Every electric circuit in a wiring system must be protected against overloads. A circuit overload occurs when the amount of current flowing through the circuit exceeds the rating of the protective devices.
The amount of current flowing in a circuit is determined by the load -- or the "demand" -- for current. For example, if a circuit is rated for 15 amps maximum, then a fuse or circuit breaker of that rating will be in that circuit. If the current exceeds 15 amps, the circuit breaker will open up, cutting off any more current flow. Without overload protection wires can get hot, or even melt the insulation and start a fire.
There are two kinds of protection for electrical units that need to be considered. The first is concerned with the protection of the actual electrical wires supplying the circuits against an overload above their carrying capacity. The second type is concerned with protecting the individual appliances and electrical equipment connected to a supply circuit from an overload. Both types of protection involve either fuses or breakers, but are based on different ideas and objectives.