In an electric power system, overcurrent or excess current causes failure or malfunction. This is a larger than intended electric current that exists on circuit conductors, leading to excessive generation of heat, and the risk of fire or damage to equipment.
Possible causes for overcurrent include:
- Incorrect design
- Short circuits
- Arc faults
- Ground faults
Electrical Terminology for Overcurrent Protection
Electrical terminology is essential for a better understanding of the functions and features of overcurrent protection.
- Ampacity: The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. The ampacity of a conductor varies with the conditions of use, as well as with the temperature rating of the conductor insulation.
- Overcurrent: Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit or ground fault. They can occur as a result of normal conditions such as a motor starting, or abnormal conditions such as a fault.
- Overload: The operation of equipment in excess of normal, full-load rating, or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating. It is important to note that an overload is not a fault. An example of a common overload is when there are too many devices on a circuit. The circuit breaker can be reset once the circuit becomes unloaded by simply unplugging the appliances from the receptacle(s).
- Short circuit: A fault usually due to insulation breakdown and improper maintenance of equipment.
- Ground fault
- Arc fault
- Bolted fault
Overcurrent Protection Devices
Standard fuses and circuit breakers are commonly used overcurrent protection devices (OCPD) to control overcurrent (overload and faults). Circuit breaker manufacturers specifically address arc fault and ground fault in their custom designs.
GFCI Circuit breakers are designed to detect an imbalance of 5 milliamps between the circuit single pole circuit breakers — L1-N, or double pole circuit breakers — L1-L2.
- GFCIs are intended for use in wet locations.
- AFCI circuit breakers are designed to detect and react to low-level arcing, indicating damage to branch circuit conductors.
- AFCIs are intended for use in dwelling units.
Caution must be taken when resetting a short circuit fault; a closer evaluation into the reason for the fault is critical. Clearing the fault can require testing by a qualified person to ensure safe operation free from fire and shock hazards.
The National Electrical Code (NEC) Article 240 provides the requirements for selecting and installing overcurrent protection devices (OCPDs) depending on your application.
Overcurrent Protective Device: Fuses and Circuit Breakers
Overcurrent protection devices must be capable of providing protection for service, feede, and branch circuits and equipment. This must be accomplished over the full range of overcurrents between its rated current and its interrupting rating.
Service, feeder and branch circuit overcurrent protective devices come with a short circuit ampere interrupting circuit rating, stamped AIC. The AIC rating must be appropriate for the intended use, but no less than 5000 amperes.
Overcurrent protective devices, such as fuses and circuit breakers, have time/current characteristics (TCC) that determine how long it takes to clear the fault for a given value of fault current. If the circuit were not opened, the excessive current would overheat the wire insulation, burn the wires and possibly start an electrical fire.
Fuses come in a variety of types and sizes depending on the application; fuses are a one-time OCPD. They are typically considered to react six times faster in a fault situation over a circuit breaker, although it must be replaced after the overcurrent (fault or overload) occurs.
Circuit breakers, similar to fuses, are designed to detect and react to the presence of excessive current. This reaction is known as “tripping,” meaning the opening of the circuit due to an overcurrent situation. Circuit breakers are resettable after the overcurrent (fault or overload) occurs.
Proper selection of an OCPD is based on the device closest to the fault that begins operating before the next device upstream. For example, any fault on a branch circuit should open the branch circuit breaker rather than the feeder overcurrent protection.
To learn more about the functions and features of overcurrent protection devices, as well as other electrical devices and how to address issues involving them, check out the SkillMill Electrical Devices course.
Interplay Learning Electrical Expert
Chad is Interplay’s electrical expert and is a Master Electrician. Chad has progressed as an electrical professional throughout his career, with early beginnings in rewiring/wiring homes to QA/Commissioning plants, honing his skills in all aspects of the electrical trade along the way. He transitioned his career through Residential, Commercial and Industrial sites, and in 2012, further expanded on his mission to lifelong learning in becoming an electrical instructor. He continued on this path as an online course developer and is steadfastly committed to electrical safety and sound adult learning theories.