Back To Basics

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

Definition

NEMA Definition: A circuit breaker is defined in NEMA standards 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.

ANSI Definition: A circuit breaker is defined in ANSI standards as a mechanical switching device, capable of making, carrying and breaking currents under normal circuit conditions. Also capable of making and carrying for a specified time and breaking currents under specified abnormal circuit conditions such as those of a short circuit.

The premise is the same for both definitions and both are accurate, but the wording is quite different. The same is true with the many types of circuit breakers. Their general purpose is the same, but there are also a number of significant differences.

The circuit breaker might be applied as an individual item in its own enclosure, or be utilized in conjunction with a wide variety of other equipment in a common enclosure such as Loadcenters, Panelboards and Switchboards.

Figure 2. Voltage Panel, Switchboard and Switchgear

No matter where or how it is applied, you will soon realize that the circuit breaker plays a pivotal role in the movement, distribution and use of electric power.

Characteristics

From a very broad perspective, all circuit breakers have the following common design and functional characteristics:

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