November 28, 2023


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How to Select Air Circuit Breakers

High Current

Introduction: Ensuring Safety and Efficiency in Your Electrical System

By protecting against overloads, short circuits, and other electrical mistakes, air circuit breakers (ACBs) are vital parts of electrical systems. Selecting the right ACB for your specific application is crucial to ensure the safety and efficiency of your electrical installation. This article provides a comprehensive guide on how to select air circuit breakers, including important factors to consider and practical calculations with examples.

Find the Rated Current: Determining the Optimal Capacity

 It should be selected based on the maximum expected load in the circuit. To determine the rated current, calculate the total load connected to the circuit by summing up the individual currents of all the devices. It is essential to consider any future expansions or additions to the electrical system.

An example is a circuit that has three devices: Device A (50 Amp), 

Device B (30 Amp), 

and Device C (40 Amp).

 There would be a 120 A total load, or 50 A plus 30 A plus 40 A. The rated current of the ACB in this situation must be 120 Amps or more.

Consider the Short-Circuit Breaking Capacity: Ensuring Effective Fault Current Interruption

The short-circuit breaking capacity (SCBC) of an ACB indicates its ability to interrupt a fault current. It is crucial to select an ACB with an SCBC that exceeds the maximum short-circuit current in the system. Calculate the prospective short-circuit current using the following formula:

Prospective Short-Circuit Current (Isc) = V / Z


V is the rated voltage of the system.

Z is the total impedance of the circuit.

Example: Assume the system’s rated voltage is 480 V and its total impedance is 0.2 ohms.

 The prospective short-circuit current would be Isc = 480 V / 0.2 ohms = 2400 A. In this case, the ACB’s SCBC should be equal to or greater than 2400 A.

Consider the Time-Current Characteristic (TCC): Analyzing Response and Tripping Time

The TCC curve represents the response of an ACB during a fault condition, indicating the time it takes for the breaker to interrupt the fault current. The TCC should be analyzed to ensure that the ACB’s tripping characteristics are compatible with the system requirements.

Example: TCC curves are typically provided by ACB manufacturers. By analyzing the TCC curve, you can determine the ACB’s tripping time for a specific fault current level and select one that meets your application’s requirements.

Evaluate Additional Features: Enhancing Performance and Functionality

Consider additional features and functionalities that may be important for your specific application. Some common features include adjustable trip settings, remote operation capabilities, fault indication, and communication interfaces. Assessing these features will help optimize the performance and functionality of the ACB in your electrical system.

Conclusion: Optimal ACB Selection for Enhanced Safety

It’s essential to choose a suitable air circuit breaker (ACB) to maintain the safety and functionality of your electrical system.

 By considering factors such as rated current, short-circuit breaking capacity, time-current characteristic, and additional features, you can make an informed decision. Remember to perform calculations, as demonstrated in the examples, to accurately determine the requirements of your system. Always consult with ACB manufacturers and electrical professionals to ensure the selected ACB is suitable for your specific application. With careful consideration and proper selection, you can effectively protect your electrical system and minimize the risk of electrical faults.