Circuit Breaker - Basic Properties, Terminology and Theory
What is a Circuit Breaker?
Circuit Breaker is a reusable overcurrent protection device. After tripping to break the circuit, the breaker can be reset to protect the circuit again. NEMA defines a circuit breaker 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 states that a circuit breaker is a mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions. Also, it is capable of making and carrying a current 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 these circuit breaker definitions, although the word is slightly different. From a very broad perspective, all circuit breakers have the following common design and functional characteristics:
- Frame
- Contacts and operating mechanism
- Trip unit (device that trips the operating mechanism in case of a short circuit or overload condition)
- Method to extinguish an arc (an effect generated when electrical current bridges the air gap between two conductors that are not touching)
- Method for mounting
- Compliance with specific standards
Circut Breaker Frame
The rigid circuit breaker frame provides the strength 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 (l²), which could be quite large and potentially destructive. The frame also provides for insulation and isolation of the current path, offering personnel protection near the equipment during operation. The frame also plays a critical role in the circuit breaker's ability to comply with applicable standards
Frames are available in:
- Metal (for power circuit breakers above 600 volts)
- Molded insulating materials (low voltage and insulated case breakers - also above 600 volts for power circuit breakers)
Circut Breaker 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. A contact 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. The simple function they perform is opening and closing.
Circuit breakers require some type of operating mechanism to open and close contacts. It can be mechanical or a combination of mechanical and power. Examples include:
- Open and close contacts manually
- Open and close contacts on demand
- Open contacts automatically
Three phase circuit breakers are designed so all three sets of contacts open or close simultaneously.
Circuit breakers are available in various trip curves/characteristics, ac or dc rated. Some circuit breakers are listed as finger-safe, din-rail mountable or quick-connect. There are several types of breakers available including low voltage at 1,000V and below, molded case that range from 600V and under, as well as miniature and sub-miniature breakers at 240v or less. Miniature types range from 0.2-63 amps, Sub-miniature in 0.5-35 amps with push button reset in the offering, and molded case with 10-2500 amps availability.