Think like a GFCI

May 1, 1999
With ever increasing locations that need GFCI protection, knowing how to select, install, and test these devices is crucial.Today's ground fault circuit interrupter (GFCI) is the result of a convergence of technology with need. In the late 1960s, over 600 people were electrocuted in the United States using consumer products every year. By 1988, that number had been reduced to fewer than 290 (according

With ever increasing locations that need GFCI protection, knowing how to select, install, and test these devices is crucial.

Today's ground fault circuit interrupter (GFCI) is the result of a convergence of technology with need. In the late 1960s, over 600 people were electrocuted in the United States using consumer products every year. By 1988, that number had been reduced to fewer than 290 (according to the Consumer Product Safety Commission). Other safety improvements, such as battery powered tools and double insulation, also have played an important role. Nonetheless, the importance of GFCI protection is clear.

A constant over the years has been the need for installers to understand how GFCIs function in order to provide the customer with the best possible installation and to properly test to assure correct installation. The more installers "think like a GFCI," the better the GFCI installation and the fewer the call-backs.

To get a better understanding, let's review the need for GFCIs and how they function. Then we'll explore practical installation and testing techniques.

Why GFCIs?

The effects of an electrical current passing through the body vary significantly, from merely the barest perception of a shock to ventricular fibrillation. In Fig. 1, the key physiological thresholds are shown, together with the maximum response times permitted by Underwriters Laboratories (UL) and the typical response times of Class A GFCIs. It's important you note that the "let-go" threshold is between 6mA and 18mA. The trip level of 6mA required for personnel-protection GFCIs by UL is based on this let-go current threshold.

When installing and testing GFCIs, you should know that they do not trip instantaneously. In fact, while they typically trip in 25 ms or so at fault currents exceeding 20 to 30mA, they are permitted by UL to take several seconds to trip at fault currents in the 6mA range. This is important to recognize when selecting GFCI testers, for some have timing circuits in them that may limit the test current to a time less than necessary to trip a GFCI. (We'll cover this in more detail in the testing recommendations, which follow.)

It's also important to remember that a GFCI does not limit the magnitude of the fault current. Instead, it limits only the duration of the fault. Therefore, it's possible for a person to receive a shock on a circuit protected by a GFCI.

The magnitude of any fault current is limited by the supply voltage and the impedance in the fault circuit. In most electric shock scenarios, the person is the principal impedance source, and the magnitude of the shock can therefore vary considerably, depending on the condition of the skin, contact area, body weight, and current path through the body.

How GFCIs function

Although GFCIs have been miniaturized and fine tuned since the early versions, the basic operation is the same. The better an installer understands what makes a GFCI tick, the easier it is to satisfy the customer with a safe and convenient installation.

Sensing function. Fig. 2 (see page 86) is an internal schematic wiring diagram of a typical GFCI receptacle. Because of its name, it's frequently believed that a GFCI detects fault current. In fact, it detects differential current.

A GFCI can be viewed as an "adding machine," constantly summing the current flowing through the differential current transformer to the load and then subtracting the current returning through it from the load.

If a ground fault exists with some of the current flowing to ground and not returning on the neutral through the transformer, then the sum of the current flowing on the hot and neutral will not be zero and differential current will be detected. The GFCI's electronic circuitry then measures its magnitude. If it reaches a predetermined level (the GFCI trip threshold) for a given duration, a signal causes the trip coil to energize and the circuit to open.

Differential current can be caused by grounded tools and appliances having low levels of harmless leakage current. UL typically requires that leakage levels on tools and appliances be less than 0.5mA (about one-tenth the level necessary to trip a GFCI). Also, a differential current can result from other causes such as sharing the neutral with another ungrounded conductor or from capacitive leakage between the ungrounded circuit conductor and ground.

Equipment grounding conductor. Referring to Fig. 2 again, we see that the hot and neutral conductors pass through the differential current sensor. The equipment grounding conductor does not. Therefore, fault current flowing from the hot conductor to ground using either the equipment grounding conductor or another ground path not involving the equipment grounding conductor will be detected as differential current.

Since the GFCI does not use the equipment grounding conductor in the sensing circuit, GFCIs will protect non-grounding circuits and can replace two-wire (ungrounded) receptacles as permitted in Sec. 210-7(d) Ex. of the NEC.

Grounded neutral. In a situation where the load side neutral is grounded and a ground fault also occurs, a parallel path through the GFCI for the ground fault current could exist. The portion of the ground fault current returning on the neutral conductor will not be sensed as differential current. This has the effect of desensitizing the GFCI. Therefore, the UL standard requires that GFCIs trip with a 6mA ground fault even when the neutral and ground are connected. To meet this requirement, GFCIs trip when the load side neutral and equipment grounding conductors are joined, even if there is no ground fault.

Test circuit. When the "test button" on a GFCI is pushed, a circuit is closed from the hot conductor on the load side of the current sensor to the neutral conductor on the line side of the current sensor. The current flow in the test circuit is limited to slightly more than 6mA. When the "test" button is pushed, the GFCI must detect this differential current, measure it, and signal the trip coil to energize and trip the GFCI. Pushing the "test" button tests not only the trip mechanism but also the complete operation of the GFCI.

Line-load terminals. All GFCI receptacles have two sets of terminals having screws or wire leads. One set is typically identified as "LINE," the other as "LOAD," again as shown in Fig. 2. Conductors from the branch circuit overcurrent device are intended to be connected to the "LINE" terminals, and conductors feeding other downstream receptacles or loads (if intended to be protected) are connected to the set of terminals marked "LOAD."

Note that the receptacle outlets integral with the GFCI receptacle are on the load side of the current interrupting contacts and thus protected by the GFCI when properly wired. If the "LINE" conductors are connected to the "LOAD" terminals (essentially back-feeding the GFCI receptacle), then note that the integral receptacles are in the circuit before the current-interrupting contacts and are therefore not protected. If the GFCI is miswired and the "test" button is pushed, the 6mA differential current will still flow as before and the GFCI will trip, even though protection will not have been provided to the receptacles integral with the GFCI. Downstream receptacles, however, will be protected because they are on the protected side of the circuit-interrupting contacts. Therefore, it's important that you follow manufacturer's wiring instructions explicitly.

Most manufacturers call for testing an installed GFCI with a lamp or appliance. If the test button is pushed, the reset button should pop out and the lamp or appliance plugged directly into the GFCI receptacle should turn off.

On all production of GFCI receptacles after July 4, 1995, anew label will be affixed to the load terminals reading:

ATTENTION. The load terminals under this label are for feeding additional receptacles. Miswiring can leave this outlet without ground fault protection. Read instructions prior to wiring.

GFCI installation lips

Again, the best GFCI installations are those where you "think like a GFCI" and lay out the installation to minimize "nuisance tripping." At the same time, you should "think like the owner" and consider the GFCI's location, visibility for monthly testing, and resetting convenience should it trip.

Length of circuit. A GFCI is subjected to tests that simulate long branch circuits. While there are no specific rules concerning the length of the circuit protected or the number of receptacles on the protected circuit, remember that the GFCI will add up all the harmless leakage currents and capacitive leakages. Under extreme circumstances, this could "preload" the GFCI and make it appear overly sensitive or, worst case, result in nuisance tripping. Therefore, you should minimize the length of circuits to the degree possible.

Do not protect some appliances. While the NEC requires a number of receptacles in the kitchen, garage, and basement to be GFCI protected, protection is not required on those serving refrigerators, freezers, or sump pumps. In addition to the obvious reasons of not wanting to interrupt power to these important appliances, some older types of frost-free refrigerators and freezers have relatively high leakage currents when in the defrost cycle. GFCI circuits should be routed so these appliances are not on protected circuits.

The Code requires lighting fixtures and exhaust fans in bathrooms to be grounded, but does not require them to be GFCI protected. Exhaust fans accumulate dust and moisture, possibly increasing leakage levels. Some types of fluorescent fixtures may generate switching transients. This type of equipment should not be wired on the protected portion of the circuit.

The effect of design

Some GFCI terminal designs make it easier to be selective in the layout of protection than others. For example, some receptacles have both screw terminals and clamp-type back-wiring capabilities. With this type of terminal arrangement, you can terminate the line side conductor coming from the branch circuit overcurrent device in one back wire terminal opening and use the other to feed appliances or outlets that are to be left unprotected. The load terminals are then used for protecting additional downstream loads or outlets as desired. If the GFCI receptacle has only terminal screws, then a splice would need to be made.

An added benefit of clamp-type terminals is the ease with which they contain the strands of stranded conductors commonly used on non-residential projects.

Line-load connections. Reversing the line-load connections will result in leaving the GFCI receptacle's integral outlets unprotected. Proper installation requires two actions:

* The "line" side conductors (those coming from the branch circuit over-current device) must be identified. On new installations, this should be done during the initial installation. Use tape or some other means of identification. When retrofitting, there may well be two or more sets of conductors in the box. Nonetheless, each set must be checked and the "line" side conductor identified.

* The "line" side terminals on the GFCI receptacle must be determined. Since GFCI terminal positions differ, the marking must be checked with each installation. Receptacles made after July 4, 1995 will have a label covering the "load" terminals.

Location of GFCI receptacles. While you may be tempted to put as many of the receptacles required to be protected on the fewest number of GFCIs, some real inconvenience can result. GFCIs protecting outdoor receptacles should be outdoors, not in the upstairs bathroom. While this may seem obvious, there are some real horror stories of people wrapped in towels going to the garage to reset a GFCI.

Multiwire circuits. Multiwire circuits are sometimes used in dwellings, particularly for serving kitchens. Even more frequently, multiwire circuits are used in the wiring of hotels, dormitories, hospitals, nursing homes, and other types of buildings.

Multiwire circuits have two or more ungrounded conductors sharing a common neutral, as in a 120/240V, single-phase circuit or a 208Y/120V, 3-phase circuit.

GFCI receptacles can be used on multiwire circuits, but they must be wired such that the neutral on the load side of the GFCI is not shared by two ungrounded conductors. Failure to observe this requirement will result in a differential current any time a load served by an ungrounded conductor not connected to the GFCI is energized, immediately tripping the GFCI.

Note that multipole GFCI circuit breakers are available for use on multiwire circuits. The GFCI sensor mechanisms in these breakers operate on the same principle as the GFCI receptacle, except that the current from all ungrounded conductors and the neutral pass through the "adding machine." As in the case of the receptacles, the unit will trip when it detects unbalanced current over the 4mA to 6mA trip range.

On retrofit projects in older installations, this can be particularly troublesome. This occurs when a previous modification or extension was made to a circuit and the connection was inadvertently made to the wrong neutral conductor, thus resulting in a neutral serving two ungrounded conductors. This could go unnoticed until a GFCI is installed, resulting in nuisance tripping and a difficult troubleshooting project.

Grounded neutral. At one time, it wasn't uncommon for electrical tradesmen to be instructed to reduce the possibility of accidentally shorting out the hot conductor to the bare grounding conductor in the box by placing the bare ground on the side of the receptacle with the neutral screws. When the receptacle is placed in the box, the bare ground is less likely to accidentally short out against the hot screws. This may have prevented shorting, but when GFCIs came along with the grounded neutral detection circuitry, the bare ground touching the neutral terminal screws caused an automatic trip of the GFCI. The solution here is to dress the wires carefully to assure that the bare ground does not contact any of the terminal screws.

Wet and damp locations (outdoors). When GFCI receptacles are used in wet or damp locations, the directions provided with the product instruct you to install them in accordance with NEC Sec. 410-57. This means that the appropriate cover needs to be properly installed. If decorative lighting, Christmas lights, or other equipment is expected to be plugged into the outdoor outlet and left unattended, then the cover should be of the type that maintains its weatherproof integrity when the attachment plug is inserted. [ILLUSTRATION FOR PHOTO 1 OMITTED]. The plate should be properly mounted (don't use covers designed for vertical mounting for devices oriented horizontally, or vice-versa), gasketed, and sealed to the wall. All other outdoor receptacles should be treated with the same care. Failure to do so can result in dust and moisture getting into the outlet, resulting in leakage current and unwanted tripping. In the case of the GFCI, it can also result in exposing the GFCI mechanism to corrosion.

Construction sites. GFCIs are required to protect 125v, 15A and 20A receptacles on construction sites. The receptacle outlets, which are not part of the permanent wiring of the building and which are in use by personnel, must have GFCI protection. This is frequently accomplished by feeding these circuits from temporary power panels using GFCI circuit breakers or using power outlet centers designed specifically for this purpose.

When receptacles used for temporary power are a part of the permanent wiring of the building, then the Code requires that "GFCI protection for personnel" be provided. In other words, the permanently installed receptacles are not required to be GFCI protected, but the people using them are. GFCI plugs or portable GFCIs are frequently used for this purpose.

UL has very specific requirements for cord-and-plug connected GFCIs. It is not proper (and is a violation of the listing of the GFCI) to make up cord-and-plug connected GFCIs by installing a GFCI receptacle in a handy box and connecting a cord through a knockout.

The special requirements UL imposes on cord-and-plug type GFCIs recognizes that they can be plugged into receptacles or cord connectors that may not have a reliable neutral connection. If this should occur, the GFCI trip coil, which needs a neutral to operate, will not function and the GFCI tripping mechanism will remain latched closed, even if a fault is present. Therefore, cord-and-plug type GFCIs typically have normally open relays as their interrupting mechanism (or in combination with a latching mechanism) that will prevent the GFCI from being energized if an open neutral condition exists.

GFCI testing

Since every GFCI has an integral "test button," the first and most important test is simply to push the test button after energizing the circuit. The internal test circuit performs a complete test of the functionality of the GFCI. Since each GFCI is required by UL to undergo an end-of-line calibration test to assure that the GFCI is tripping within the prescribed 4mA to 6mA trip threshold within the defined clear time, it should not be necessary to check calibration in the field.

Line-load miswiring. You can easily verify with no special test instruments that line terminations have been made properly. Simply insert a night light (or a circuit tester) into the GFCI receptacle. Push the "test button." If the GFCI trips but the night light or circuit tester stays energized, the GFCI receptacle is wired with reverse line-load connections. The GFCI needs to be removed and properly wired.

Some GFCI receptacles have an integral light that can indicate line-load reversed wiring. With this type of GFCI receptacle, you don't even need a night light; they will indicate line-load reversal when the GFCI test button is pushed.The GFCI trips but the indicator light stays illuminated, indicating that the receptacle is energized even though the GFCI tripped. Since some GFCI receptacles have indicator lights that are normally illuminated when the GFCI trips (the reverse of the type previously described), care must be taken to read and understand the instructions of the GFCI being installed.

GFCI testers. If you're from Missouri and simply don't believe the internal test circuit, then you need to exercise care in selecting the GFCI tester and in interpreting the results. It wasn't until the 1993 Edition of UL 1436, Safety Standard for Outlet Testers and Similar Devices, that the requirements for GFCI testers were coordinated with the requirements for GFCIs themselves. Testers that are not UL listed and UL-listed testers produced prior to 1993 may contain test circuits that do not properly test the GFCI. Indeed, they may indicate an unsatisfactory test when the GFCI is functioning fully in accordance with the UL GFCI standard.

Some testers not meeting UL 1436-1993 have timing circuits that limit the test current to a duration of 200 ms. If the GFCI takes more than 200 ms to trip (as permitted by UL), these testers may never trip some GFCIs that are in full compliance with UL requirements. Therefore, if you choose to use a GFCI tester, make sure it's listed to the 1993 Edition of UL 1436.

Some GFCI testers are relatively simple circuit testers with an additional test button. Others have milliammeters or dials that can be used to select various test currents. When using any of these more sophisticated testers, do not assume that it's accurately checking the calibration of the GFCI. There is very likely some capacitive leakage and possibly some harmless ground fault leakage "preloading" the GFCI. Consequently, a GFCI that is tripping within the UL time/current requirements may, according to the tester, be tripping at 3mA. In this case, the tester is simply giving a misleading reading.

Reverse polarity. Since testers establish a test circuit between hot and equipment ground, if the receptacle into which the tester is plugged is wired with reverse polarity, there will not be a voltage across the tester and test current will not flow. The GFCI will not trip and thus the GFCI might erroneously be considered defective.

Testing GFCIs on nongrounding circuits. Testers should not be used to test GFCIs installed as replacements for two-wire receptacles on nongrounding circuits. By design, GFCI testers will not test a GFCI protecting a 2-wire circuit and can expose the user to a potential shock.

The test button integral to the GFCI applies the test current between hot and neutral. This is not the case with GFCI testers; the test current in these devices is applied between hot and the equipment ground. Therefore, if there is no equipment ground, no test current will flow. If there are any exposed metal parts connected to the receptacle grounding contact (such as a metal face plate or a weatherproof cover), they will be energized by the test device. Since some of the test devices apply up to a 30mA test current; using such a tester on a 2-wire circuit while touching a metal cover plate could result in an uncomfortable shock.

Keys to testing GFCIs.

* Use the GFCI integral test button.

* Check for line-load reversal using the GFCI integral test button supplemented with a lamp or appliance.

* Be sure that any GFCI tester used is listed to the current UL standard.

* Do not use GFCI testers on two-wire nongrounding circuits.


The use of GFCIs, both required and otherwise, has mushroomed over the years, from just underwater swimming pool lights in 1968 to 33 references in the 1993 NEC, with even more to come. Now that GFCI installations number in the millions per year, repetitiveness can result in complacency. Not all GFCIs are alike. The installation layout needs to be evaluated and the most appropriate GFCI selected for the job, and you must adhere to all wiring instructions and markings.

Jack Wells is Vice President of Corporate Development for Pass & Seymour/Legrand.

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