I understand that the green wire attaches to a bare ground wire and provides a path to ground in case the hot wire touches something that it’s not supposed to. But doesn’t this just complete the circuit? Assuming that a breaker doesn’t trip, how would I know if the ground was carrying current? Do I care? I assume that I do.
Don’t need answer fast.
You don’t know, unless you have a GFCI breaker.
It’s certainly possible to have some ground fault that is undetectable and causes no functional issues, but it would be pretty unusual. But - that’s why GFCIs were invented - to protect from ground faults that were small enough to not cause the breaker to trip, but still present a shock hazard.
A ground fault current means that the usually balanced current running between live and neutral becomes unbalanced. A GFCI watches this balance and trips if the imbalance exceeds a given (small) current. It isn’t just current running down the ground wire. Any current finding it’s way outside the live/neutral pair will cause a trip. So current running up your arm and down your leg through the physical ground. Which is why GFCIs are much more useful. Much more than 20mA can kill you. So they watch for this sort of an imbalance.
The purpose of the ground wire is to bring a low resistance high current capacity to the metal shell or components of a device. A proper fault, one that connects the power to any metal will result in a large fault current that trips an over current breaker or fuse. This fault path requires many Amps of current. Orders of magnitude more than needed to stop your heart. This protection is mostly about preventing fires or catastrophic damage. But does prevent some electrocutions.
Double insulated appliances don’t have any exposed metal and don’t get a ground wire. The GFCI in the supply circuit still acts to protect humans in case of damage exposing a live conductor.
Typically the ground conductors in a system are bonded to the neutral in the distribution system. And the ground point is bonded at the same location to a buried ground system that eventually ties the distributed ground of all the consumers and supply. This stops various parts of the system from floating to unhappy (ie high) voltages relative to ground.
That’s not the whole story. Consider all of the possibilities of two types of faults, a broken wire and a short circuit.
Let’s say you have an appliance with a metal case, and you have a 2 wire system. The metal case is connected to the neutral wire for safety.
If the hot wire shorts to the case, the breaker trips.
The neutral wire is already shorted to the case, so that’s not a fault.
If the hot wire breaks, the device goes dead, but the case is still grounded and there is no shock hazard. No biggie.
If the neutral wire breaks, and someone turns the device on, the case now becomes “hot” and is a potentially fatal shock hazard.
Now let’s change that to a 3 wire system with a separate safety ground (the green wire) instead of the neutral being used for the safety ground.
If the hot wire shorts to the case, the breaker trips.
If the neutral wire shorts to the case, you technically have a fault, but the case is still grounded so it’s not a shock hazard.
The ground wire is already connected to the case, so the ground wire shorting to the case isn’t a fault.
If the hot wire breaks, the device goes dead, but the case is still grounded and there is no shock hazard.
If the neutral wire breaks, the device goes dead, but the case is still grounded and there is no shock hazard.
If the safety ground breaks, you’ve lost your safety ground, but the device still functions normally, the case doesn’t have any dangerous potential on it, and there is no shock hazard.
So it only takes a particular single fault for the 2 wire system to kill you, but any single fault in a 3 wire system will not kill you. You need to have multiple faults before the 3 wire system becomes potentially deadly (ground wire breaking combined with hot wire shorting to the case, for example).
You wouldn’t. This is the whole reason GFCIs were invented.
A regular breaker protects the wiring. If there is too much current, the breaker will trip. Without the breaker, the wire could overheat and cause a fire that burns down your house (potentially killing you).
But a regular breaker only trips if there is a rather excessive amount of current, more than 15 amps on a typical U.S. circuit. The “safe” level of current that you can pass through the human body is 5 mA, which is 0.005 amps. That’s quite a bit less than 15. So breakers will save you from dying in a fire, but will still easily allow a fatal amount of current to flow through you.
A GFCI (Ground Fault Circuit Interrupter) measures the current through the hot and neutral. If they aren’t the same, then the electricity has found some other path back to ground. It might be through the green wire. It might be through your plumbing. It might be through your home’s aluminum siding. Who knows. But the point is that this is a bad thing. Remember how I said the “safe” current level is 5 mA? Guess what current GFCIs trip at? You guess it, anything greater than 5 mA.
But a regular breaker and a GFCI both won’t protect you from something like a frayed extension cord, which will cause a fire at significantly less than 15 amps. So this is why they created AFCIs (Arc Fault Circuit Interrupters). This protects you from bad extension cords, which historically have been one of the leading causes of house fires.
BTW, “safe” is in quotes throughout this post because it is mostly theoretical with some animal testing to back it up. There hasn’t actually been a whole lot of human testing done to verify this value for reasons that I hope are rather obvious. Most safety standards these days are built around this 5 mA value.
All of this leads to a rather interesting discussion of why do we have a grounded system in the first place? If you keep your electrical system isolated from earth ground, you now have two “hot” wires and you can safely touch either one and not get shocked. You just can’t touch both. And in fact hospital operating rooms and other “wet” locations do use isolated systems, because they are safer (there’s a whole history of how the standard for this evolved out of hospital operating rooms because patients were dying and doctors initially didn’t know why).
So why don’t we use isolated systems for residential power? The reason is that hospitals have to go through great pains to keep their isolated systems isolated, and they have to be tested every year and properly maintained. If you tried to run an entire residential power system completely isolated, what you would find instead is that mother nature would randomly insert ground connections all through your system, by doing things like blowing tree branches into wires and such. It is much easier and safer to have a dedicated safety ground than to have a randomly grounded system. It’s easier to detect ground faults too.
Some ships use isolated systems. Keeping their electrical systems isolated in a salt water environment can take a lot of effort.
Thanks for the replies. Everyone has mentioned GFCIs. My house is around 15 years old and about a third of my breakers are GFCI and are for the bedrooms and living room. All kitchen and bathroom breakers are non-GFCI but each circuit has at least one (and maybe only one) GFCI receptacle. AIUI, one receptacle protects the entire circuit. Is there a reason/advantage for this division? ISTM that it would be easier to just use all GFCI breakers. If the answer is “cost”, then I understand. If the answer is “code”, what’s the reasoning behind the code?
Excellent comments. But would like to elaborate a bit on this.
(I know you’re aware of all of this, ecg, but am sharing this for the benefit of some of the other folks.)
If you’re in the U.S., the transformer for your house - the one that’s hanging out there on the pole - has two terminals on the primary winding and three terminals on the secondary winding. The primary voltage is around 7200 VAC. In addition, one of the primary terminals is connected to the earth (literally, the dirt). For the secondary winding, there is a terminal on each end of the winding, and one “tap” in the center. The nominal voltage between the two end terminals is 240 VAC, and the nominal voltage between either end terminal and the center tap is 120 VAC. The secondary winding connects to your house using three wires: L1 (which connects to one of the “end” terminals on the transformer’s secondary winding), L2 (which connects to the other “end” terminal on the transformer’s secondary winding), and N (a.k.a. neutral, and connects to the center tap of the transformer’s secondary winding). 120 VAC receptacles and loads in your house are connected between L1 and N or L2 and N. 240 VAC receptacles and loads in your house are connected between L1 and L2.
Finally, transformers have galvanic isolation between the primary and secondary windings. For an ideal transformer, you can think of the secondary winding as a battery sitting on a table that produces AC voltage.
Let’s say nothing on the secondary side of the transformer is connected to the earth. In other words, let’s say we do not connect a copper wire between L1 and the earth, or L2 and the earth, or N and the earth. We would then say the entire secondary side of the transformer, including all the receptacles, wiring, electrical appliances, etc. in your home, would be “floating” above ground. As if the entire house is being fed from an AC battery that is isolated from ground.
So what would happen if you did this?
Well, all the receptacles in your house would still work fine - they would still have 120 VAC. In fact, pretty much everything would work fine, e.g. lights, 240 VAC loads, etc.
Now let’s say you are standing barefoot on a concrete floor in the basement, and you accidently touched the “hot” terminal in a receptacle (which is connected to ether L1 or L2). What would happen? Would you feel a shock? Would you feel nothing?
Well, that depends.
If the transformer out on the pole has perfect isolation between the primary and secondary windings, and there’s a guarantee that none of the wiring, receptacles, appliances, etc. are making a connection to the earth, then you won’t feel a thing.
And… the previous paragraph describes what hospitals essentially do. But you don’t live in a hospital, and the transformer out on the pole has pretty lousy isolation; the resistance between the primary and secondary windings is not super high, and (more importantly) the capacitance between the primary and secondary windings is not super low. Which means that, even though all of the receptacles and appliances and stuff in your house would be fed the proper differential voltage of 120 VAC or 240 VAC, the entire secondary circuit could “float up” to a very high voltage relative to the earth due to the non-infinite impedance between the transformer’s primary and secondary windings. In other words, L1, L2, and N could each be at thousands of volts relative to the earth. Not good.
So how do we fix this problem? Simple: by connecting L1, or L2, or N to the earth using a copper wire and ground rod that is pounded into the earth. This will keep everything from floating up to a dangerous voltage. Of the three, N is the best one to connect to the earth, since it means the max voltage between L1 and the earth (and L2 and the earth) will never be higher than 120 VAC.
One answer is convenience. Far easier to reset a tripped GFCI right next to the sink than it is to hoof it to the breaker box (which in my house is outside, so means putting on some clothes).
Even worse is when the builder installs a GFCI outlet in one bathroom and puts the outlets in other bathrooms downstream of that outlet. Cheaper, and to code, but inconvenient.
By the way, if you are ever unfortunate enough to be in a hospital, the outlets are color-coded.
The orange outlets are the ones that are isolated. They have to be tested and certified every year to make sure that they maintain their isolation.
Red outlets are connected to backup power of some sort. They might be on a UPS or they might be connected to a backup generator. If you have a critical medical device that needs to function even if the power goes out, plug it into one of these red outlets.
White outlets are just standard outlets, the same as what you have in your home. Nothing special about them. They aren’t isolated and they aren’t connected to backup power. If the outlet has a green dot on it, then it is “hospital grade” which means it conforms to higher standards for durability (in other words, it’s a step up in quality from the el-cheapo outlets you get in the bargain bin at Lowes or Home Depot).
But what if I want backup power that’s also isolated? 
I just realized something… my previous post focused on the primary purpose of connecting the center tap on the transformer’s secondary winding to a ground rod that is pounded into the earth. But the OP is referring to something different, namely devices, loads, fixtures, appliances, etc. that have a “ground” wire. This wire serves a different purpose, which you and others have explained very well.
Unfortunately the term “ground” is used in many different ways in electrical / electronics systems. It’s so damn confusing.
That’s why you distinguish between the EGC - Equipment Grounding Conductor and the GEC - Grounding Electrode Conductor.
When I was a kid in the 80s, my folks had a bathroom redone and it had I guess an early GFCI. We quickly figured out that you had to turn the hair dryer on slowly, from Off to Lo to Hi to Max Power where to is a half second pause. Being familiarvwith fuses and breakers, I remember thinking at the time it must be because of the high heat/high power (1500 Watts!) of the appliance but that was a misdirection.
What I now know is that the GFCI was tripping when the current that energizes the blower motor wasn’t simultenously returned to the neutral. But doing it in steps as described kept the current differential small enough that its rise with respect to time stayed below the GFCI’s threshold.
It’s been a useful, practical memory to help me illustrate or visualize motor currents and magnetic fields and junk. I picture the mag field inflating and the current to do so won’t fully return on neutral until the motor reaches steady state.
Yep, nuisance trips from motors were very common in early GFCIs. Some brands were worse than others.
I know a few people who had treadmills in basements that would often trip the GFCI.
Hmm, that would better say “won’t fully return to balanced until ..” The current is itself returned as the field collapses/deflates when power is removed.
Those and vacuum cleaners & blenders & air compressors at least have big motors. I don’t remember that old hair dryer moving much air and the motor and associated reactance couldn’t have been very big, it was just a sensitive circuit interrupter.
I recently got a new audio amplifier that noticeably dims the lamp on the power strip when I turn it on. I wince and think, ‘Thaaat can’t be good,’ and it isn’t but it’s also not bad. I low-level wish it didn’t do that but sounds great sooo I’ll live with it.
A related question: I’ve seen some school buildings with different outlets in those colors. I highly doubt that a school would actually be maintaining an isolated power system, at least in ordinary classrooms (maybe in some of the labs). It’s a code violation to use one of those colors on an ordinary outlet, right?
I understand that this is very common with AFCIs, which is part of the reason why they aren’t used very much.
It is not. The NEC does not prescribe any specific coloring scheme for receptacles. It just requires that isolated-ground receptacles have a “distinct color or marking”, and life-safety “Critical Operations Power Systems (COPS)” need both the “distinct color or marking” and an indicator light to show the presence of power.
Facilities such as hospitals may generally use the same conventions, but ultimately it’s officially facility-specific. If you want to install receptacles in a rainbow of colors in your house, that’s fine.
I vaguely recall that some computer facilities used a different colour for receptacles that were connected to a surge suppression system. Sometimes theres were also power backup (battery or generator) capable too.
I guess for the OP the question would be… how? Your ground is carrying a current? Either the current source is a segment of some electronics that is power limited, or it would quickly burn out the electronics. Obviously, a connection of minimal resistance between the 120V live and ground would produce some special fireworks; or melt something in short order. Hopefully, trip the breaker first. Maybe someone more up to date on electronics can answer, but it seems to me that most power sources for electronics are not designed to accomodate an extremely low-ohm load -something will go pop. Ditto if it’s a short from somewhere on the circuit board?
One of the possible fault conditions is a neutral wire shorting to a metal case. The return current would then be split between the neutral wire and the green safety ground wire.
It’s entirely possible that this type of fault might not burn anything out, and there wouldn’t be any “fireworks” like you’d get if the hot wire shorted to the case since the neutral and the safety ground are both at essentially the same electrical potential.
Nope, they’re common now. According to the 2023 NEC, AFCI protection is required for
• Family room
• Dining room
• Living room
• Bedroom
• Sunroom
• Library
• Den
• Office hallways
• Closets
• Recreation room
• Kitchen (except where otherwise noted)
GFCI protection is required for
• Bathroom
• Garage
• Porch
• Pool area
• Clothes dryer
• Kitchen
• Outdoors (except for use with listed HVAC equipment)
Dual function (DFCI) breakers protection can be used for
• Kitchen
• Dishwasher
• Clothes washer
• Laundry room or area
Well, those big ol’ caps on the DC rails gotta get charged up somehow.
(I’m also annoyed at big amps that have big inrush currents. The designers could have easily fix the problem by charging the caps through resistors at power-up, and then shorting across the resistors after a couple seconds using a time-delay relay.)
Would a receptacle tester identify the issue mentioned in the OP?