In an electric power system, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment. There are two types of overcurrents:
- overload currents;
- short-circuit currents.
Possible causes for overcurrent include short circuits, excessive load, incorrect design, an arc fault, or a ground fault. The electrical conductors present in electrical systems are protected from the presence of any overcurrents by using protective equipment such as fuses and magneto-thermic switches, circuit breakers, and current limiters are commonly used overcurrent protection (OCP) mechanisms.
The fuse is the simplest among the protection devices and is generally used to protect a line from short-circuit currents. It is placed in series with the line to be protected and intervenes by interrupting the line when the value of the current that passes through it is such as to cause the melting of a fuse element, hence the name of the fuse.
Circuit breakers are mechanical power cut-off devices. They are equipped with a special release device which causes the automatic opening of the contacts when the current flowing through it exceeds a certain value for a certain time. The tripping time depends on the value of the fault current.