected to form a three-stage protection, or only long delay and instantaneous two types of protection can be selected as two-stage protection. The short circuit instantaneous opening time is generally within 20-30ms, and circuit breakers with only instantaneous quick break protection can also be selected.

5. Rated working voltage Ue. Generally refers to the voltage between phases. For a three-phase four wire neutral grounding system, it refers to the phase to ground voltage, including the phase to phase voltage (e.g. 277/480V). For a three-phase three wire ungrounded or impedance grounded system, it refers to the phase to phase voltage (e.g. 480V)

Rated frequency of circuit breaker. If it is specified that the circuit breaker is only used for one frequency, the rated frequency should be indicated（

6. Other factors include: usage category, structural form, number of poles, installation method, installation size, rated working system, protection level (if not IP20), reference ambient air temperature (if not 30 ℃); Isolation effect.

The meaning of isolation: for safety reasons, cutting off the power supply of the entire device or one independent part by separating it from all power sources.

Meaning of High Voltage Circuit Breaker Parameters

1. Rated voltage (nominal voltage): It is a parameter that characterizes the insulation strength of a circuit breaker and is the standard voltage for long-term operation of the circuit breaker. In order to meet the requirements of power system operation, circuit breakers also specify the maximum operating voltage corresponding to the rated voltage of each level. For levels 3-220KV, the maximum operating voltage is about 15% higher than the rated voltage; For 330KV and above, the maximum operating voltage is about 10% higher than the rated voltage. The circuit breaker should be able to operate reliably for a long time at the highest operating voltage.

2. Rated current: It is a parameter that characterizes the long-term current capacity of a circuit breaker, that is, the maximum current that the circuit breaker allows to pass continuously for a long time.

3. Rated breaking current: It is a parameter that characterizes the breaking capacity of a circuit breaker. At rated voltage, the maximum current that a circuit breaker can ensure reliable breaking is called the rated breaking current, which is expressed in kiloamperes as the effective value of the periodic component of the short-circuit current at the moment of contact separation of the circuit breaker. When a circuit breaker operates in a power grid below its rated voltage, its breaking current can increase. However, due to the mechanical strength limitation of the arc extinguishing chamber, there is a maximum value for the breaking current, which is called the limit breaking current.

4. Dynamic stable current: It is a parameter that characterizes the ability of a circuit breaker to pass short-term current, reflecting the ability of the circuit breaker to withstand the electrodynamic effects of short-circuit current. The maximum peak current allowed to pass through a circuit breaker in the closed state or at the moment of closure is called electric stability
5. Closing current: It is a parameter that characterizes the closing current capability of a circuit breaker. Because when the circuit breaker is connected, there may be a short-circuit fault in the circuit, and the circuit breaker will close a large short-circuit current. In this way, on the one hand, the electric force of the short-circuit current weakens the closing operation force, and on the other hand, due to the breakdown of the contacts before they make contact, an arc may occur, which may cause the contacts to melt and damage the circuit breaker. The maximum peak current that a circuit breaker can reliably close is called the rated closing current. The rated closing current and dynamic stability current are numerically equal, both equal to 2.55 times the rated breaking current.

6. Thermal stability current and duration of thermal stability current: Steady current is also a parameter that characterizes the ability of a circuit breaker to withstand short-term current, but it reflects the ability of the circuit breaker to withstand the thermal effects of short-circuit current. Thermal stable current refers to the effective value of the maximum periodic component of the current allowed to pass through a circuit breaker in a closed state for a certain duration, at which point the circuit breaker should not be damaged due to short-term heating. According to national standards, the rated thermal stability current of a circuit breaker is equal to the rated breaking current. The duration of the rated thermal stability current is 2 seconds. If it needs to be greater than 2 seconds, it is recommended to use 4 seconds.

7. Closing time and opening time: These are parameters that characterize the operational performance of circuit breakers. The opening and closing times of various types of circuit breakers are different, but they all require quick action. The closing time refers to the time from the closing coil of the circuit breaker operating mechanism to the contact of the main contact. The opening time of the circuit breaker includes two parts: the inherent opening time and the arc extinguishing time. The inherent opening time refers to the period from the opening coil of the operating mechanism being connected to the separation of the contacts. The extinguishing time refers to the period from the separation of the contact to the extinguishing of the arc in each phase. So, the opening time is also known as the full opening time.

8. Operation cycle: This is also an indicator of the performance of circuit breaker operation. Short circuit faults in overhead power lines are mostly temporary, and the fault quickly disappears after the short-circuit current is cut off. Therefore, in order to improve the reliability of power supply and the stability of system operation, circuit breakers should be able to withstand one or more closing, opening, or immediately opening actions after closing. This type of operation that involves multiple opening and closing operations at certain time intervals is called an operation loop.

The rated operating cycle of circuit breakers in China is as follows:

Automatic reclosing operation cycle: open - t '- close/open - t - close/open

Non automatic reclosing operation cycle: open - t - close/open - t - close/open

Among them, 'open' represents the opening action;

Closing and Opening - Refers to the action of opening the circuit immediately after closing; T '- No current interval time, which refers to the time from the arc extinguishing when the circuit breaker disconnects the fault circuit to the automatic reconnection of the circuit. The standard time is 0.3 seconds or 0.5 seconds, which is the time for the reclosing action.

T - Strong power transmission time for operators, with a standard time of 180 seconds