Introduction

What is a circuit breaker? Definitions vary depending upon where you read the definition.

Definition

 A circuit breaker is defined as a device designed to open and close a circuit by non-automatic means, and to open the circuit automatically on a predetermined overcurrent without injury to itself when properly applied within its rating.

Characteristics

all circuit breakers have the following common design and functional characteristics:

• A frame
• Contacts and operating mechanisms
• Trip Units
• A method to extinguish an Arc
• A method for mounting
• Compliance with specific standards

The first four characteristics are components of the circuit breaker, which can be seen in the figure below. Let's look at each component separately.

Figure 3. Characteristics of a Circuit Breaker

1-Frame

The rigid circuit breaker frame provides a method by which all the required components can be mounted and kept in place, ensuring the proper operation of the circuit breaker.

The circuit breaker frame provides the rigidity and strength required to successfully deal with the interruption process and achieve the desired Interrupting Ratings. The frame's mechanical strength must be sufficient to withstand the forces created by the square of the current (I²), which could be quite large and potentially destructive.

Figure 4. Circuit Breaker Frame

The frame also provides for insulation and isolation of the current path, offering personnel protection near the equipment during operation

Historically, there are two types of frames:

• Metal Frame
• Molded Insulating Material

2-Contacts and Operating Mechanism

Contacts

Contacts in a circuit breaker provide a method for connecting the circuit with the system. They also provide a method for isolating a part of a circuit from the rest of the system.

Figure 7. Contact and Operating Mechanism

A contact set contains a fixed and movable contact. As a circuit breaker opens or closes, the fixed contact maintains its position while the movable contact moves to close (make) or open (break) the circuit. When all is said and done, contacts perform a simple function; they open and close.

Operating Mechanism

Circuit breakers require some type of operating mechanism to open and close the contacts. This operating mechanism can be mechanical or a combination of mechanical and power. Depending upon the type of circuit breaker being considered, the operating mechanism could be called upon to:

• Open and close the contacts manually
• Open and close the contacts on demand
• Open the contacts automatically

Let's consider a basic Three-Phase circuit breaker. It is designed such that all three sets of contacts open or close simultaneously.

This requires that all the contacts be linked together in some manner. This part of the mechanism might be connected mechanically to a common handle. The handle, when operated, puts the mechanism into motion and opens or closes the circuit breaker by opening or closing the contacts.

Figure 9. Three-Phase Contacts

This additional assistance takes the form of springs. Springs play a big role in the precise functioning of circuit breaker mechanisms. Springs are stretched or compressed to provide the energy necessary to assist with the proper opening or closing of the contacts.

3- Trip Units

For a circuit breaker to be effective, it needs to have some intelligence to enable it to perform automatically or respond to a command. Without this capability, a circuit breaker would just be a fancy switch. A trip unit is the circuit breaker's intelligence. We will discuss the function and the types of trip units.

Figure 12. Circuit Breaker Trip Units

The trip unit's function is to trip the operating mechanism (open the circuit) in the event of these overcurrent conditions:

• Thermal Overload
• Short Circuit Currents (Fault Current)
• Ground Fault