GFCI or Ground Fault Current Interruptor is a practical application of different current protection. In this article, you’ll learn the basics of GFCI protection.
It is noteworthy that a form of differential current protection also finds application in American households, where electrical codes require the installation of Ground Fault Current Interruptor (GFCI) protected circuits in areas where contact between electrical appliances and water is likely (e.g. bathrooms, kitchens). GFCI receptacles function by sensing any difference in current between the “hot” and “neutral” conductors carrying current to and from any load plugged into the receptacle:
A single current transformer (CT) within the GFCI unit senses any differential current by sensing the net magnetic field around both current-carrying conductors. If the “hot” and “neutral” currents are equal, their opposite directions will produce opposing magnetic fields, with zero net magnetic field sensed by the CT. If, however, a ground fault exists at the load plugged into this receptacle, these two currents will be unequal and the CT will detect a net magnetic field. These protective devices are extraordinarily sensitive, tripping the contacts with differential current values in the milliamp range. This is important, as a ground fault existing in an electrical appliance may very well pass through the body of a person or an animal, in which case mere milliamps may prove harmful or even fatal.
If a GFCI receptacle trips, it may be reset by pressing a “reset” button on its face. GFCI units may also be manually tested by pressing a “test” button also mounted on the front face.