Overcurrent is just like it sounds: It's an excess of current—or amperage—in an electrical circuit. An overcurrent occurs when the current exceeds the rated amperage capacity of that circuit or of the connected equipment (such as an appliance) on that circuit. An overcurrent can be caused by overloading the circuit or by a short circuit, a ground fault, or an arc fault. Circuit breakers and fuses protect circuit wiring from damage caused by overcurrent.
OCPDs—Breakers and Fuses
Circuit breakers and fuses are two types of overcurrent protection devices or OCPDs. Every electrical circuit in a home must be protected by its own OCPD that is properly rated for the circuit wiring. Most homes today have circuit breakers, located in the home's main service panel, or "breaker box." Older homes that haven't been updated may have service panels with fuses instead of breakers. Fuses work just as well as breakers, but like breakers, they must be properly sized for each circuit to protect against overcurrent.
A circuit overload is an overcurrent that occurs when more current (amperage) is drawn from a circuit than the wiring of the circuit can safely handle. If you've ever plugged in too many holiday lights in the same outlet and caused the breaker to trip, you've overloaded the circuit.
Another common type overload is a surge power draw. This happens when a large motor, such as a refrigerator compressor, draws a surge of power to start up. If the circuit capacity is exceeded for more than a brief moment, it can trip the breaker. Circuits are usually designed to handle motor startup, and the motor's demand, or load, goes down after startup, but in some cases, it's still too much for the circuit.
Short circuits occur when a "hot" wire (an ungrounded wire, usually black or red) touches another hot wire or comes in contact with a neutral wire (a grounded wire, usually white). Shorts can also happen if there is a break in a wire in the circuit. The short circuit path has lower resistance the normal path of the circuit, allowing a great deal of current to flow through the short path, overheating the wires.
Ground Faults and Arc Faults
Ground faults and arc faults are similar to short circuits but have their own characteristics. A ground fault commonly occurs when a hot wire contacts a grounded object, such as a metal electrical box (when it is installed as part of a grounding system) or the metal case of a tool or appliance. This is an overcurrent situation that can energize the grounded object and deliver a dangerous shock. Ground-fault circuit-interrupter, or GFCI, circuit breakers are special OCPDs designed to protect against the hazards of a ground fault.
An arc fault is an electric discharge—a very hot spark—that jumps from one conductor to another. This can happen when a hot wire has a small break in it and makes contact only intermittently or when a hot wire touches a neutral or ground wire. A loose wire connection on an outlet or another device also can cause arcing. Arc faults create high current flow and tremendous amounts of heat, which can melt wire insulation or start fires. Arc-fault circuit-interrupter, or AFCI, circuit breakers are special OCPDs designed to protect against the hazards of an arc fault.