In San Bernardino County, CA the answer is yes, the metal box should be connected to the green ground wire.
If you have an electric drill, say with a metal housing the housing is connected to the safety ground, never, never, never to the white or neutral wire. The metal housing is a grounded chassis.
But, as I’m sure the OP will ask when (s)he next posts, why then join them at the fuse box? Surely that makes them the same circuit throughout the house? How can they then not be at the same potential (presumably 0)?
They are at the same potential at the place where they are joined. However, as was pointed out by engineer_com_geek the green ground carries no current ideally, and very little in actuality barring a defect on the wiring or an appliance and so it is at a uniform potential throughout.
However, the white, neutral ground does carry current, sometimes a lot. For example suppose we have two circuits of #12 (approx 2 mm dia.) copper wire, one that runs for a distance of 35 ft from the fuse box and is carrying 6 amp. and the other runs for 40 ft from the fuse box and is carrying 15 amp. The voltage between the green and wite wires at the far end of the first circuit will be 0.325 V. The voltage between the green and white wires at the far end in the second case will be o.93 V. All through the white, neutral wire network the voltage between green and white wires will vary from 0 at their point of connection at the fuse box to whatever the current*wire resistance is from the fuse box to the point in question.
That’s what I’m asking, not sure if it does or not. I just know that sometimes making the hot first can damage some delicate equipment.
Open Neutral;
This condition has received virtually no national attention, but it can literally ruin almost every appliance in the home. It occurs almost exclusively on residential 120/240V systems. The electric service includes two ‘hot’ legs, L1 and L2, and a neutral, N. Either hot leg and neutral provide 120V power to the home. Power for 240V equipment, typically air conditioners and clothes dryers, is obtained between L1 and L2.The neutral is grounded at the utility transformer and again at the service entrance. The National Electrical Code®, NEC®, identifies this as the grounded conductor since it is required to be grounded to the grounding electrode (i.e. ground rod) via the grounding electrode conductor (GEC).When this connection becomes loose, voltages in the house may fluctuate up and downdepending on the balance of load between the two phases. They are rarely balanced. When a customer reports that their lights are “bright and dim”, it is almost a sure sign that the neutral is loose. Sometimes the neutral connection is totally broken. This may happen over time due to a tree limb rubbing through it or corrosion in the connector. It can happen abruptly due to heavy icing or a large branch falling on the conductor.This condition can subject your 120V electrical equipment to voltage fluctuations between zero and 240V. In PQ terms, this is NOT a surge; it is an overvoltage. Most TVSS are as helpless in this condition as the appliance itself. Some TVSS literally explode, emitting smoke and flames. This can be even more dangerous than the overvoltage itself. Cases have occurred where carpet and wallpaper has been ruined because the fire department was called to put out a fire in the TVSS.Some TVSS actually sacrifice themselves and at least protect downstream equipment. Most TVSS utilize metal oxide varistors (MOV) for clamping transients. The TVSS has a clamping voltage based on peak voltage for milliseconds. --------------------------------------------------------------------------------
All I’m trying to get at is if the plug makes the hot connection first, you will have a split second of ‘open neutral’. Don’t know if it’s a big deal and I don’t mean to drag the point out, but that’s why we have this oportunity here, might as well use it to learn something (I’m referring to myself).
What Mr. Simmons said and maybe even more simply stated; because the point of conection at the fuse box is bonded to the ground rods outside the home and ALSO to the main water supply (if it’s in a metal conduit). This is supposed to create a ground connection with very little resistance, one which the electrons are sure to flow to. If we bond the neutral and the ground anywhere BUT the service then there is the potential for current on the safety ground somewhere else in the home. We don’t want that, ever.
Because the bonding is done at the service only, then we are mostly assured that the system will function safely and consistently.
A grounded chasis (also called the yoke) may also refer to the support structure of the device itself (switch or outlet) which are made up of metal parts and those metal parts are supposed to be bonded to the box, the green ground wire and then back to the panel through the use of the grounding conductor (green, or bare copper wire).
You speak of current and then you drift into fluctuating voltages, sort of confusing.
Dumbed down:
The neutral is designed to carry the RETURN current in any given circuit. Sort of like the unused energy, but not really, but for the sake of explanation we can call it that. If you have a single circuit using 10 amps at 120 volts then you will have 10 amps and whatever the voltage is calulated using E=I*R on the neutral. So, on this particular neutral you will have 10 amps and very little voltage of unused energy returning on the neutral. We’ll pretend the voltage got used up in the light bulbs and the current is trying to get back home.
If you have two circuits sharing the same neutral then you will have the DIFFERENCE between the two currents on the neutral. Circuit A is using 6 amps and circuit B is using 14 amps. The neutral will have 8 amps returning home on it and whatever the voltage is calculated at, but probably not much. This difference is due to the two circuits being 180 degrees out of phase with each other, basically canceling each other out.
Now, this is all good and well untill you OPEN the neutral at some point and piss off the current electrons that are trying to go home. You get fluctuating voltages throughout the curcuit too. Not a good idea.
If the engineers would like to go into more detail and discuss stuff like harmonics, I’m all ears. 
Return? From where?
Current also returns on the hot. Many times per second. It’s AC, after all. 
I think it’s easier to describe it this way:
Let’s say you have a 120 V battery setting on the grass in the middle of your lawn. This means there’s 120 VDC between the two terminals. Note also that it’s an isolated voltage; the entire battery (including both terminals) is isolated from earth ground. Consequently, grounding yourself and touching either terminal (but not both!) will not give you a shock. If someone asks you what the voltage is between the positive terminal and earth ground, or the negative terminal and earth ground, you would answer as follows: “I have no idea. It’s probably very small. Or it could have a sizable static voltage between the terminal and earth ground. At any rate, it doesn’t matter.”
Now let’s connect one of the terminals to earth ground. It really doesn’t matter which terminal we choose; we’ll pick the positive terminal just for the heck-of-it. Note that the situation has now changed: if your feet are touching earth ground, and you touch the battery’s positive terminal, you will not get shocked. This is because there’s a wire between earth ground and the positive terminal. But if your feet are touching earth ground and you touch the battery’s negative terminal, you will get shocked.
Your home wiring system is basically the same set up, except that instead of a DC battery you have an AC battery that never needs recharged (a.k.a. the secondary of a transformer). An AC battery doesn’t have terminals we can permanently designate as “positive” and “negative.” This is because they’re constantly switching hats. (Terminal A is positive and terminal B is negative. And then a few milliseconds later Terminal A is negative and terminal B is positive. And then a few milliseconds later Terminal A is positive and terminal B is negative. And then a few milliseconds later Terminal A is negative and terminal B is positive…) But like our DC battery example, we still need to connect one of the terminals to earth ground. But which one? Does it matter?
If you have a regular 'ol two terminal AC battery (a.k.a. the secondary of a transformer w/o a center tap), then the answer is “No” – go head and connect either terminal to earth ground. But if you have a fancy-dancy three-terminal battery (a.k.a. the secondary of a transformer w/ center tap), you could still connect any one of the three terminals to earth ground, but it would be best to connect the “center.” That way, the highest voltage relative to earth ground is only 120 VAC. (If you connect either of the other two terminals to earth ground, the highest voltage relative to earth ground would be 240 VAC, which would be less safe.)
I knew I’d get hung for using that term, but some (most) people can grasp the concept better if you walk them through the circuit in a manner that resembles something of a line (from point A to point B).
Then you can whip 'em up after they grasp the general idea.
When it comes down to it, the only difference between the hot and neutral is that the neutral is referenced to earth ground.
And of course, there’s the question of why we connect the neutral to earth ground. I mean, wouldn’t your home’s AC system be safer if it were not referenced to earth ground?
There is a reason it is done, but it is not obvious…
Alright, I’ve now gone over this thread several times and I think I’m getting it. A couple statements are still unclear, though.
There seem to be 2 main reasons for the ground.
= The safety issue I kind of understand- if it gets interupted or if you touch it somewhere if there’s current in it, the you could get zapped. I have no intention of joining ground to neutral before the fuse box, but it still seems like a low probability type of event
= The electrical armegeddon I still don’t understand.
I installed GFCI outlets in the ungrounded outlets. Is there a safety benefit to installing GFCI in all ungrounded outlets (about 5), instead of just the first one?
The transformer outside of your house is going to have the effect of isolating you from the power system’s ground system. You need to ground your side of the transformer.
The power plant also grounds its side of things, and different segments of the transmission system (power lines, transformers, etc) will also be grounded.
Technically, an ungrounded (or isolated) system is safer. In an isolated system, you can touch either wire and earth ground and not get shocked. The problem with running large complex isolated systems it that mother nature tends to occasionally find some way of grounding part of your system for you, so what you end up with is a randomly grounded system instead of an isolated system. A predictably grounded system is better than a randomly grounded one, so that’s what we use. There are places where isolated power is used though. Hospitals use isolated power. If you are unfortunate enough to be in a hospital, look for the red outlets. They are isolated. Hospitals go through great pains to keep their isolated power systems isolated, and its difficult enough for them to keep it isolated and they only have one building to deal with, not thousands of buildings like the power company does.
FYI there’s a handbook of isolated power, similar to the handbook for the national electric code. The introduction of the book spends several pages on the history of isolated power, and on electrical safety and grounding in general, and hits on many of the topics discussed in this thread.
I now this sounds strange, but current never really flows into the 4-foot ground rod that attaches to your breaker box. At least not much. The rod and copper wire are installed so that your home wiring system is “referenced” to earth ground.
To expand on what ECG said, the real reason we ground the secondary of the transformer is because, if we didn’t, the common-mode voltage of the secondary winding could float up to the primary voltage. This would not be a good thing, as you might image; while there would still be 120 VAC between your hot and neutral lines, there would be thousands of volts between either conductor and earth ground. And why would it float up? Because transformers do not exhibit perfect isolation; there’s always a little bit of leakage or coupling impedance between the primary and secondary windings.
My question was not supposed to be “why join them at the fusebox rather than elsewhere”, but “why join them at all (in practice this happpens to be at the fusebox)”.
As Crafter_man said one side of the power line is connected to an earth ground for safety reasons. Without that ground there is no way of knowing what the voltage of the power lines into your house would be relative to the ground voltage. It could be hundreds of thousands of volts, or whatever the insulating properties of the air and building materials will allow. And that’s important because you are quite often electrically connected to an earth ground and in proximity to the electrical outlets in your house. So you want to ensure that no part of your wiring is at any voltage different from that of the earth around you by more than the nominal system voltage, or 120 V in the case of US residences.
Oops. The peak voltage relative the earth ground for US residences is 170 V.
Another reason to Bond the Neutral at the service;
According to the NEC, metal parts of service equipment shall be grounded to the earth. The purpose of grounding the metal parts to the earth is to protect persons and property from fires by limiting voltage on the metal parts from lightning [250.4(A)(2)].
In addition, the grounded (neutral) service conductor shall be grounded to the earth at service equipment. For the purpose of limiting the voltage imposed by lightning, line surges, or unintentional contact with higher voltage lines. Grounding is intended to shunt potentially dangerous energy into the earth from the system [250.4(A)(1)]. Brackets are NEC code sections.
Also;
One last point, if the grounded (neutral) service conductor (which serves as the effective ground-fault current path) is opened or not provided at all (this happens when people think that a neutral is not required if there are no line-to-neutral loads), a ground fault cannot be cleared. The result is that metal parts of electrical equipment, as well as metal piping and structure steel will become and remain energized at line voltage. In this case 120V.
If the grounded (neutral) service conductor is open, neutral current will flow onto the metal parts of the electrical system. When this occurs in a wood frame construction building, neutral current seeking a return path to the power supply will travel into the moist wood members. After many years of this current flow, the wood will be converted into charcoal (wood with no moisture) and ultimately it can result in a fire. This condition is called pyroforic-carbonization.
Sorry, if I’m dwelling too much on OPEN NEUTRALS but I have seen my share of them and the dangers that they pose, both for the equipment and the persons involved.
No, as long as they are in the same two wire circuit and ‘downstream’ from each other you only need the one. Adding more will NOT increase the safety factor and I believe will only cause nuisance tripping of the GFCI’s.
If you need a diagram I can find one online and link to it.
David’s response in the second post was directly referring to the conditions in the OP and he is correct. By using the neutral as the ground on a GFCI you are essentially making the ‘neutral to ground bond’ outside of the main service.
You are also now combining the neutral and the ground in the system and in the electroncs of the GFCI, thereby defeating the design of the GFCI.
This has to do with the way the GFCI itself works and Crafter and/or David would know more about that.
As far as grounding a GFCI receptacle, I see nothing wrong with it… if there’s a ground wire already run to the outlet box, then you will want to hook it up to the GFCI receptacle’s ground screw. That way, you’re protected in two ways.