How to read the parameters of the Air circuit breaker ACB?

Let’s look at the photo below:

The following explains the meaning of these parameters.

1. Understanding the Internal Structure of ACB.

Let's look at the diagram below:

In Figure 2, we see the internal structure of a ACB. Inside the body of the frame circuit breaker, there are moving contacts, flexible connections for moving contacts, conductive busbars for moving contacts, stationary contacts and their conductive busbars, operating mechanisms, and control panels. We can also see the frame and internal rails of the ACB. Because the body of the frame circuit breaker can be withdrawn from the frame, giving it an isolation function, the ACB shown in Figure 2 is called a "with-drawable ACB."

2. Two National Standards Closely Related to the Questioners Topic

The two standards are:

· GB/T 14048.1-2023: Low-voltage switchgear and controlgear – Part 1: General rules

· GB/T 14048.2-2020: Low-voltage switchgear and controlgear – Part 2: Circuit breakers

Note 1:?The IEC60947-2:2019 notation at the bottom of Figure 4 explains the source of the national and IEC standards referenced in the questioner's Figure 1.

Note 2:?Many definitions and parameters in GB/T 14048.2-2020 directly use those in GB/T 14048.1-2012, now updated to GB/T 14048.1-2023. Therefore, this post interprets the parameters of ACBs based on GB/T 14048.1-2023.

3. Electrical Clearance and Rated Voltage Ue

In GB/T 14048.1-2023, the definition of rated voltage for switching devices is:

The rated voltage Ue, in combination with the rated current, determines the voltage value for which the device is intended. For single-pole devices, the rated voltage refers to the voltage across the poles. For multi-pole devices, it refers to the phase-to-phase voltage or line voltage.

Let's revisit the ACB in Figure 2:

The width of the gap between the moving and stationary contacts in Figure 5 is called the breaking gap. When voltage is applied, this gap ensures that the air within it can withstand the applied voltage without breakdown. When the circuit breaker interrupts a circuit, an arc forms between the moving and stationary contacts. As the arc is directed into the arc chute and extinguished, the contact gap gradually increases until it reaches the maximum breaking gap, ensuring the air within the gap is not broken down.

The distance between conductors of different poles, between conductors and the metal casing, and the breaking gap between the moving and stationary contacts are collectively referred to as electrical clearance.

The rated voltage Ue corresponds to the electrical clearance required to prevent air breakdown.

4. Rated Insulation Voltage Ui

Theory and practice show that when a circuit breaker conducts current, the heating of the conductor reduces the breakdown voltage of air. Therefore, rated voltage and rated current are typically paired values. As the rated current decreases, the rated voltage can increase. The maximum value of the rated voltage is referred to as the rated insulation voltage, denoted as Ui.

The definition Ui of in GB/T 14048.1-2023?is:

The rated insulation voltage Ui is essentially the maximum value of the rated voltage Ue.

When using a frame circuit breaker, the applied voltage must not exceed Ui.

5. Rated Impulse Withstand Voltage Uimp

The definition of Uimp in GB/T 14048.1-2023 is:

Notice that Uimp is also related to electrical clearance.

6. Rated Current Ie

The definition of rated current (or rated operational current) in GB/T 14048.1-2023 is:

When rated current flows through the conductive structures of a ACB, heat is generated, raising the temperature of the conductive structures and the overall device. This increase in temperature affects the mechanical strength of the conductive structure and the insulating capability of insulating materials.

In Figure 10, the external connection terminals refer to the primary terminals of the moving and stationary contacts of the circuit breaker. We see that the temperature rise of the primary terminals is 80 K. Assuming the ambient temperature is 35°C, the maximum temperature of the terminals would be 80 + 35 = 115°C.

Reminder 1: Temperature rise refers to the difference between the surface temperature of an object and the ambient temperature. If the temperature is measured in Celsius (°C), the unit for temperature rise is also °C. If the temperature is measured using the Kelvin scale (K), the unit for temperature rise is K.

Regardless of the unit (°C or K), the numerical value of the temperature rise remains the same. For example, if the surface temperature of an object is 40°C and the ambient temperature is 35°C, the temperature rise τ=40?35=5°C. Converting this to Kelvin:

τ=(40+273.15)?(35+273.15)=313.15?308.15=5K

As shown, although the units differ, the numerical value of the temperature rise is identical. Therefore, the temperature rise value in Kelvin can be added to the temperature value in Celsius to determine the surface temperature of an object.

Reminder 2: The temperature rise of the conductive structure in a circuit breaker is highest at the connection terminals. Hence, national standards specify the temperature rise at the connection terminals as the maximum allowable temperature rise for circuit breakers.

The rated current and rated voltage of a ACB are interdependent. Specific technical parameters of the circuit breaker are required to determine these values. For the circuit breaker in figure 1, the rated current at a rated voltage of 690 V is 1000 A.

7. Ultimate Short-Circuit Breaking Capacity Icu, Operational Short-Circuit Breaking Capacity Ics, and Short-Time Withstand Current Icw.

The Icu value of a frame circuit breaker is highly significant and directly influences the selection of the circuit breaker. Let us consider an example:

Assume a power transformer with the following specifications:

• Capacity: 2000 kVA
• Low-voltage side line voltage: 400 V
• Impedance voltage: 6%

We will calculate the transformer’s rated current (In) and short-circuit current (Ik), and determine a suitable frame circuit breaker for the low-voltage incoming line.

Calculating the Rated Current (In):

Calculating the Short-Circuit Current (Ik):

Based on these calculations:

• The rated current of the selected frame circuit breaker must be greater than or equal to 3000 A.
• The ultimate short-circuit breaking capacity (Icu) must be greater than 48.1 kA.

Therefore, the selected frame circuit breaker must have a rated current greater than or equal to 3000 A, and its ultimate short-circuit breaking capacity Icu must be greater than 48.1 kA.

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