This chapter is in no way intended to be a comprehensive coverage of wiring issues but includes a discussion of a few of the common residential wiringFor more information, see the official Usenet Electrical Wiring FAQ or a DIY book on electrical wiring. The NEC (National Electrical Code) handbook which is updated periodically is the 'bible' for safe wiring practices which will keep honest building inspectors happy. manual is not what you would call easy to read. A much more user friendly presentation can be found at the CodeCheck web site: This site includes everything you always wanted to know about construction codes (building, plumbing, mechanical, electrical) but were afraid to ask. In particular, the following series of sections on Ground Fault Circuit interrupters is present at the CodeCheck web site and includes some nice 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

GFCIs typically test for the following condition: 1. A Hot to Ground (safety/earth) fault. Current flows from the Hot wire toac unit for 1500 sq ft home Ground bypassing the Neutral. how split system air conditioners workThis is the test that is most critical forcentral air handling unit 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. 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. 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 theThe 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 carryingIf 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). 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 faultsThey 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 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 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. 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 :-).