Hey Ray, Good posting on ground fault interrupters. Maybe you could find something on arc fault interrupters. In the last few years they have become a new code requirement for protecting outlets in sleeping areas. Last time I cheched some time ago information was pretty sketchy on how they worked. Thanks, Ron ----- Original Message ----- From: "Ray Boyce" <[EMAIL PROTECTED]> To: <[email protected]> Sent: Sunday, July 30, 2006 11:11 PM Subject: [BlindHandyMan] Ground Fault Circuit Interrupter
> 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. > > > > To listen to the show archives go to link > http://acbradio.org/handyman.html > or > ftp://ftp.acbradio.org/acbradio-archives/handyman/ > > The Pod Cast address for the Blind Handy Man Show is. > http://www.acbradio.org/news/xml/podcast.php?pgm=saturday > > The Pod Cast address for the Cooking In The Dark Show is. > http://www.gcast.com/u/cookingindark/main.xml > > Visit The New Blind Handy Man Files Page To Review Contributions From Various List Members At The Following Address: > http://www.jaws-users.com/handyman/ > Visit the new archives page at the following address > http://www.mail-archive.com/[email protected]/ > For a complete list of email commands pertaining to the Blind Handy Man list just send a blank message to: > [EMAIL PROTECTED] > Yahoo! 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