Hi What is a GFCI? A Ground Fault Circuit Interrupter (GFCI) is a device to protect against electric shock should someone come in contact with a live (Hot) wire and a path to ground which would result in a current through his/her body. The GFCI operates by sensing the difference between the currents in the Hot and Neutral conductors. Under normal conditions, these should be equal. However, if someone touches the Hot and a Ground such as a plumbing fixture or they are standing in water, these currents will not be equal as the path is to Ground - a ground fault - and not to the Neutral. This might occur if a short circuit developed inside an ungrounded appliance or if someone was working on a live circuit and accidentally touched a live wire.
The GFCI will trip in a fraction of a second at currents (a few mA) well below those that are considered dangerous. Note that a GFCI is NOT a substitute for a fuse or circuit breaker as these devices are still required to protect equipment and property from overloads or short circuits that can result in fire or other damage. GFCIs can be installed in place of ordinary outlets in which case they protect that outlet as well as any downstream from it. There are also GFCIs that install in the main service panel. Note that it may be safe and legal to install a GFCI rated at 15 A on a 20 A circuit since it will have a 20 A feed-through. Of course, the GFCI outlet itself can then only be used for appliances rated 15 A or less. Many (if not most) GFCIs also test for a grounded neutral condition where a low resistance path exists downstream between the N and G conductors. If such a situation exists, the GFCI will trip immediately when power is applied even with nothing connected to the protected outlets. GFCIs, overloads, and fire safety A GFCI is NOT a substitute for a fuse or circuit breaker (unless it is a combined unit - available to replace circuit breakers at the service panel). Therefore, advice like "use a GFCI in place of the normal outlet to prevent appliance fires" is not really valid. There may be some benefit if a fault developed between Hot and Ground but that should blow a fuse or trip a circuit breaker if the outlet is properly wired. If the outlet is ungrounded, nothing would happen until someone touched the metal cabinet and an earth ground simultaneously in which case the GFCI would trip and provide its safety function. See the section: " Why a GFCI should not be used with major appliances" for reasons why this is not generally desirable as long as the appliance or outlet is properly grounded. However, if a fault occurs between Hot and Neutral - a short in the motor, for example - a GFCI will be perfectly happy passing almost any sort of overload current until the GFCI, wiring, and appliance melts down or burns up - a GFCI is not designed to be a fuse or circuit breaker! That function must be provided separately. How does a GFCI work GFCIs typically test for the following condition: 1. A Hot to Ground (safety/earth) fault. Current flows from the Hot wire to Ground bypassing the Neutral. This is the test that is most critical for safety. 2. A grounded neutral fault. Due to miswiring or a short circuit, the N and G wires are connected by a low resistance path downstream of the GFCI. In this case, the GFCI will trip as soon as power is applied even if nothing is connected to its protected (load) circuit. To detect a Hot to Ground fault, both current carrying wires pass through the core of a sense coil (transformer). When the currents are equal and opposite, there is no output from its multiturn sense voltage winding. When an imbalance occurs, an output signal is produced. When this exceeds a threshold, a circuit breaker inside the GFCI is tripped. GFCIs for 220 VAC applications need to monitor both Hots as well as the Neutral. The principles are basically the same: the sum of the currents in Hot1 + Hot2 + Neutral should be zero unless a fault exists. To detect a grounded neutral fault, a separate drive coil is continuously energized and injects a small 120 Hz signal into the current carrying conductors. If a low resistance path exists between N and G downstream of the GFCI, this completes a loop (in conjunction with the normal connection between N and G at the service panel) and enough current flows to again trip the GFCI's internal circuit breaker. GFCIs use toroidal coils (actually transformers to be more accurate) where the core is shaped like a ring (i.e., toroid or doughnut). These are convenient and efficient for certain applications. For all practical purposes, they are just another kind of transformer. If you look inside a GFCI, you will find a pair of toroidal transformers (one for H-N faults and the other for N-G faults as described above). They look like 1/2" diameter rings with the main current carrying conductors passing once through the center and many fine turns of wire (the sense or drive winding) wound around the toroid. All in all, quite clever technology. The active component in the Leviton GFCI is a single chip - probably a National Semiconductor LM1851 Ground Fault Interrupter. For more info, check out the specs at National'a web site at: http://www.national.com/pf/LM/LM1851.html. More on how the GFCI detects a N-G short To detect a Neutral to Ground fault there is a second transformer placed upstream of the H-G sense transformer (see the illustration of the internal circuitry of the GFCI at: http://www.national.com/pf/LM/LM1851.html). A small drive signal is continuously injected via the 200 T winding which induces equal voltages on the H and N wires passing through its core. * If N and G are separate downstream (as they should be), no current will be flow in either wire and the GFCI will not trip. (No current will flow in the H wire as a result of this stimulus because the voltage induced on both H and N is equal and cancels.) * If there is a N-G short downstream, a current will flow through the N wire, to the G wire via the short, and back to the N wire via the normal N-G connection at the service panel. Since there will be NO similar current in the H wire, this represents a current unbalance and will trip the GFCI in the same manner as the usual H-G short. * Interestingly, this scheme automatically detects a H-H fault as well. This unlikely situation could occur if the Hots from two separate branch circuits were accidentally tied together in a junction box downstream of the GFCI. It works the same way except that the unbalance in current that trips the GFCI flows through the H wire, through the H-H fault, and back around via the Hot busbar at the service panel. Of course if the two Hots are not on the same phase, there may be fireworks as well :-). GFCIs and safety ground Despite the fact that a Ground Fault Circuit Interrupter (GFCI) may be installed in a 2 wire circuit, the GFCI does not create a safety ground. In fact, shorting between the Hot and Ground holes in the GFCI outlet will do absolutely nothing if the GFCI is not connected to a grounded circuit (at least for the typical GFCI made by Leviton sold at hardware stores and home centers). It will trip only if a fault occurs such that current flows to a true ground. If the original circuit did not have a safety ground, the third hole is not connected. What this means is that an appliance with a 3 prong plug can develop a short between Hot and the (supposedly) grounded case but the GFCI will not trip until someone touches the case and an earth ground (e.g., water pipe, ground from some other circuit, etc.) at the same time. Note that even though this is acceptable by the NEC, I do not consider it desirable. Your safety now depends on the proper functioning of the GFCI which is considerable more complex and failure prone than a simple fuse or circuit breaker. Therefore, if at all possible, provide a proper Code compliant ground connection to all outlets feeding appliances with 3 wire plugs. Where are 3 wire grounded outlets required? If you move into a house or apartment where some or all of the outlets are the old 2 prong ungrounded type, don't panic. There is no reason to call an electrician at 2:00 AM in the morning to upgrade them all at great expense. You don't need grounded outlets for two wire appliances, lamps, etc. They do essentially nothing if the third hole isn't occupied :-). A GFCI will provide much more protection! You should have grounded outlets for the following: * Computers in order for the line filters and surge suppressors to be most effective. * High-end entertainment gear if it uses 3 prong plugs for similar reasons. * Microwave ovens. For safety, these really should be on a grounded circuit. (A GFCI will not protect against a fault on the high voltage side of a microwave oven, though this sort of fault is extremely unlikely). * Large appliances including refrigerators, clothes washers and dryers, dehumidifiers, window air conditioners, etc. In most cases, there will only be a few circuits where this is needed and only these need to be upgraded. To what extent the wiring plan of your residence separates lighting type circuits from those with outlets that will be used for 3 wire equipment will determine how easy it is to upgrade only those outlets that are affected. It may be cheaper to just add new branch circuits for specific equipment needs. 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