plase…place; bother…both; I think I missed a question mark or two, as well.
Okay, so I butchered that post. I gotta learn to preview. 
No, the neutral should never be used as a ground.
There is current in the neutral wire at all times while the appliance is in service.
The ground wire will never have a current unless the case of the appliance becomes in contact with the 115 V hot wire and for a split second until the breaker trips there will be current flowing.
If there is no ground wire and the case is connected (grounded) to the neutral and for some reason the neutral wire becomes unconnected while the appliance is in service the case of the appliance would show 115 V. This **would not **throw the circuit breaker thus there is no protection from someone receiving electrical shock.
Does that make sense?
Electrician here;
Man, I love these threads, I get to answer some Q`s finally.
EngineerCompGeek, Some people may get Isolated GROUND outlets confused with an Isolated Electrical System, which has no grounds. The isolated Ground outlets are orange with a green triangle stamped on them. The isolated electrical system outlets are red. The reason I bring this up is I would never say it is safe to touch any hot wire - isolated or not. You just never know. Although in thoery you are correct.
Also, today you need TWO ground rods at a residential service. The water main must also be bonded to the earth ground at the same point in the panel.
As far as I know, you could never legally have used the water source as the only ground - at least one ground rod has always been required.
This is to assure that the entire power grid is at the same potential. This way your neihbor next to you or the business across the street isn`t sending stray ground currents into the earth that could find their way back to your ground rods or water supply. Everything originating from the same source should have the same earth or ground potential.
Basically what the Nuetral wire is is a controlled short. - The electrons get directed through some resistive element and need an earth ground as the potential difference to complete the circuit. In this case all current is directed through some resistive device first (toaster) and than to ground.
The actual ground wire acts as a safety shield for the metallic raceway or anything that may become accidentily energized (the toaster enclosure) and causes the circuit breaker or fuse to open. This is an uncontrolled short (opposite of the nuetral) and is not supposed to occur. There is very little resistance in this situation - causing the current to skyrocket and the circuit breaker or fuse to open.
Even though the nuetral and the ground wires are bonded together at the service panel and are at the same potential they are designed for different uses.
The nuetral for the controlled direction of electons to earth and the ground for the uncontrolled, unwanted current to earth.
I understand all that, olefin. I know that the ground wire is different from the neutral because it’s not meant to carry current, while the neutral one is.
But, to rephrase my point: Aren’t the ground and neutral wires electrically equivalent? They both are at 0V, and both end up in the ground. If you check resistance between the ground wire and neutral wire (in a circuit that’s been turned off at the breaker, of course), you find almost no resistance between those two receptacle terminals. It’s not quite zero, but pretty low. It at least shows they’re electrically connected.
So, if the ground and neutral wires effectively form a single node…what’s the point?
Well, I see that Uncommon Sense has answered my question.
Thanks…that’s been bugging me forever!
Huh? I think you’re confusing a few things here. If you are referring to delta-wound three phase motors, then yes, there is no neutral; each winding is connected between a pair of lines: A-B, B-C and C-C. If the line-to-neutral voltage in such a system is 120 V, then the line-to-line voltage will be 208 V (120 x SQRT(3)). However, a ground is not required for operation. As in other electrical installations, it is included for safety purposes. OTOH, a wye-wound three phase motor does require a neutral, with each of the windings connected between it and one of the lines: A-N, B-N and C-N. Again, a ground is included for safety.
If you are referring to single-phase 220 V motors, these will generally be wired similarly to other 220 V residential appliances. The two opposite-phase hot lines coming into the building from the pole transformer will have 220 V between then, with the center tap being bonded to Earth and serving as the neutral, thus providing two 110 V circuits. This is assuming such motors exist. I’ve never personally encounterd a single-phase 220 V motor.
They`re out there. Most residential well pumps are 220v single phase.
Here is one.
Well, there you go.
I was thinking the change happened in the late 60’s or early 70’s, but it looks like the change happened around 1960. Homes older than this will have only the water pipe ground, and then only if the electrical system hasn’t been upgraded since then. If a new panel was put in (to replace the old fuses with breakers, for example) then usually a ground rod will be installed at that time.
From here: http://www.ci.kirkland.wa.us/depart/fire_bldg/bldg/electricity_waterpipes.htm
Huh? 
No, I’m not talking about three phase motors… the discussion was about home electrical systems.
Your comment, “assuming such motors exist”. :eek:
Well, my air central conditioning unit, sewage lift pump and yard sprinkler pump all are wired with 2 110 V lines, no neutral and of course a ground to the motor case.
I’m only familiar with 2 central air units and both are wired the same.
BTW, I’m very familiar with 3 phase 12 KV 9000 HP motors.
To add another point in the discussion vis-a-vis ground and neutral:
Although the ground is connected at the panel and at the transformer, the ground conductor is soil; which has varying resistance depending on mineral makeup, moisture content, etc. The neutral, on the other hand, is a low resistance conductor. The lower the resistance of the conductor, the less line loss or to put it a bit more clearly, the less load on the circuit. One of the reasons that the utility runs a neutral as opposed to just using the ground is that using the ground would add additional load to the system that is of no benefit either to the customer or the utility.
More importantly, having a high resistance ground removes the reference point of the 120v circuits. This is a bit hard to explain, but if you’ve ever had a bad neutral connection in your panel or the service feeding your house, you would have experienced very marked fluctuations in your 120v circuits. Generally, you will see your lights alternately get very dim and the very bright. While this may or may not have an adverse affect on your lights themselves, these fluctuations in voltage wreak havoc on your appliances, generally burning out motors and frying electronic components.
The neutral point (or grounded center tap) on a 120/240 system provides a reference point on the coil of the transformer that will allow one half of the coil to carry more load that the other without adversely affecting the voltage (as engineer_comp_geek pointed out the 120v is the result of the ratio between the number of turns on the primary and secondary side of the transformer coils). Remove the reference point, and the single coil of the transformer will have an apparent midpoint (i.e. where load on one side of this midpoint equals load on the other side) that will affect the voltage to the extent that this apparent midpoint is removed from the physical center of the coil.
Electrical circuits are all about balance; the same amount coming out as is going in. Remove the reliable path for the neutral current to get back to the transformer, and lose control over the voltage.
In an ideal system, with an ideal transformer, you wouldn’t need a connection to earth ground.
But your system is grounded. This is because the transformer hanging out on the pole isn’t perfect. When I say it isn’t “perfect,” I’m saying that it doesn’t have perfect isolation. If we allowed the secondary to float, there’s a chance it could float up to the primary voltage. If this happened, the normal mode (i.e. differential) voltage would still be 120V/240V in the secondary, but the common mode voltage could be as high as the primary voltage (7000V or so).
To fix this problem we ground one of the secondary legs. Theoretically, it doesn’t matter which leg we connect to earth ground (hot-1, hot-2, or the center tap). But the NEC says we should always connect the center tap (i.e. neutral) to earth ground. That way, the highest voltage above earth ground is only 120 VAC RMS.
Many single phase motors are built to use either 110 or 220 by simply changing a connection in the motor. For example this one.
OK Crafter_Man, i’ll bite:
If, as you say, floating the secondary would cause the common mode voltage to drift up to the primary voltage, why don’t we see this in straight 3Ø delta (i.e. ungrounded) power applications?
The only commonplace instances of primary voltage on the secondary side that i am aware of occur in open-circuit conditions on a regulated output transformer (street-lighter) or current transformer.
If you could point me to an article or instance of this, i’d be much appreciative.
I installed a pump in the lake for yard sprinkler system… the motor was one of those that will run on 110 or 220. My choice was 220 due to wire size and efficiency.
I think probably the most common single phase 220 motors are home central air conditioning.