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Electrical safety at a bargain price.

While brushing your teeth each night, perhaps you've noticed a slightly odd receptacle in the wall near the bathroom sink, the one with the tiny buttons that say "test" and "reset." This device is a GFCI, the mysterious acronym for a small package of electronic genius that adds an entirely new level of electrical safety to our homes.

GFCI stands for "ground fault circuit interrupter," a mouthful of jargon that almost everyone avoids, preferring the acronym. Researchers developed GFCIs about 20 years ago to solve a serious problem: Homeowners often received potentially dangerous electrical shocks while in damp or wet areas of their homes and yards.

Part of the problem was moisture itself. Dampness in the soil or in concrete resting upon the soil makes both surfaces very good electrical conductors. So if you touch an electrically charged (hot) wire while standing on soil or concrete in bare feet, electricity will flow right through you and into the ground, shocking you in the process.

The other part of the problem was the plumbing system. Most fresh water comes to our homes in metal pipes, and drain systems are often metal too. Both enter the house underground, so they offer a perfect grounding path for electricity. If you touch a metal faucet or sink while holding a faulty electrical appliance, you could get a shock. And of

course, the potential for getting shocks has risen with the wave of new hand-held electrical appliances that we use in the kitchen, bathroom, basement and outdoors.

Now GFCIs reduce the danger. Yet as commonplace and valuable as they are, most of us don't have a clue how they work. In this article, we'll open up a GFCI to see how it operates and point out where to install them.


Fig. A illustrates the scenario for a dangerous shock. With one hand you pick up the hair dryer to move it away from the sink, while at the same time you shut off the water faucet. In lifting the hair dryer, you accidentally touch an exposed live (hot) wire in the frayed cord near the handle. An electric current immediately flows from the cord, through your body, through the plumbing system, and eventually to the earth.

Meanwhile, the circuit breaker in the main panel might never shut off the electricity, nor will a fuse burn out. Circuit breakers react when too much current flows through them, enough to overheat the wires, melt the insulation, and perhaps cause a fire. Their job is to protect the wires, not you.

In contrast, a GFCI instantaneously senses misdirected electrical current, and reacts within 1/40th of a second to shut off the circuit before a lethal dose escapes. This process explains the ground fault circuit interrupter's name. Unintended current leakage from an electrical circuit is called a "ground fault." Upon sensing the ground fault, the device "interrupts" the circuit by switching it off.


A look inside a GFCI receptacle (Fig. A) shows how the device senses an electrical leak. Normally, the currents passing through both the hot and neutral wires are equal, though they travel in opposite directions. The doughnut-shaped transformer senses both currents, but since they're equal and opposite, their effects cancel each other out. However, when some current "leaks" out, through a worn cord or a faulty power tool, the current through the hot wire will be larger than through the neutral. The transformer immediately senses this imbalance. When the difference exceeds .005 amphere (an amphere is a measure of current; for instance an electric drill usually runs on 3 amps), a switch inside the GFCI reacts and "breaks" the circuit, shutting it down instantly.

If you happened to cause the imbalance by touching a frayed wire or holding a bad electric tool, for example, you might feel a light shock, that initial .005 amp of current before the GFCI reacts. Engineers didn't pull that figure out of thin air. It takes about .005 amp of current passing directly through the human heart to stop or scramble its steady beat. So this is the limit at which GFCIs are set. (Fortunately, electrical shocks rarely pass directly through the heart, so even powerful shocks aren't always fatal.)

To make sure a GFCI works, manufacturers added the "test" and "reset" buttons that you see on the face of a receptacle. Pushing the test button creates a small electrical fault, which the GFCI circuitry should sense and immediately react to by shutting off the circuit. The reset button restores the circuit. Test your GFCIs weekly and replace them if they don't function properly.


GFCIs have a relatively short history in the National Electrical Code. The 1971 code initially required them on the circuitry controlling the lights and other electrical equipment for swimming pools. The 1971 code also listed deadlines for including GFCIs in outdoor locations (1973) and around construction sites (1974). The 1975 code followed up and included GFCI receptacles in bathrooms.

Initial GFCI locations were undoubtedly limited to the most dangerous areas by the relatively high price of the device, about $25. However, as the price dropped to below $10, the cost became insignificant compared to the safety gained. More recent codes expanded GFCI use (Fig. B). The 1990 code requires GFCI protection for readily accessible receptacles located outdoors, in garages, in crawl spaces and unfinished basements, and above the countertop within 6 ft. of a kitchen sink. Code officials intended these rules to protect the users of portable power tools and hand-held kitchen appliances from receiving shocks in these damp, high-risk areas.

Incidentally, the code treats spas, hot tubs and Jacuzzis like swimming pools. Receptacles, lights and electrical equipment within a certain distance of pools all require GFCI protection.


GFCIs can be especially useful when you want to update an older electrical system and make it safer. Home electrical systems installed since about 1962 have a third wire running alongside the hot and neutral wires, called an "equipment ground wire," which is either bare copper or covered by green insulation. (Don't confuse this ground wire with the more general term "ground" in "ground fault.") The third, round blade on a three-prong plug and the third hole in a standard receptacle represent this wire. But older wiring systems usually do not have an equipment ground wire (unless they were updated), and have receptacles with only two slots. So you can't plug three-prong plugs into those older two-slot receptacles, which is a big inconvenience.

What's more, by code you cannot replace two-slot receptacles with the standard three-prong grounded type, unless you also install the third ground wire. Tearing out walls and ceilings to put that third wire in is rarely worth the cost and trouble.

However, you can replace an old two-slot receptacle with a three-slot GFCI, even though you don't have a ground wire. The GFCI will protect the circuitry and detect current leaks even better than the ground wire, and it will shut off the circuit should a problem arise. This is a handy way to update an older system safely and inexpensively.

The code even allows you to connect other ungrounded receptacles to the GFCI (Fig. C). So you can go ahead and replace other two-slot receptacles with the standard three-slot, as long as you connect them properly to the original GFCI and don't add any ground wires.

Installing a GFCI is almost as easy as installing a regular receptacle. First, turn off the circuit breaker or fuse at your main electrical panel that supplies the receptacle. The GFCI has two pairs of terminals (screws on some and wires on others). One pair is labeled "load" (Fig. C), which you connect to the wires coming from the main panel; white (neutral) to the silver terminal; and black (hot) to the brass terminal. Use the other pair of terminals, labeled "line," to connect other standard outlets to the GFCI.

However, a GFCI requires more box space than a standard receptacle. So if the GFCI and its wires don't fit, be prepared to install a larger box.

As usual, be sure to obtain an electrical permit if you do your own work, so you can check the local codes with your electrical inspector and have your work inspected afterward.
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Title Annotation:use of the ground fault circuit interrupter
Author:Johnson, Duane
Publication:The Family Handyman
Date:Oct 1, 1992
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