Ground fault circuit interrupter (GFCI)

Friday, 02 February 2007

A ground fault circuit interrupter (GFCI) is an electrical device that is operable to de-energize a circuit in response to the detection of a ground fault condition at an AC load. Ground fault circuit interrupters were developed to meet a great need for a device which is capable of detecting the presence of abnormal current flow, e.g., current flow from a phase line to ground, and immediately interrupting power to a faulted line in which the abnormal current is detected to protect persons from electric shock, fire and explosion. With the increasing use of household electrical appliances, more attention is paid to the safety of using such appliances. Many electrical wiring devices including receptacles have a line side that is connectable to an electrical power supply, and a load side that is connectable to one or more loads and at least one conductive path between the line side and load side. When a person accidentally comes in contact with the line side of the AC load and an earth ground at the same time, a serious injury may occur because the human body forms another conductive path for the electrical current to flow through. The danger of electrocution or of serious shock to an individual coming in contact with an appliance surface having an electrical potential differing from ground potential when the individual was also touching a grounded surface created the need for ground fault circuit interrupters. Ground fault circuit interrupters are utilized in power distribution systems to protect against personal injury and property damage which may result from ground fault-type short circuit current flow. A ground fault condition exists when there is a current imbalance between the current-carrying phases and neutral above a designated threshold specified in electrical codes. Electrical codes also mandate a maximum amount of time after power up of a ground fault circuit interrupter in which the unit must trip in response to a ground fault condition. Thus, ground fault circuit interrupters are designed to break the electrical continuity upon detecting a ground fault condition occurring at an alternating current (AC) load. GFCIs are widely employed in both commercial and residential environments.

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.

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