Circuit Breaker Basics: Understanding How They Work

This page covers circuit breaker basics, including application notes and a list of manufacturers/vendors. As we all know, circuit breakers are essential devices for our safety at home and in the office.

This device cuts off the supply of AC power to the premises when it notices any unexpected situations such as short circuits, equipment failures, or heavy current in the electric wiring. Without circuit breakers, our lives would be in danger as it may lead to fires. It may also damage costly equipment and other devices connected to the AC power lines during untoward situations.

Circuit Breaker Application Note

Before diving into how circuit breakers work, let’s understand situations like overload and short circuit, which necessitate the use of fuses and circuit breakers.

• Overload: This situation occurs when more than one device operates in a single electric circuit. An overload can also arise when equipment is driven beyond its rated designed capabilities. When this situation arises, the circuit should be shut down or cut off by the overcurrent protection device; otherwise, it will damage the equipment connected in the electric circuit line.
• Short Circuit: This situation occurs when a low-resistance path gets created due to the touching of open conductors. A decrease in resistance results in a short circuit current, which will be thousands of times greater than the normal condition current. Due to this, heat gets generated and will often result in a fire in the premises. Under this condition, current should be immediately stopped from flowing.

This situation can be understood with Ohm’s law as explained below.

Ohm’s Law describes the relationship of current, voltage, and resistance. For example, a 240-volt circuit with 24 Ω of resistance draws 10 amps of current. When a short circuit develops, resistance drops. If resistance drops to 24 milliohms, current increases to 10,000 amps.

To protect a circuit against over-currents, a protection device must detect the faulty situation and automatically disconnect the electrical equipment from the voltage source. An overcurrent protection device must be able to recognize the difference between overloads and short circuits and respond accordingly. Small overloads can be allowed to continue for a short time, but larger overloads require a quicker response, and short circuits must be interrupted instantaneously.

• Fuse: It is a one-shot device. The heat produced by overcurrent causes the current-carrying element to melt open, disconnecting the load from the source voltage.

• Circuit Breaker: Circuit breakers provide a manual means of energizing and de-energizing a circuit and automatic overcurrent protection. Unlike fuses, which must be replaced when they open, a circuit breaker can be reset once the overcurrent condition has been corrected. Pushing the handle to the “OFF” position then back to the “ON” position restores the circuit. If a circuit reopens upon reset to the “ON” position, the circuit should be checked by a qualified electrician.

All circuit breakers perform the following functions:

• SENSE when an overcurrent occurs.
• MEASURE the amount of overcurrent.
• ACT by tripping in a timely manner to prevent damage to the circuit breaker and the conductors it protects.

Figure-1 depicts the application of a circuit breaker. As shown in the figure, it is connected in series with the electric circuit. The Circuit breaker is usually in a closed condition, and hence AC current is being provided to the motor. The Circuit breaker gets open when heavy current flows or short circuit condition occurs and hence prevent damage to the motor.

Circuit Breaker Manufacturers/Vendors

Following are the circuit breaker manufacturers or vendors:

• E.T.A.
• Altech Corporation
• LS Industrial Systems
• Precision Controls Co.
• Mechanical Products Co.
• Carling technologies
• Airpax
• Schurter electronics Pvt. Ltd
• American Electrical, Inc.
• MITSUBISHI ELECTRIC