GFCI by Sam Goldwasser - Masterworks Services

1 GFCI by Sam Goldwasserfile:///C:/Documents%20and%20 Settings/Tim%20 Lorenson/ of 68/4/2007 6:52 PMThis page is from the original Code Check website. To see the latest version choose "Home Page New"Code Check 1998 by Redwood Kardon|| Home Page New || Search || by Sam Goldwasser -- exerpts from: FAQ: is a GFCI?:A Ground Fault Circuit Interrupter (GFCI) is a device to protect against electric shock should someone come in contactwith a live (Hot) wire and a path to ground which would result in a current through his/her body. The GFCI operates bysensing the difference between the currents in the Hot and Neutral conductors. Under normal conditions, these shouldbe 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.

2 This might occur if ashort circuit developed inside an ungrounded appliance or if someone was working on a live circuit and accidentallytouched a live GFCI will trip in a fraction of a second at currents (a few mA) well below those that are considered that a GFCI is NOT a substitute for a fuse or circuit breaker as these devices are still required to protectequipment and property from overloads or short circuits that can result in fire or other can be installed in place of ordinary outlets in which case they protect that outlet as well as any downstreamfrom it. There are also GFCIs that install in the main service by Sam Goldwasserfile:///C:/Documents%20and%20 Settings/Tim%20 Lorenson/ of 68/4/2007 6:52 PMNote 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 Afeed-through.

3 Of course, the GFCI outlet itself can then only be used for appliances rated 15 A or (if not most) GFCIs also test for a grounded neutral condition where a low resistance path exists downstreambetween the N and G conductors. If such a situation exists, the GFCI will trip immediately when power is applied evenwith nothing connected to the protected does a GFCI work:GFCIs typically test for the following condition:A Hot to Ground(safety/earth) fault. Current flows from the Hot wire to Ground bypassing the Neutral. This is the test that is mostcritical for Grounded Neutralfault. Due to miswiring or a short circuit, the N and G wires are connected by a low resistance path downstreamof the GFCI. In this case, the GFCI will trip as soon as power is applied even if nothing is connected to itsprotected (load) detect a Hot to Ground fault, both current carrying wires pass through the core of a sense coil (transformer).

4 Whenthe currents are equal and opposite, there is no output from its multiturn sense voltage winding. When an imbalanceoccurs, an output signal is produced. When this exceeds a threshold, a circuit breaker inside the GFCI is detect a Neutral to Ground fault there is a second transformer (left toroid in the illustration below) placed upstreamof the H-G sense transformer (in the illustration above). A small drive signal is injected via the 200 T winding whichinduces equal voltages on the H and N wires passing through its N and G are separate downstream (as they should be), no current will be flow in either wire and the GFCI willnot trip. (No current will flow in the H wire as a result of this stimulus because the voltage induced on both Hand N is equal and cancels.)

5 If there is a N-G short downstream, a current will flow through the N wire, to the G wire via the short, and backto the N wire via the normal 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 there is a H-H[Incidently, a type A GFCI will detect a "hotted hot" <G> as well as a gounded neutral. If there is aparallel path path from the load side hot back to line side hot, it will trip via the same mechanism as theload grounded neutral trip. So, a GFCI won't work on a "double ended" works pretty simply when you study that circuit you pointed us to to. The second coil has as it's primarythe ufiltered output of the full-wave rectifier.]

6 If a closed loop condition exists between any of the twowires going through the coil, this will induce a ~120Hz current in that closed loop. Ingenious!.. "RichardG. Jones" GFCI by Sam Goldwasserfile:///C:/Documents%20and%20 Settings/Tim%20 Lorenson/ of 68/4/2007 6:52 PMGFCIs for 220 VACapplications need to monitor both Hots as well as the Neutral. The principles are basically the same: the sum of thecurrents in Hot1 + Hot2 + Neutral should be zero unless a fault detect a grounded neutral fault, a separate drive coil is continuously energized and injects a small 120 Hz signal intothe current carrying conductors. If a low resistance path exists between N and G downstream of the GFCI, thiscompletes a loop (in conjunction with the normal connection between N and G at the service panel) and enough currentflows to again trip the GFCI's internal circuit use toroidal coils (actually transformers to be mroe accurate) where the core is shaped like a ring ( , toroid ordoughnut).

7 These are convenient and efficient for certain applications. For all practical purposes, they are just anotherkind of transformer. If you look inside a GFCI, you will find a pair of toroidal transformers (one for H-N faults and theother for N-G faults as described above). They look like 1/2" diameter rings with the main current carrying conductorspassing once through the center and many fine turns of wire (the sense or drive winding) wound around the of pageAll in all, quite clever technology. The active component in the Leviton GFCI is a single chip - probably a NationalSemiconductor LM1851 Ground Fault Interrupter. For more info, check out the specs at National'a web site at: or go to illustration 1851 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 notcreate a safety ground.

8 In fact, shorting between the Hot and Ground holes in the GFCI outlet will do absolutelynothing if the GFCI is not connected to a grounded circuit (at least for the typical GFCI made by Leviton sold athardware stores and home centers). It will trip only if a fault occurs such that current flows to a true ground. If theoriginal circuit did not have a safety ground, the third hole is not connected. What this means is that an appliance witha 3 prong plug can develop a short between Hot and the (supposedly) grounded case but the GFCI will not trip untilsomeone touches the case and an earth ground ( , water pipe, ground from some other circuit, etc.) at the same that even though this is acceptable by the NEC, I do not consider it desirable.

9 Your safety now depends on theproper functioning of the GFCI which is considerable more complex and failure prone than a simple fuse or circuitbreaker. Therefore, if at all possible, provide a proper Code compliant ground connection to all outlets feedingappliances with 3 wire installed GFCIs: [More on GFCI testing]The built-in tester is supposed to actually introduce a small leakage current so its results should be valid. Therefore,testing a single GFCI outlet with an external widget is not really necessary except for peace-of-mind. However, such adevice does come in handy for identifying and testing outlets on the same circuit that may be downstream of the external tester is easy to construct - a 15 K ohm resistor between H and G will provide a 7 mA current.

10 Wire it intoa 3 prong plug and label it "GFCI Tester - 7 mA"[Caution see GFCI testing article. rk]. The GFCI should trip as soon as you plug the tester into a protected outlet. On a GFCI equipped for grounded neutral detection, shorting the N and Gconductors together downstream of the GFCI should also cause it to suppose you can purchase suitable low cost testers as well. Try your local home center or electrical supply of pageDetermining wiring of a 2-wire outlet: [More on GFCI testing]Connect a wire between one prong of a neon outlet tester and a known ground - cold water pipe if copper throughout,heating system radiator, ground rod, by Sam Goldwasserfile:///C:/Documents%20and%20 Settings/Tim%20 Lorenson/ of 68/4/2007 6:52 PM(Experienced electricians would just hold onto the other prong of the tester rather than actually grounding it.)