A Ground Fault Circuit Interrupter (GFCI) is a device that detects a mismatch in the current flowing in the supply (hot) line and the current returning in the neutral line of an AC electrical circuit. The purpose of the device is to protect people from harm in the case of an electric shock.
Note as of June, 2015: There have been major changes relating to the requirement for marina, boatyard, condo docks, wharves, and other non-residential shore facilities to install GFCI devices on dock feeders. Please refer to my article on this emerging concern, here: https://gilwellbear.wordpress.com/category/boat-technical-topics/electrical-topics/boat-ac-topics/emerging-ac-electrical-concern/. What these changes mean is that boats with ground and leakage faults aboard will encounter loss-of-power situations more frequently, not less frequently.
Cruising boats and boaters may on occasion encounter docking situations where normal 30A or 50A shore power is not available, but where residential standard 120V, 15A/20A power is available. Connecting a cruising boat – with all of the household and marine electrical equipment it carries aboard – to such an outlet may prove frustrating and unworkable. The GFCI device protecting the shore circuit will repeatedly disconnect power to the outlet. However frustrating that behavior may be to the cruiser, it is “normal operation,” and does not represent a fault in the supply circuit. Further, it DOES NOT ALWAYS mean there is a problem on the boat. Read on for a better understanding of the reasons.
In the United States, various local, state and national electrical codes, and the Canadian Electric Code in Canada, require that GFCI-protected utility outlets be installed in certain locations in residential, office and commercial buildings. These locations include bathrooms, kitchens, basements, garages and of course, docks. Similarly, the ABYC Electrical Standard, E-11, requires GFCI-protected utility outlets in certain locations on boats, including galleys, heads, “machinery spaces” (engine room, thruster/windlass locker, lazarette, etc) and weather-decks. These are all “wet” locations where unknown, unexpected, electrically viable leakage paths could be present. Several factors contribute to electrical leakage currents, including the placement, handling and condition of electric cords and/or electric appliances, the design of the appliance itself, capacitative coupling within motors, and even atmospheric humidity. So in wet locations, there is the risk of coming into unexpected and unintentional contact with a live electric circuit. If contact with live electrical circuits were to occur, current could flow through your body to some nearby ground point.
With GFCI protection in the case of unexpected contact, the current flowing from the AC power source to the circuit would be higher by some small amount than the current returning to the source VIA THE GFCI DEVICE; that difference is a leakage current. That small difference in current would activate the GFCI device and the device would disconnect the power to protect persons or pets from potentially dangerous or lethal electric shock.
Household GFCI devices are intentionally designed to be very sensitive. They are built to detect a very small imbalance in current of 5 milliamps (mA, thousandth of an amp) lasting for only a very short time of 7 milliseconds (mS, thousandth of a second). Industry and medical experts assert that this level of shock for such a short time interval will not injure people or pets.
Residental GFCI protective devices come in two types. One type of GFCI is built into an Overcurrent Protective Device and is installed as a circuit breaker in the electrical distribution panel of the building. The other more common type is built into individual utility outlets. GFCI devices have a “test” and “reset” push-button.
Because of the very nature of GFCI devices, they are subject to a phenomenon known as “nuisance trips.” Nuisance trips result from small leakages in components within the attached circuit that are otherwise allowable within the design of a residential electrical system. Why? All electrical devices have some very small allowable leakage values associated with them. Insulating materials are not “perfect.” The specific values for a given device are buried deep in the manufacturing specifications and relevant Underwriter’s Laboratory (UL) testing specifications. Some electrical system attachments (ex: TVs, entertainment system components, computer and peripheral device power bricks, rechargeable battery chargers, surge suppressors, powered appliances, powered tools like drills, saws and polishing buffers), heat pump compressors and water pumps can cause false trips when connected or when power switches are turned on, and particularly when they are all turned on at the same moment.
The nature of boats adds to the available number of leakage paths not generally associated with residential dwelling units, offices or light commercial buildings. On boats, “reverse polarity” indicators are required by the ABYC Electrical Standard, E-11, on 120V, 30A shore power circuits. Reverse polarity devices connect between the neutral and the green safety ground. To avoid nuisance trips caused by returning current diverting through reverse polarity alarms, the ABYC E-11 Standard specifies that the minimum net electrical impedance from reverse polarity devices must be at least 25K ohms at 60Hz. But, not zero, and again, some leakage is allowable. And, multiple reverse polarity devices (common on boats today) reduces that impedance significantly. Household surge suppression devices use in-line inductive components and bypass capacitors from the neutral to safety ground. Modern electronic power inverters (particularly switch-mode, “modified square wave” devices) generate a lot of undesirable electrical “noise” which affects the HF and VHF radio frequency spectra. Suppressing this “rf noise” requires the use of inline inductive components and bypass capacitors from neutral to safety ground and on the DC input lines. GFCI devices, themselves, have inherent allowable leakages, so a boat with, for example, 10 GFCI-protected outlets presents 10 times the allowable per-unit leakage to the shore power device at power-on. So, all these devices each have tiny, “normal,” allowable amounts of leakage current, which are cumulative on the circuit that powers the boat.
At the moment shore power cords are connected/disconnected from their dockside shore power receptacles and the boat initially becomes energized, all of the electrical devices that possess “normal, allowable” leakage from the neutral to the safety ground, as discussed above, are energized at once. There are several variables that all play a role in what happens next, (capacitive leakage in motor windings, humidity) and at least some of them conspire to create a “surge” in leakage currents that can easily exceed the residential GFCI specification of 5 mA for 7 mS. The result is, a residential 120V, 15A/20A GFCI outlet will disconnect the complex circuit represented by the boat with all its installed electrical equipment. Cruising boats will frequently be unsuccessful in connecting to these power outlets. Dockside 120V, 15A/20A GFCI outlets are intended for electric trimmers, weed eaters and edgers, not entire complex sub-panel systems like cruising boats.
Notwithstanding the above, if a GFCI outlet experiences frequent trips, that symptom should always be investigated as a potentially valid problem.
GFCI utility outlets can be wired to protect additional “downstream” non-GFCI outlets. This is a common method used to minimize installation cost while achieving GFCI code compliance coverage. Because leakage currents are cumulative, however, this method of wiring can aggravate nuisance (false) tripping.
The first step in diagnosing GFCI tripping for nuisance tripping is to change the point-of-attachment of electrical equipment to another circuit. This is to determine if the trip follows the attachments. If so, investigate that symptom as a problem with the attachment(s). Remove any power strips with multiple power bricks and re-test. Reduce the number of power bricks or electronic devices connected to the GFCI-protected circuit by setting the feed circuit breaker “off.” Replace “downstream” non-GFCI utility outlets with outlets having built-in GFCI protection. Doing so requires re-wiring the original feeding outlet, too. Finally, if a particular GFCI circuit breaker or outlet causes frequent nuisance (false) trips, replace the outlet unit with a new GFCI device.
The internal mechanism of the GFCI devices are subject to deterioration over time due to vibration, wear and exposure to environmental contaminants like airborne moisture, oils, dust, etc. All GFCI device manufacturers recommend testing these devices monthly.
An excellent reference article on GFCI, ELCI and GFPE technologies can be found here: http://www.ecmag.com/section/codes-standards/differences-between-gfci-idci-and-gfpe.