I live in an older home that doesn't have any ground wiring. Is it safe to just install 3-prong GFCI outlets and use devices and appliances that expect a ground wire connection when I just have the GFCI outlet and no actual connection to ground?
What are the exact differences between a no-ground GFCI outlet and a grounded GFCI outlet in terms of safety?
Absolutely!
There are a number of different functions that an ordinary ground wire performs. A GFCI, despite the "G", does not actually need a ground wire to function. In fact, a properly installed GFCI will provide protection similar to (and actually in some ways better than) a regular ground. That doesn't mean you shouldn't have ground. But it means that if you don't have ground, GFCI can protect you anyway.
When you install GFCI in this way, you do everything the same as a regular installation except that you don't connect anything to the ground screw. You are supposed to label them no equipment ground.
How does this actually work?
The traditional ground wire has no power flowing over it in normal usage, with the exception of some special types of switches or other equipment that are allowed to send a very small amount of current over ground (this generally has to do with smart/dimmer/timer/etc. switches with no neutral available - not ordinary receptacles). The ground gets power in two ways:
The second case - actual faults in wiring or equipment - is designed in a way that if lots of power goes over ground, the breaker (which is on the hot wire) will trip, shutting off the circuit.
A GFCI takes this "fault" a few steps further. Unlike a regular breaker:
A GFCI also doesn't really "care" whether the current is going to an actual ground wire (and directly back to the breaker panel) or if it is going through a person to the ground under their feet.
The end result is that a GFCI can effectively substitute for a ground wire, since the same faults that would normally use ground will instantly trip a GFCI, and additional safety because it trips on small amounts of current and very quickly.
Note that a GFCI does not substitute for the regular breaker in every aspect. In particular, a short circuit where all current goes back on neutral (and nothing on ground or through a person) will have no effect on a GFCI because the GFCI still sees the hot & neutral as balanced.
Note also that "better protection" does not, generally speaking, mean that "everyone should put GFCI everywhere". GFCIs are not free, so there is a cost-benefit analysis involved. GFCI also can have some drawbacks (an unnoticed GFCI trip turning off a refrigerator can result in spoiled food). Since the extra protection provided by GFCI is most useful in wet areas, GFCI protection is required in kitchen, bath, outdoor (rain) and certain other locations, but not "everywhere". That being said, using it in place of retrofitting ground is often practical and cost-effective protection.
In terms of safety, GFCI is all you need. The ruling factor for safety is that the GFCI will shut off if more than 5ma leaks (e.g. via a human being shocked). That is better human protection than grounding, actually.
Lack of ground, however, reduces the chance of a shock happening (and thus a shock being detected). If a human touches hot and ground and the system is grounded, current will flow and the GFCI will trip. If the human touches hot and ground and the ground is isolated, then all of it - hot, human, chassis ground, and any grounds in that "island" (i.e. grounded power strip) -- will cheerfully float at 120V from neutral, like a bird on a wire. No one will ever know there was a problem.
Note that GFCIs do nothing about a human getting between hot and neutral. But that is highly improbable with good equipment design.
GFCI is not a receptacle, it's a protection system. Any GFCI device can protect a downline part of a circuit. (GFCI receptacles protect their own sockets, obviously). So in fact, you can have full circuit GFCI protection without a single GFCI receptacle being involved.
Anyway, GFCI receps give you the choice whether to protect the downline or not. If you don't want to protect a downline, you use only the "Line" terminals - and they are designed to accept 2 wires (check the instructions). If you do, then you remove the warning tape and use the "Load" terminals. In that case, you must label the downline receptacles "GFCI Protected". (you don't need to label a GFCI receptacle that, because it's obvious).
Regardless, if the GFCI is not grounded, you also needs a "No Equipment Ground" label both on the GFCI and any downline receptacles.
The reason for the "No Equipment Ground" label is to warn users that there is no ground to assist in surge suppression. This can be a threat to electronics.
What are the exact differences between a no-ground GFCI outlet and a grounded GFCI outlet in terms of safety?
Consider what happens when a wire comes loose inside an appliance and touches the case metal case. In the design of a "Class 1" appliance this would be considered a "single fault condition".
If there is a proper ground connection then current will flow through the ground connection and cause a trip almost immediately. Since most items are not being touched most of the time, the chances are it will disconnect the fault before anyone gets a shock.
If there is no ground connection then there will initially be no trip, the case of the appliance will remain live until someone (or something) touches it. Only after they have touched it and are getting a shock can the GFCI detect there is a problem and disconnect the power. In most cases, it disconnects the current before the shock becomes fatal but it would still have been vastly preferable not to have had the shock in the first place.
So why do codes in the USA allow fitting 3 pin GFCIs, in place of regular 2 pin outlets in retrofit applications (but not in new installations)? Because it's a lesser evil. In the real world people will use their electrical appliances even if that means cutting off the earth pin or using a cheater plug. It's much safer to have a missing ground and a GFCI than a missing ground and no GFCI.
Summing up and coloring in
Here are several common failure scenarios and the behavior of various grounding configurations. A GFCI without a ground is much safer than a regular grounded outlet or an ungrounded one, and almost as safe as a grounded GFCI. Red is bad. Yellow is undesirable as it employs the user to trip the GFCI but it does so quickly and without injury.
Referring to the Question we are moving from an entirely red column to a slightly yellow one. A good investment for just swapping out some outlets.
There are other situations where a ground is needed eg some medical devices, surge arrestors, etc. I have not tried to account for those.
This is exactly what the electricians did for a few outlets in my home.
I had most of them grounded for the sake of the computers, but there were two or three where it was extra difficult to fish a ground wire and they installed ungrounded GFCI outlets. (House built circa 1951, this work done in 1995 or so.)
Others have explained well why this works, and the limitations, so I won't repeat that; just giving a data point that it is done by licenced professional electricians.
Yes!
GFCI does not need a connection to Earth to work. If you put a GFCI in your main electrical panel, it measures the difference in current between Live and Neutral wires, and if that difference exceeds a threshold like 30mA, it will trip.
The idea is, under normal conditions, all the current that flows from the power company's cable through the live wire then flows through the various appliances in your home then back through the neutral wire. If there is a difference between these two currents, it means some current is returning through a path that it shouldn't take, for example through someone's fingers, their body, their feet, the floor, then through Earth, then back to the power company's transformer neutral which is usually connected to Earth. Or something like that. So it trips.
However:
if someone cleans the toaster in the sink and then plugs it, or if the washing machine has a leak that sprays water into the electric bits, or any other similar problem that causes a current leak from live to accessible metal, then the metal enclosure of the appliance could carry dangerous voltage.
If the appliance is earthed, and the earth wiring in the house works properly, then this causes a current to flow in the Earth wire instead of returning via the neutral wire, and the GFCI trips before someone touches the dangerous device. If the appliance is not earthed (or there is no Earth in the house) then that won't happen, but the GFCI will still save your skin if you touch the appliance and get zapped. In this case you act as the Earth conductor, and you get zapped, but only for about 10 milliseconds.
In theory, a GFCI does NOT protect from electrocution when touching both live and neutral, because in this case there is no way to distinguish a finger from a legit electrical load consuming current. However, if you are standing barefoot on a somewhat conductive floor, enough current will still leak through your feet to make it trip. Same if you dump the hairdryer in the tub, and the tub is earthed, it should trip.
So, basically, if you don't have Earth, the GFCI will not provide an early warning when an appliance is unsafe, but it will still protect you from surprise electrocution. But it is not a magic bullet nor idiot proof.
Safer? Yes. Safe? No! Think for a minute and ask yourself "Why does the NEC require a sticker that says 'No Equipment Ground' if the receptacle is safe?".
The key to this answer is your question asks about "appliances that expect a ground".
The sticker is there to warn you to not use the receptacle where a ground is required.
Here is the edited NEC section that allows GFCI protection for 3 wire receptacle replacement for ungrounded receptacles, note most carefully the Informational Notes at the end.
406.4(D)(2) Non–Grounding-Type Receptacles. Where attachment to an equipment grounding conductor does not exist in the receptacle enclosure, the installation shall comply with (D)(2)(a), (D)(2)(b), or (D)(2)(c).
(b) A non–grounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit interrupter-type of receptacle(s). These receptacles or their cover plates shall be marked “No Equipment Ground.” An equipment grounding conductor shall not be connected from the ground-fault circuit-interrupter-type receptacle to any outlet supplied from the ground-fault circuit-interrupter receptacle.
Informational Note No. 1: Some equipment or appliance manufacturers require that the branch circuit to the equipment or appliance includes an equipment grounding conductor.
Informational Note No. 2: See 250.114 for a list of a cord-and-plug- connected equipment or appliances that require an equipment grounding conductor.
Informational notes remind you to refer to other codes or parts of this code that have bearing. Note 1 generally refers to NEC 110.3(B).
110.3(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.
So if you have a window AC unit, and the user instructions (which are part of the UL Listing) or tag on the cord says to use a grounded outlet then it is not certified for use on an ungrounded circuit, and use of a labelled receptacle does not change that.
NEC 250.114 goes further explaining certain equipment, labelled or not, is not safe enough without a ground.
250.114 Equipment Connected by Cord and Plug. Exposed, normally non-current-carrying metal parts of cord-and-plug-connected equipment shall be connected to the equipment grounding conductor under any of the following conditions:
(3)In residential occupancies:
(a)Refrigerators, freezers, and air conditioners
(b)Clothes-washing, clothes-drying, and dish-washing machines; ranges; kitchen waste disposers; information technology equipment; sump pumps; and electrical aquarium equipment
(c)Hand-held motor-operated tools, stationary and fixed motor-operated tools, and light industrial motor-operated tools
(d)Motor-operated appliances of the following types: hedge clippers, lawn mowers, snow blowers, and wet scrubbers
(e)Portable handlamps and portable luminaires
So an ungrounded GFCI receptacle will provide protection in case of fault to ground with a two-wire circuit, and allows polarized plugs to replace old non-polarized plugs, but it is not legal to use those receptacles for many applications, which is the main reason you are required to label it "No Equipment Ground".
And you asked difference about grounded or ungrounded GFCI's, at least one aspect is if a fault develops to the ungrounded chassis the chassis will remain energized until something or somebody completes the path to ground. A GFCI functions by running both wires through a current sensor (current transformer). If no current is leaking to ground the magnetic fields cancel each other and nothing is detected in the current transformer. If there is an imbalance then magnetic fields don't cancel, current is developed in the CT, and the device is triggered to open. So just energizing the chassis doesn't upset the current flow, a grounded chassis provides that path. I think most of the items listed in 250.114 are there because human contact with the chassis is the likely event to complete the path.
Interesting that this was migrated from the EE SE.
I remediated this problem in my first house. It's very common. Two-prong outlets in an old house and some nitwit puts in three-prong sockets without a ground. The local authority (Ontario ESA in this case) says "No good, either ground it or put GFCIs on every socket" He also said I could put back 2-prong outlets, but I would have to go to an antique shop to find them.
In regards to safety, GFCI ungrounded is IMHO far safer than no GFCI with ground. The GFCI works with a toroid which is electrically balanced between the current in the live and the neutral. If there is an imbalance, a relay triggers and breaks the circuit.
Chassis grounds on things like fridges and dishwashers etc can be dangerous as in the event that a low enough current leaks through the chassis somewhere in the circuit, then every fricking chassis of every appliance with a chassis ground on that circuit will electrify. This isn't even that weird given the service neutral and ground are electrically bonded at the panel. In modern houses, large appliances have their own circuits, so only the appliance itself will electrify, but in old houses, i.e., yours, it's not always clear. In terms of this causing a fuse to blow, it would require a catastrophic short for live to hit ground and put 15A through the chassis. If the neutral shorted to the chassis, you wouldn't even notice a current change in the appliance, but the chassis would still be electrified.
The danger of current passing through your body is all about finding the path of least resistance to ground. If some current takes a shortcut through your body to get to ground, you may be in trouble. When this happens with a GFCI, the current of the live and neutral will differ and that relay will trip. With a well-grounded outlet?... your body will still be part of the circuit, unless you're in the bathtub, you might only feel a buzz, as you have a relatively high resistance compared to the copper wire.
To throw a wrench into the whole thing, large electric motors imbalance the live/neutral current too, spinning up that motor takes current, and when the motor stops, the momentum of the motor can send that current back. Washing machines and electric lawnmowers can trip GFCIs, so there are limits to their usefulness.
In Ontario if you do any work on your own house where you live, you need the electrical inspectors to give you a pass before it's ok. If the house is not yours or you don't live in it, you're not allowed to do the work.
All this said, grounding equipment as a non-safety issue is desirable as it eliminates problems caused by floating grounds https://en.wikipedia.org/wiki/Floating_ground ... but if you're having problems like this, you're on GFCIs and you're not relying on the ground for safety, then you can run a wire from the chassis to a pipe or radiator. Most stuff doesn't even have a three prong plug these days anyway.
Only rewire the house if the walls are already open, you're worried about resale or have problems with insurance.
In the end, I put in GFCIs and the ESA inspector passed me.
Do yourself a favor and install some kind of ground.
There is no such thing as absolutely safe electricity. Any electricity is potentially unsafe, which means anything being safe in electrical systems is always relative to how you treat it. I went to school in a building that had open metal power lines running along the ceiling. That was considered acceptable then, but only safe if you didn't jump for it.
In your house, a GFCI will be an improvement over no GFCI, but installing any ground wiring will be still safer than that. Proper ground wiring has to live up to local regulatory standard, but 'improper' ground wiring doesn't. Electricity will prefer running along a home made ground connection over running through you any time. Provided you stay away from the electrical system itself, you can ground pretty much anything you like completely unrestricted and it does pay off.
A GFCI may be what keeps you alive, but the trip current is still far from enjoyable and that current will pass through you unless it finds a shorter way home.
