preface This article is about the use of Ground Fault Circuit Interrupters (GFCIs), also called Ground Fault Interrupters (GFIs), in homes in the United States. As with all the articles on this web site, the information gathered here is not intended to be instructions for do-it-yourselfers. There are two general categories into which GFIs are placed: equipment protection, and personnel protection. The method of operation is the same for both kinds of GFI, but the trip parameters are different. A GFI for personnel protection is more sensitive than the type for equipment protection. In this article, I will be concerned with the type for personnel protection, as it is used in a residence in the United States. what is a GFI and what does it do for me? A Ground Fault Interrupter (GFI) is a device that senses the presence of any stray ground path on a power distribution circuit, and interrupts the power to that circuit. The NEC, and many others, use the (perhaps more accurate) term "Ground Fault Circuit Interrupter" (GFCI) to mean the same device. GFIs are there to protect people against electrical shock. They also provide some measure of fire and equipment damage protection. Electrical systems, tools, and appliances are designed to prevent contact by people with the electricity. However, accidents happen, and equipment gets damaged or wears out. A typical example is a power tool being used outdoors. If the tool becomes wet, for instance, a path can be established that bypasses the tool's insulation and allows the metal case of the tool, in contact with the user, to become electrified. The user becomes part of the return path for electrical current by way of the earth on which he is standing. If a GFI is protecting the circuit, it will sense this condition and immediately remove the power. The user will be annoyed, but unhurt. A GFI for personnel protection also provides some additional equipment damage and fire protection. A partial ground fault to the hot wire can cause dangerously high temperatures to develop at the point of contact, and may not draw enough current to trip the circuit breaker. A GFI will detect the condition and protect against a fire hazard when a circuit breaker alone might not. GFIs found at homes come in two styles: outlets and circuit breakers. Both perform the same job in the same way, but differ in their physical construction. Both kinds have a "test" button, and a means to reset the unit after it trips. things that go wrong with GFIs the GFI trips immediately when reset, or won't reset This is by far the most common complaint about a GFI. There are just two possible causes: either the GFI has failed, or there is a ground fault. Failure among newer (under around 10 years old) GFIs is rare, but it happens. What follows is an outline of how to find the source of the trouble. 1. Identify all the outlets that are affected. Usually these can be identified by virtue of being non-functional for no apparent reason. Don't forget to check the outlets outdoors, and in the basement. 2. Eliminate cord-connected equipment: Once the outlets are all located, unplug everything connected to these outlets. This, of course, is to eliminate connected devices as the source of the trouble. If the GFI can now be reset, just plug everything back in one thing at a time, until it trips. That should identify the culprit. If the GFI will still not reset, don't plug things in again just yet. Proceed with the next step. 3. Proceed beyond here with caution: Since we have eliminated things plugged-in, we are left with either the wiring or the GFI itself. Going beyond this point involves taking apart the outlets, so decide whether you should continue on your own or call an electrician. 4. Check the GFI: The next step is to remove the load from the GFI. Remember to turn off the power, first. If the GFI is an outlet-type device, it will have a pair of "line" terminals and a pair of "load" terminals. The "line" terminals are the unprotected (by the GFI) incoming power from the circuit breaker, while the "load" terminals are the protected power going from the GFI to protected outlets elsewhere. There must always be wiring connected to the "line" side, and may be wiring connected to the "load" side. Disconnect the "load" side, and temporarily insulate the loose wire ends by twisting-on (gently, it's temporary) a twist-on connector, or with electrical tape. Make sure the connections to the GFI cannot touch anything, including you, and turn the power on. If the GFI will now reset, the problem is with the wiring connected to the load side. If it won't, then replace it. The process is similar for a circuit-breaker-style GFI, except that instead of "line" terminals, there are the buss connector and the white pigtail (the pigtail is the line-side neutral connection). Most of the electrical panel will be "live", so it is a good idea to turn off the main power before proceeding. Disconnect the two (both hot and neutral) load-side wires from the GFI breaker, and temporarily insulate them. Turn the power back on, and try to reset the breaker. If it will reset, the problem is in the wiring or outlets. If it won't, the problem is with the breaker, and it should be replaced. If the GFI outlet or GFI circuit breaker will reset with no load connected, the problem is not with the GFI, so put it all back to gether and keep looking. 5. There remain the wiring and the outlets as potential locations of the problem. I will list things to look for in the order I think is most likely: Outlets — outdoor outlets first: By far the most frequent source of problems is the common practice of protecting the outside outlets with the same GFI as the bathroom outlets. If the outside outlets are "dead", the next thing to do is to turn off the power and take each box apart, inspecting carefully for insect nests, webs, dirt, moisture, loose connections, corrosion, and anything that looks ugly. Occasionally the bare ground wire makes accidental contact with a current-carrying wire. If it touches the not wire, the circuit breaker may trip, but if it touches the neutral, it won't. Either situation will trip the GFI. In cable-wired systems, rough handling during installation sometimes tears the cable, allowing the clamp or box to touch the wires inside, causing a ground fault which will trip the GFI. A damp (not wet) cloth and old toothbrush, and perhaps some sparingly-applied contact cleaner or WD-40 will help clean the outlets and the inside of the box. If the outdoor outlets are a few years old, consider replacing them -- outlets are cheap. Outdoor outlets should have rain tight covers. If equipment is continuously plugged-in, spring for the rain tight-while-connected covers -- they are well worth the investment of $15 to $20. Indoor outlets: Indoor outlets, being better protected than outdoor outlets, are much less likely to develop ground faults. However, if the outside outlets have been eliminated, the next thing to check is the indoor outlets. GFI-protected indoor outlets are likely to be located in the bathroom or kitchen, both of which can be sources of high humidity. Again, inspect for dirt, webs, moisture, and so forth, as described for outdoor outlets. This is a good time to replace any outlets that seem to be aging, corroded, or otherwise questionable. Light fixtures and switches: Bathroom light fixtures, associated switches, and sometimes exhaust fans and other things used to be frequently supplied by the bathroom GFI. These devices are subject to the same kinds of problems as outlets. Disassemble each in turn and inspect and repair as for outlets. While disconnecting equipment and switches to eliminate trouble sources, be sure to disconnect both the hot and neutral wires. If the wires are of substantially different length, the GFI may trip. (Occasionally, amateur electricians become creative with wire routing, and route the hot wire and neutral via different routes. This is a code violation, and might also cause a GFI to trip.) Wiring: If you have thoroughly inspected the outlets, switch, and fixtures, and haven't found the problem, it must be in the wiring. Inspect all the wiring to which you can gain access for damage, too-tight staples, penetration by nails, rodent tooth-marks, and the like. For those sections that are not accessible, disconnect the wiring at each outlet in the string starting at the outlet farthest from the GFI, and proceeding towards the GFI, checking whether the GFI will reset at each step. Stranger than fiction: Here are a few things I have actually found while tracking down GFI problems: - Fire ant, wasp, and spider nests and webs.
- Corrosion sufficient to bridge from the screw terminals to ground.
- Ground wires touching a current-carrying wire.
- Wiring to the backyard fountain with a leaking buried splice.
- One current-carrying wire connected, but not the other.
- Buried cable cut off and re-buried by landscapers.
- Buried cable damaged by shovels and picks.
- Buried cable stripped of insulation by gophers.
- Outdoor outlet inundated by sprinklers.
- Corroded outdoor lighting.
- Various objects inside outlet boxes.
- Wiring penetrated by nails during re-roofing, re-shingling, deck building, and the like.
- Metal staples driven too tight, gradually penetrating the cable sheath.
- Rodent damage in attics, in crawl spaces, and inside walls.
- Water from leaking plumbing inside walls and ceilings.
- Metal box clamps too tight, eventually penetrating the cable.
- Various defective tools and appliances connected to the circuit.
- Once in a while, a defective GFI.
the GFI doesn't trip when the test button is pressed First, make sure the power is on. The GFI test circuit won't operate without power. If power is on (plug something into one of the GFI-protected outlets to check), and it still won't trip, replace the GFI. Make sure you are pressing the correct button – the labeling is usually very hard to read. Sometimes, the connected devices (try it with an incandescent lamp) will turn off while you are pressing the button, but come back on again when you release it. If this happens, you are pressing the reset button, not the test button. GFI circuit breakers, of course, do not have a reset button. how a GFI works  Figure 1 — Pressing the reset button "hooks on" to the contacts.
Incoming power arrives at the interrupter contacts through a spring-brass metal strip. In its relaxed position, the contacts are open, and no current flows. The first of two steps in closing the contacts is to press the red reset button. A small brass link with a hook on it is attached to the button. Pressing the button pushes the link down so that the hook catches under the contact. One can hear a tiny "click" (actually two clicks — there are two contacts) when this happens. |  Figure 2 — Releasing the reset button closes the contacts.
As the reset button is released, two small springs under the button push it upward, pulling the link and the attached contact with it. This arrangement ensures that the springs, and not outside forces, exert a controlled amount of pressure on the link. The contacts are now closed, and power is applied to the protected outlets. The GFI will remain in this state until it is tripped. |  Figure 3 — Under normal conditions, current flowing to the load is equal to current returning from the load.
As illustrated in this sketch, electrical current flows in a closed path from the source (labeled "line"), through the load, and back to the source. The current flowing toward the load is exactly equal to the current returning from the load. Current in both directions is made to pass through the center of a detection coil (L1). Current passing through the detector coil produces a voltage at the output terminals of the coil, but because the outbound and inbound current is exactly equal and opposite, the two currents together exactly cancel and produce no output voltage from the detector coil. |  Figure 4 — Close-up of the detection transformers.
The differential current is sensed by the upper coil (green). The lower coil injects a signal into both conductors to detect a grounded neutral. The "hot" wire is the insulated conductor shown passing vertically through both coils, and the "neutral" wire is the bare wire. The coils are wound on a torroidal (donut-shaped) core made of a material called "ferrite", which is an efficient conductor of magnetic flux. The arrangement, along with the many turns of wire wound around the core, is called a torroidal transformer. The very large number of turns produces a useful detection voltage from the magnetic field of the line conductors passing through the center. |  Figure 5 — When a ground fault occurs, part of the current returns via the ground path.
When some fault in the insulation of the conductors or the appliance, or accidental contact with a person or animal causes leakage of current to ground, that part of the current does not flow in the intended path, so the current in one conductor does hot quite equal the current in the other. In this condition, the magnetic fields of the hot and neutral conductors do not exactly cancel, and the detection coil produces a voltage. The ground fault detector circuit senses the voltage from the detection coil and sends a pulse of current through the trip solenoid. |  Figure 6 — When a ground fault occurs in the neutral wire, neutral fault detection signal current flows.
Neutral-fault detection is an additional feature of the GFI. If no load is connected, a ground fault in the neutral conductor could escape detection. This situation is not nearly so dangerous as a fault in the hot conductor, but could become dangerous if further faults develop. To detect the condition, the GFI generates a signal, which is induced into both conductors (i.e. it is a common-mode signal). If no ground fault exists, there is no closed path for the signal to follow, hence it causes no current and is not seen by the detection coil. When a fault develops, the signal returns to its source via the grounding system, producing a current which is detected, causing the GFI to trip. The voltage and current levels at which this signal operates are very small, and have no effect on electrical equipment. |  Figure 7 — When the GFI is tripped, the load contacts open to stop all current.
The solenoid unlatches the interrupter contacts, which spring open to disconnect the protected wiring. The GFI will remain tripped until it is manually reset, so that the cause of the trip can be corrected safely. |  Figure 8 — A signal from the ground fault detector trips the mechanism.
If the detection circuit senses a ground fault, the circuit will apply a pulse of current to the trip solenoid. The solenoid exerts force on the brass link, pulling it away from the incoming power contact, and releasing the contact from the hook. Because the contact is a spring, it snaps downward to its resting position, opening the interrupter contacts and removing power from the protected circuits. |  Figure 9 — The trip solenoid.
The trip coil is the white cylindrical object at the center. It is located just beneath the test and reset buttons shown in Figure 1b. When activated by the Ground Fault Detector, the coil pulls the metal rod protruding from the left side of the coil, which releases the spring-loaded interrupter contacts. This view is from the front side of the GFI with the buttons and metal mounting strap removed. |  Figure 10 — Close-up of the detection circuit.
With amazingly few electronic parts, this circuit detects ground faults, generates the neutral-fault detection signal, and operates the trip solenoid. The simplicity is made possible by the integrated circuit (the black device with the printing on it), which contains most of the circuitry. |  Figure 11 — The test function.
The test button connects a resistance between the load side of the "hot" wire and the source side of the "neutral" wire, so that a small, calibrated amount of current is added to the current carried by the "hot" wire, and not to that carried by the "neutral" wire. The GFI should trip in the presence of this small imbalance, demonstrating that the device is working. |  Figure 12 — Close-up of the TEST and RESET buttons.
The red reset button mechanically latches the interrupter contacts. The black test button closes the contact in the foreground, activating the test circuit, which causes the interrupter contacts to trip. The interrupter contact for the hot side of the circuit is just below the test circuit contact. The neutral contact is on the other side of the black button out of view here. | where is a GFI required? Use of a GFI is prescribed in numerous situations by the NEC. Some of the most common requirements in homes include bathroom outlets, outdoor outlets, kitchen countertop outlets, outlets in a garage or outbuilding, and a few more. In older wiring, there may be no grounding method at the outlet boxes. These older systems use two-prong receptacles with no grounding conductor. It is often desirable to replace one or more of these with three-prong grounding-type outlets to accommodate equipment with a three-prong plug. A GFI-protected three-prong outlet can replace an ungrounded two-prong outlet, and can be operated without a ground connection because it provides protection for people equivalent to that provided by the ground wire. A GFI outlet used in this manner must be labeled "No Equipment Ground". In addition, other outlets that are protected by a GFI (either a GFI outlet or a GFI circuit breaker) may be replaced with three-prong outlets. These outlets must be labeled "No Equipment Ground" and "GFI Protected". Do not connect the ground wire to a GFI or outlet used in this way. where does the NEC require a GFI? This is an index to numerous rules in the 2002 edition of the National Electrical Code. Refer to the NEC for the exact meaning of these rules. | section | item | 210.8(A)(1)
210.8(B)(1) | in homes: outlets in bathrooms | | 210.8(A)(2) | in homes: outlets in garages and accessory buildings with the floor at or below grade level (with exceptions for dedicated outlets) | | 210.8(A)(3) | in homes: outdoor outlets (with exceptions for deicing equipment) | | 210.8(A)(4) | in homes: crawl space outlets | | 210.8(A)(5) | in homes: outlets in unfinished basements | | 210.8(A)(6) | in homes: kitchen counter-top outlets | | 210.8(A)(7) | in homes: outlets within six feet of a wet bar sink | | 210.8(A)(8) | in homes: outlets in boathouses | | 210.8(B)(2) | except in homes: outlets on rooftops | | 210.8(B)(3) | except in homes: all kitchen outlets | 426.28
426.32 | fixed deicing and snow melting equipment (with exceptions) | 427.22
427.27 | pipeline heaters (with exceptions) | | 511.12 | commercial garages (a.k.a. repair shops) where portable equipment is to be used | | 527.6(A) | construction sites (see also OSHA and other agency rules) | 550.13(B)
550.32(E) | mobile homes: outlets outdoors, compartments accessible from outdoors, bathrooms, kitchen countertops, within 6 ft. of a wet bar sink (with exceptions) (Note: The state of California regulates most mobile homes in the state, and does not use the NEC for mobile homes.) | | 551.40(C) | recreational vehicles: In an RV having just one branch circuit, that branch circuit must be protected by a GFI. | | 551.41(C) | recreational vehicles: outlets adjacent to a bathroom lavatory, outlets serving the countertops and outlets within 6 ft. of any lavatory or sink (with exceptions) | | 551.71 | RV park: all ordinary outlets for RV connection or equipment | | 552.41(C) | park trailer: outlets adjacent to a bathroom lavatory, outlets serving the countertops and outlets within 6 ft. of any lavatory or sink (with exceptions) | | 555.19(B) | marinas and boatyards | | 680.22(5) | pools, fountains, etc: outlets within 20 ft. of a pool or fountain, and receptacles that supply pool pump motors (with exceptions) | | 680.23(A)(3) | pools, fountains, etc: underwater lighting fixtures over 15 volts | | 680.27(A)(5) | pools, fountains, etc: electric pool covers | | 680.32 | pools, fountains, etc: all electrical equipment used with storable pools | | 680.43(2) | indoor hot tubs: outlets within 10 ft. of an indoor hot tub | | 680.43(3) | indoor hot tubs: outlets that supply power to an indoor hot tub | | 680.44 | hot tubs: outlets that supply power to any hot tub (Note: Most rules that apply to pools also apply to hot tubs.) | | 680.51(A) | fountains: fountain equipment over 15 volts | | 680.57 | fountains: signs near a fountain | | 680.62 | therapeutic pools and tubs | | 680.71 | hydromassage bathtubs | | 690.5 | roof-mounted solar photovoltaic systems must have DC GFI protection for fire prevention | | 700.26 | emergency systems do not require GFI protection, but do require ground fault indication | Other GFI Rules | section | item | | 215.9 | For 15- and 20-amp branch circuits, protection required by 210.8 (branch circuits) and Article 527 (temporary wiring) can be provided by GFI circuit breakers | 215.10
230.95
240.13 | ground fault protection for equipment is required for certain high-power industrial three-phase systems | | 406.3(D)(2) | When replacing an existing receptacle, a GFI must be installed where required by code, whether or not the original receptacle is a GFI. | | 406.3(D)(3)(b) | Where no grounding means exists at an outlet box, a non-grounding receptacle may be replaced with a grounding-type GFI receptacle with no ground connection. The receptacle must be marked "No Equipment Ground". If the GFI protects additional loads, a ground wire must not be connected to the supplied loads. | | 406.3(D)(3)(c) | Where no grounding means exists at an outlet box, a non-grounding receptacle may be replaced with a grounding-type receptacle with no ground connection if it is protected by a GFI. The receptacle must be marked "No Equipment Ground" and "GFCI Protected". A ground wire must not be connected between grounding-type receptacles so used. | | 517.17 | special rules for feeders at health care facilities | | 647.7(A) | rules for sensitive electronic equipment on separately derived systems |
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