A friend who was buying an older house was concerned about electrical safety and asked for my opinion as an electrical engineer. All of the AC receptacles (also called outlets) in the house were the two-wire non-grounded type with only a hot (black) and a neutral (white) wire; there were no three-wire receptacles with separate Earth ground (green) as mandated by the National Electrical Code (NEC) in the US since the 1960s, Figure 1. (Other countries have similar requirements, but we’ll stick with the US NEC for this discussion.)
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Figure 1 For many decades, home AC-line wiring used a basic two-wire receptacle with hot and neutral wires, but the code was upgraded in the 1960s to mandate a three-wire receptacle with a separate ground wire. Source: NCW Home Inspections
An electrician had told him there were two safety-improvement options: 1) rewire some, or all, of the receptacles to have a true ground, a costly and messy undertaking; or 2) install receptacles with built-in Ground Fault Circuit Interruption (GFCI) functions costing about $20 each at outlets of concern. which is not messy, could be done by anyone with a screwdriver and basic ability, and no electrician needed.
My friend’s questions were these: was using a GFCI on a receptacle without a true ground just a cosmetic, feel-good thing? Did it provide any protection? Full protection? Was it code approved? Most important, would it prevent user shock in case of a fault in the wiring or the load?
My answer was simple: I didn’t know. I assumed you needed a ground for proper GFCI wiring, but the electrical code is complicated with many subtleties.
If you only learned about electricity as part of “Electronics 101” but not from the perspective of the power-electrical code and safety, you’re in for many surprises. There are often requirements that don’t make sense at first, and you are likely to have misconceptions as well. The NEC is very good at what it does and defines, and it characterizes a world which is far different than simply using a qualified AC/DC supply to power your lower-voltage circuits.
I did some research and found that, contrary to my intuition, a GFCI without a formal third-wire ground does provide some protection against some types of faults, but not all. Incidentally, we are talking about a real Earth ground here, not the circuit “common” which is often referred to as “ground” even though it has nothing to do with the Earth ground—a misnomer that is not only widely used but easily leads to sloppy and sometimes dangerous assumptions. Most electronic-circuit “grounds” are not grounds at all, end of story.
A little background: The consumer GFCI was developed in the 1960s; there were earlier designs, but they were subject to false tripping and had higher tripping thresholds. Use of GFCIs was mandated by the NEC since 1968, when it first allowed for GFCIs as a method of protection for underwater swimming pool lights. Throughout the 1970s, GFCI installation requirements were gradually added for 120-volt receptacles in areas prone to possible water contact, including bathrooms, garages, and any receptacles located outdoors. The 1980s saw additional requirements implemented. During this period, kitchens and basements were added as areas that were required to have GFCIs, as well as boat houses, commercial garages, and indoor pools and spas. New requirements during the ’90s included crawl spaces, wet bars and rooftops.
How it works: The operating principle of the GFCI is clear, although implementation has subtleties, of course. The GFCI function is usually built into the AC receptacle and is connected across the three AC-line wires, Figure 2; it is “invisible” to the person doing the installation. Portable and external versions are also available and authorized by the NEC for some situations, but the principle is the same.
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Figure 2 Wiring of a GFCI receptacle is the same as for a non-GFCI unit, as the GFCI function is embedded and invisible to the user. Source: PDH Online
In normal operation, current flows between the hot and neutral wires with the load in between the two, and there is no current flow through the ground wire. When there is a fault such as current leaking from one of the active conductor through the load (appliance, tool, hair dryer) and possibly through a user and then to ground—a potential shock situation—the current instead goes directly to ground, as that is the path with far lower impedance than through a person. The safety and shock risk from current flow is reduced to non-dangerous levels.
If there is no ground connection, or the ground wire is defective (thus, a “ground fault”), the user is at risk. The reason is that the fault current no longer has a low-impedance path to ground, and instead goes through the user, Figure 3. At the same time, the current flowing through the hot conductor is not the same as the current returning through the neutral conductor.
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Figure 3 If a direct, low-impedance path to ground is absent, fault currents may instead flow through the user to ground, establishing a shock risk. Source: Pressbooks/Douglas College, Canada
This is where the GFCI comes into action: it detects this hot/neutral current imbalance and disconnects the hot and neutral lines from the load. When it senses that imbalance of current, a sensor coil within the GFCI generates a small current that is detected by a sensor circuit. If the sensed current is above a preset threshold, the sensor circuit releases a solenoid, and the current-carrying contacts open (“trip”).
How much imbalance is tolerated? The NEC dictates that residential GFCIs designed to protect people (rather than electrical infrastructure) interrupt the circuit within 25 milliseconds if the leakage current exceeds a range of 4 to 6 milliamps. (The GFCI manufacturer chooses the exact setting.) For equipment-only receptacles, the limit is higher at around 30 milliamps.
Note that GFCIs can’t protect against faults which do not involve an external leakage current, as when current passes directly from one side of the circuit through a victim to the neutral wire. They don’t protect against overloads or short circuits between the hot conductor and neutral.
What about non-grounded GFCIs?: The NEC is an evolving document that is updated every few years to allow new technologies and configurations while disallowing others. GFCI’s provide protection whether or not the house wiring is grounded—that’s why they are called “ground fault” devices and not “shock-protection” ones.
Over the years the NEC has mandated use of grounded GFCIs in new installations, but also formally allowed for retrofit installation without a third-wire ground. In such cases, the three-wire GFCI receptacle or its cover place must be marked “no equipment ground.”
A GFCI will help to protect against electric shock where current flows through a person from a hot or neutral phase to Earth, but it cannot protect against electric shock where current flows through a person from phase to neutral or phase to phase. For example, if someone touches both live and neutral wires the GFCI cannot differentiate between current flows through an intended load versus flows through a person.
When you think about it, not having a third-wire ground at all is the ultimate ground fault. A GFCI does not require an equipment-grounding conductor (green wire) since the GFCI detects an imbalance between the “hot” (black) conductor and the “neutral” (white) conductor.
In short: using a GFCI on a non-grounded receptacle does, indeed, provide some level of protection, even though there is no “ground in which a fault can develop”. The GFCI doesn’t magically produce a ground; it just interrupts power when abnormal current flow is detected. Your electronic devices won’t be protected if there’s a ground fault, for example, and a standard plug-in tester won’t work on the non-grounded GFCI outlet (that can be confusing). Still, an ungrounded GFCI outlet will still shut off in the event of a current-flow fault, so it can help keep users safe.
The answer to the question of using a GFCI in a non-grounded receptacle rather than adding a ground wire is easy: do it. The GFCI provides some protection when the ground wire is faulty, and the absence of a ground wire is certainly a clear fault. It provides some level of protection again user shock under the most common wiring and load failure modes.
Dealing with power-line wiring, faults, regulations, and codes is not trivial, but the rules are based on basic and solid electrical principles. It’s easy to think you understand more about it than you actually do, when you don’t grasp the reasoning behind many of the mandates of the code. While ignorance may be bliss, here it can be dangerous, especially when based on overconfidence or misconceptions.
Bill Schweber is an EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features.
Related Content
- (Under)standing Your Ground
- AC grounding: essential, dangerous, or both?
- AC Grounds: So Essential, Except When They’re Not
- When Poor Grounding Leads to More Grounding
- Mistakes were made, even in a simple 3-wire AC hookup
References
- Angi, “Does a GFCI Outlet Need to be Grounded?”
- com, “406.4(D)(2) Non-Grounding-Type Receptacles”
- NCW Home Inspections, LLC, “3 Prong Grounding Type Receptacles on 2 Wire Ungrounded System: A little history”
- EE World Online, “Basics of ground fault interrupters”
- PDH Online Course E321, “Ground Fault Circuit Interrupters”
- Douglas College/BC/Canada, “7 Electrical Safety: Systems and Devices”
- CPSC Fact Sheet, “What is a GFCI?”
- Harvard University Campus Services, “Ground Fault Circuit Interrupters (GFCI)”
- International Association of Certified Home Inspectors, “Ground-Fault Circuit Interrupters (GFCIs)”
- The Holmes Group/Make it Right, “Ground Fault Circuit Interrupters (GFCIs)”
- This Old House, “GFCI Receptacles” (video)
- Independent Alliance of the Electrical Industry (IAEI), “GFCI and AFCI Basics”
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