Circuit breakers are automatic electrical switches designed to interrupt current flow in the event of an overload or short circuit.

Electrical circuits can experience several conditions which pose danger to associated equipment or the circuit itself, namely current overload and short circuits. An overload occurs when a larger than anticipated electric current flows through a circuit, generating excess heat and the possibility of a fire. Short circuits (which are a main cause of current overload) occur when current is allowed to travel along an unintended, low impedance path in the circuit, resulting in rapid heat buildup, explosions, and fire.

Fuses — which melt and "blow" in the presence of excess heat from overcurrent — offer effective circuit protection, but must be replaced after each circuit interruption occurs. Circuit breakers perform a similar circuit protection function but can be reset after each interruption and reused indefinitely.

(Other circuit protection devices which are related to circuit breakers include thermal overload relays and ground fault circuit interrupters [GFCI].)

Construction and Operation

Circuit breakers are typically electromechanical switches in which the circuit's hot wire connects to both ends of the switch. When the switch is "on" electricity flows from the input terminal through the switch components and through the output terminal, completing the circuit. In the event of overcurrent the switch linkage breaks, interrupting the circuit.

When a circuit breaks, an operator must determine the source of the overcurrent and correct the problem. Overcurrent may be caused by introducing new devices into a circuit, causing an increase in the cumulative current and overloading the circuit; in this case simply removing the extra device would eliminate overcurrent. Circuit breakers are equipped with a manual actuator, so that an operator may easily reconnect the circuit and restart current flow after it has been interrupted.

Electromechanical circuit breakers found in consumer and commercial applications typically accomplish circuit interruption using either electromagnets or bimetallic strips.

Electromagnetic designs rely on an integral magnet's ability to become increasingly magnetized as associated electrical current increases. When current reaches an unsafe level, it magnetizes the electromagnet to a sufficient degree that it is able to pull down a lever attached to the switch linkage, breaking contact.

An electromagnetic circuit breaker. Image credit: HowStuffWorks

The second design uses a bimetallic strip, which changes in response to heat, to mechanically open the contacts. Bimetallic strips consist of two conjoined pieces of metal which expand at different rates when exposed to heat. This difference forces the strip to bend when heated. In a circuit breaker, the strip is attached by a spring to a contact. When excess heat from overcurrent causes the strip to bend and pull the spring, the contacts are pulled apart and the circuit is broken.

A simplified bimetallic strip design. Image credit: Georgia State University

Arc Protection

Circuit breakers must include some means to extinguish electric arcs which occur between open contacts when current flow is interrupted. The method of containing and cooling the arc varies based on the type and voltage ratings of a breaker; for example, a low voltage device may simply use air, while higher voltage breakers use vacuum, gas, or oil to extinguish arcs. For specific methods please reference the Type section below.

Applications

Circuit breakers are used in a wide variety of scenarios and applications requiring circuit protection. Small, low-voltage breakers are found in every modern residential and commercial building, as well as in DC power systems such as subway lines. Electromagnetic circuit breakers are used in power systems which permit very brief overcurrents, such as motor circuits and energizing equipment. Industrial power distribution substations may use a wide variety of medium- and high-voltage circuit breakers to protect and control power transmission networks.

Types

It is most helpful to classify circuit breakers based on the voltage levels they are capable of handling. Low-voltage and high-voltage devices, for example, are two product types with differing construction and operation techniques.

Note: A device's rated voltage and rated current refer to values experienced during normal operation. A circuit breaker's rated breaking capacity (I_cn ) describes the maximum interruption current the device can tolerate.

Low Voltage

Low voltage breakers protect circuits using less than 1 kV (1000 V) of electricity. Miniature circuit breakers (MCB) are very common low voltage devices which can handle rated current of < 100 A; they typically have static (non-adjustable) trip settings. Most consumer distribution boards are made up of a series of MCBs, with one for every protected circuit. Molded case circuit breakers (MCCB) are a specialized type of low voltage device rated to 2500 A. MCCBs typically feature thermal or thermal-magnetic actuation like MCBs, with the added advantage of allowing adjustable trip currents.

A 3-pole MCB; a 2-pole MCCB; miniature breakers within a residential panel.

Image credit: Miniphysics | Electrical Engineering Portal | Eugbug

Medium Voltage

Medium-voltage circuit breakers are rated between 1 and 72 kV (the standard medium-voltage range). These may be employed indoors within a switchgear panel, or may be installed as part of a power substation. MV breakers typically use current-sensing relays to switch circuits, and may use one of the following means to extinguish arcs:

Vacuum: the arc is contained and extinguished within a vacuum bottle. Vacuum circuit breakers are effective up to approximately 40.5 kV and are rated up to about 6300 A.

Air: the arc is extinguished by air. These devices are rated to 6300 A or higher and are often used in industrial plants or any location where ease of maintenance is key. Air circuit breakers are gradually being replaced by vacuum types due to the latter's longer lifespan.

Sulfur hexafluoride (SF6): the arc is extinguished within a chamber filled with sulfur hexafluoride gas, which has a high affinity for absorbing free electrons. While SF6 is an effective arc extinguisher, it is also a greenhouse gas and its emission is becoming increasingly regulated. Additionally, pumping the gas requires more mechanical energy than other types.

A vacuum circuit breaker (left); operation of an SF6 breaker

Image credit: Electric Energy Online | Solon

High Voltage

High-voltage breakers are typically installed in electrical power transmission networks, which involve voltages in excess of 72 kV; they are typically controlled by heavy duty solenoids. Specialized modern HV breakers are rated to 1000 kV or above. These devices may use air, vacuum, and SF6 methods to extinguish arcs; older devices may also use oil, although this practice is in decline due to spills and other hazards. Along with medium-voltage breakers, high-voltage switches are an integral part of switchgear panels, which are used to control, monitor, and protect transmission networks.

An oil-filled circuit breaker (left); three large high-voltage breakers.

Image credit: Open Electrical | Idaho National Laboratory

Switch Specifications

Like all switches, circuit breakers may be described using the number of circuits they control (referred to as "poles") and the number of switch positions they can adopt ("throws"). Because circuit breakers are relatively simple, "on-off" devices, they almost always have a single throw. The device below is a 3-pole, single-throw (3PST) breaker. Note that it can connect and control three separate circuits due to its three sets of terminals, while the long manual actuator (the single throw) controls all three circuits at once.

Image credit: ASI

Standards

IEC 60898

IEC 60898-1 — titled Circuit Breakers for Overcurrent Protection for Household[s], [etc.] — is a widely-followed international standard which lays out preferred methods of rating and labeling low-voltage circuit breakers. IEC 60898 defines "rated current" as the maximum current which can be continuously carried at an ambient temperature of 30° C. The standard defines preferred current ratings as (in amps): 6, 10, 13, 16, 20, 25, 32, 40, 50, 63, 80, 100, and 125. It also establishes a trip current rating signified by a letter; the trip rating is based on a factor of rated current.

• Type B: breakers trip when exposed to 3-5 times normal load current.
• Type C: trip when exposed to 5-10 times normal load current.
• Type D: exposure to 10-20 times normal current triggers switching.
• Type K: specialized breakers designed for loads which experience frequent current peaks during normal operation. They trip when subjected to 8-12 times normal load current.
• Type Z: trip when exposed to sustained currents which are 2-3 times the rated value. 

Common Standards

The following are commonly-referenced circuit breaker standards:

• BS 7671 -- Requirements for electrical installations / wiring regulations
• IEC 60898  -- Circuit Breakers for Overcurrent Protection for Household[s], [etc.] 
• NFPA 70 -- National electrical code

References

EE Portal - Circuit Breaker Types

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