For questions generally relating to safety devices used to automatically stop the flow of electric current in the event of a fault condition. Use this tag for questions about fuses, too.
Circuit breakers are devices designed to automatically stop the flow of electrical current in the event of a fault condition. They are similar to a fuse, although circuit breakers are resettable and typically offer more than just thermal overcurrent protection.
Circuit breakers use various techniques to offer different types of protection, often providing at least thermal and magnetic protection.
A bimetallic device is the most common way to provide thermal protection. As current flow it generates heat: the higher the current, the more heat that is generated. At a specific ambient temperature, a specific current will generate a specific amount of heat. Knowing the specific values, a bimetallic device can easily be designed to react to temperatures above a predetermined range. As current flows through the bimetallic device, the device heats up. If the temperature gets too high, the device opens the contacts and stops the flow of electricity.
The current at which the breaker will trip is often listed on the handle of the circuit breaker. However, since this is a thermal device, there will be a time delay between the overcurrent and the opening of the circuit. This time delay will be described by what's known as a trip curve, and will be documented by the manufacturer.
This type of protection is typically handled by a solenoid, and may be referred to as instantaneous trip. This type of protection is designed to detect very high current short-circuits and open the circuit quickly when they are detected. When the high current flows through the circuit breaker, it generates a large magnetic field. The magnetic protection device uses this magnetic field to physically open the contacts, stopping the flow of electricity.
The magnetic (instantaneous) trip setting is documented for most breakers as a "jump" or "plateau" in the breaker trip curve. Some circuit breakers are available with differing magnetic trip settings in order to avoid nuisance trips on high-inrush-current loads.
There also exist circuit breakers that provide ground-fault circuit interruption (GFCI), arc-fault circuit interruption (AFCI), and combination arc-fault circuit interruption (CAFCI), as well as dual function circuit interruption (DFCI) which provides both AFCI and GFCI protection. These devices typically offer the specialized protections in addition to thermal and magnetic protection.
Overcurrent (and other types of interrupting) devices have what is called an interrupting rating, which is the maximum current the device's contact system can safely interrupt (stop the flow of). This interrupting rating is printed on the breaker's approval label -- for North American branch circuit breakers, it is typically 10,000 amps, or 10 kiloamps (kA). Higher ratings are found on breakers used in commercial applications and on panelboard main circuit breakers.
This rating exists because when any set of mains-voltage contacts opens, an arc forms and must be extinguished by the contacts in order to stop the flow of current. Circuit breakers contain sophisticated devices such as magnetic blowouts and arc chutes to stretch and break the arc; however, if the capacity of the breaker to stop the arc is exceeded, the high temperatures and pressures generated (like a tiny thunderclap) can damage the breaker internally or even completely destroy it.
If you still have fuses...
Some older installations, or some disconnecting devices in newer installations, still use fuses. These use a thin wire or metal strip calibrated to melt in an overcurrent or short-circuit situation, with thermal mass elements or calibrated solder joints used to provide time delays.
Fuses are commonly available in a variety of current ratings and also can be found in fast-blow or time-delay types. Time-delay or time-lag fuses are commonly used in residential work due to the prevalence of high-inrush motor and lighting loads; however, fast-blow fuses may be used to protect electrical heating loads or other loads where inrush currents are not present.
Most fuses are sealed units, intended to be replaced wholesale if they operate. Modern fuse types such as type S and CC are equipped with rejection features to avoid installing an incorrect fuse or non-fuse object into the fuse base. Older fuses such as Midget and Edison base lack this, and thus were vulnerable to being defeated by the use of the incorrect fuse.
Renewable fuses and fuse wire were used in old UK installs. They are universally obsolete and hazardous due to the ease of inadvertently or intentionally defeating the fuse protection of the circuit by installing the wrong fuse wire.
Sealed, low-voltage fuses may take the form of a cylinder or block that clips into a holder, or a plug that screws into a base. The former are used for higher current or 240V applications, as they can use a fiber or ceramic body filled with quartz sand to help suppress the parting arc of the fuse element blowing. Some fuse elements are sprung to help them part as well -- this is especially common in glass-bodied, air-filled fuses such as plug-style (Edison base and type S) or supplemental (electronic) fuses.