|Ground fault circuit interrupter (GFCI)|
|Friday, 02 February 2007|
The GFCI devices can detect a ground fault condition and break the electric power supply by employing a sensing transformer to detect an imbalance between the currents flowing in the phase (hot) and neutral conductive paths of the power supply. Prior to the development of GFCI devices, differential circuit breakers were known and used to provide ground fault protection to circuit systems. Differential circuit breakers include a differential transformer with a core through which two conductors which act essentially as primary windings relative the core. The differential circuit breaker also includes current interrupting contacts, which, in the event of a detected short or abnormal leakage current, are forced to a high impedance or "off" state. GFCIs evolved from differential circuit breaker technology. Ground fault circuit interrupters essentially comprise a current sensor with a circuit breaker connected between neutral and phase conductors, interposed between a power source and a load. GFCIs also include a differential transformer circumscribing the neutral and phase conductors. A ground fault condition happens when the current is diverted to the ground through another path such as a human body that results in an imbalance between the currents flowing in the phase and neutral conductors. Upon detection of a ground fault condition, the circuit breaker within the GFCI devices is immediately tripped to interrupt the electrical continuity and removes all power supply to the loads. Ground fault circuit interrupters provide the required protection by de-energizing the electrical connection between the power source and load circuit when an undesired loop connection is detected between the hot lead of the power source and some other circuit which is typically to ground via an individual's body that forms part of, and completes, the circuit. One important function of a GFCI is to provide electric shock protection to individuals that come into contact, or close proximity, to an unintentionally exposed wire, or to a conductive component of an ungrounded appliance in which an internal wiring fault undesirably provides an electrical connection between the wiring and the ungrounded component. The extent of damage to the human body caused by such a ground fault depends on the relative amount of fault current and the length of time of the application of the current through the body. Ground fault circuit breakers must be able to detect current flow between line conductors and ground at current levels as little as 5 milliamperes, which is much below the overload current levels required to trip conventional circuit breakers. In addition to ground fault detection/protection, GFCI devices provide protection from miswiring. A ground fault circuit breaker can trip upon occurrence of an inadvertent short between the neutral conductor and ground that may occur due to a fault such as a wiring error by the electrician installing the circuit breaker. Many types of GFCI devices are capable of being tripped not only by contact between the line side of the AC load and ground, but also by a connection between the neutral side of the AC load and ground.
A GFCI generally includes a housing, a tripper, a reset button, a test button, a mounting strap with grounding strap and banding screw, a pair of movable contact holders with contacts, a pair of fixed contact holders with contacts, and a control circuit. A differential current transformer, referred to as the ground fault or sense transformer, is normally used to sense these ground fault currents. The sense transformer has as its primary windings the conductors of the distribution circuit being protected, which are encircled by the core, and a multi-turn winding wound on the core. The circuit breaker is actuated when the differential transformer senses that more current is flowing into the load from the source through the phase conductor than is flowing back to the source through the neutral conductor. A trip coil of a circuit breaker having a plurality of contacts in line with the conductors of a distribution circuit is energized with a minimum current. A pulse generator is coupled to the neutral conductor for producing a high frequency current therein upon grounding of the neutral conductor between the differential transformer and the load. An additional current transformer, referred to as the ground neutral transformer, is commonly used to detect neutral-to-ground faults. A ground fault interrupter generally includes an operational amplifier which amplifies the sensed ground fault signal and applies the amplified signal to a window comparator which compares it to positive and negative reference signals. A GFCI device may be used in a single phase circuit, such as a single phase 120V AC circuit, or in a polyphase circuit, such as 120/240V AC circuit. More commonly, GFCI devices are incorporated into electrical receptacles that are designed for installation at various locations within a building. Such GFCI receptacles are generally adapted for securely mounting on walls of a building. Typically GFCIs are four terminal devices, two phase or AC leads for connection to AC electrical power and two LOAD leads for connection to downstream devices. Some GFCI receptacles have a user accessible load in addition to the line side and load side connections. Users can connect other household appliances to the power supply through plug entries on the receptacle. GFCI devices may be connected to fuse boxes or circuit breaker panels to provide central protection for the AC wiring throughout a commercial or residential structure.