Fuses and circuit-breakers are designed to break the circuit if current flow is excessive. The most common kinds are fuses, fusible links, and circuit breakers. They are all rated in amperes. Their ratings are usually marked on them.
Fuses are typically used in lighting and accessory circuits where current flow is usually moderate.
A fusible link is typically placed near the battery, and, except for the starter motor, it carries the current needed to power an individual circuit, or a range of circuits.
Circuit breakers are not destroyed by excess current. A bimetallic strip heats up and bends, opening a set of contacts and breaking the circuit. In most types, as the strip cools, it resumes its original shape. The contacts close, completing the circuit once more.
Fuse
In electronics and electrical engineering a fuse, short for 'fusible link', is a type of overcurrent protection device. It has as its critical component a metal wire or strip that will melt when heated by a prescribed (design) current, opening the circuit of which it is a part, thereby protecting the circuit from an overcurrent condition.
A practical fuse was one of the essential features of Edison's electrical power distribution system. An early fuse was said to have successfully protected an Edison installation from tampering by a rival from a gas-lighting concern.
Fuse characteristics
Each type of fuse has a time-current characteristic which shows the time required to melt the fuse for any given level of overload current. In power system design, main and branch circuit fuses can be coordinated for best protection by plotting the time-current characteristics on a consistent scale, making sure that the source fuse curve never crosses that of any of the branch circuits. To prevent damage to fuses, both "maximum clearing" and "minimum melting" curves are plotted.
Fuses are often characterized as "fast-blow" or "slow-blow," according to the time they take to respond to an overcurrent condition. Fast-blow fuses (sometimes marked 'F') open quickly when the rated current is reached. Ultrafast fuses (marked 'FF') are used to protect semiconductor devices that can tolerate only very short-lived overcurrents. Slow-blow fuses (often marked 'T') can tolerate a transient overcurrent condition, but will open if the overcurrent condition is sustained.
A fuse also has a breaking capacity. This specifies the maximum current the fuse can safely interrupt. Generally this should be higher than the prospective short circuit current though it may be lower if another fuse or breaker upstream can be relied upon to take out extremely high current shorts. Breaking capacities range from about 10 times rated current for some types of glass fuses through about 15KA for household HRC fuses to tens of kiloamps for some industrial fuses.
A fuse should normally be selected with a rating just over the normal operating current of the downstream wiring or equipment which it is to protect. Properly-selected fuses (or other overcurrent devices) are an essential part of a power distribution system to prevent fire or damage due to overload or short-circuits. Usually the maximum size of fuse for a circuit is regulated by law. For example, the Canadian Electrical Code, the United States National Electrical Code, and the UK Wiring Regulations provide limits for fuse sizes for a given conductor, and local authorities will incorporate these national codes as part of local law.
Circuit breaker
A circuit breaker is a piece of equipment which is designed to protect an electrical apparatus from damage caused by overload or short circuit. Unlike a fuse which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation.
Circuit breakers are often implemented with a solenoid (electromagnet) whose strength increases as the current increases and eventually trips the circuit breaker. Alternatively a bimetallic strip may be used which heats and bends with increased current. Some circuit breakers incorporate both techniques. This allows the properties of the circuit breaker to be tailored to suit the application, with the electromagnet generally responding to short, large surges in current (short circuit) and the bimetallic strip responding to smaller but longer-term (overload) overcurrent conditions. Circuit breakers for larger currents are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism.
Under short-circuit conditions a current of many times greater than normal can flow. When a circuit breaker tries to interrupt this current, an arc may form allowing the flow of current to continue even though the contacts of the circuit breaker are open. Circuit breakers incorporate features to divide and extinguish the arc. In air-insulated and miniature breakers an arc chute structure consisting (often) of metal plates or ceramic ridges cools the arc, and blowout coils deflect the arc into the arc chute. Larger circuit breakers such as those used in electrical power distribution may use vacuum, an inert gas such as sulfur hexafluoride or have contacts immersed in oil to suppress the arc. The maximum short-circuit current that a breaker can interrupt is determined by testing. Application of a breaker in a circuit with a higher prospective short-circuit current may result in failure of the breaker to safely interrupt a fault.
Small circuit breakers are either installed directly in equipment, or are arranged in a breaker panel. Power circuit breakers are built into switchgear cabinets. High-voltage breakers may be free-standing outdoor equipment or a component of a gas-insulated switchgear line-up.
Source: CDX Global & Wikipedia - en.wikipedia.org
