Question about the grounded neutral

Thread Starter

RickinFlorida

Joined Dec 28, 2008
1
In a typical split-phase AC circuit, one leg is grounded. I understand why it is. What I don't understand is why grounding one leg of an AC circuit doesn't short circuit the entire circuit. In other words, why don't all available electrons in the circuit, including from the hot leg, rush to the nearest ground electrode? Why would they continue to flow through the circuit, through the house and back to the ground tap on the distribution transformer? There are other grounding opportunities in the house for electrons to travel to. These grounding opportunities offer a low impedance to the flow of electrons.

Second, what would happen in your basic household AC circuit if both legs were grounded instead of just one? How would that affect the operation of appliances? Since we ground one for safety, why not ground both?

Thanks!

Rick
 

jpanhalt

Joined Jan 18, 2008
7,698
Second, what would happen in your basic household AC circuit if both legs were grounded instead of just one?
Rick
It would be a short lived show. BTW, I am not sure by leg whether you meant L1 or L2; neither of them is grounded.

Back to your original question. Consider typical DC wiring in airplanes, cars, etc. There is only one hot wire. The chassis is ground. Making the hot wire AC doesn't change that situation.

In homes, one could in theory run only a black wire, and ground all appliances at the point of use. That of course is not code and would be inconvienent at best to get a good ground at each appliance, so we run a white (grounded wire).

John
 

Wendy

Joined Mar 24, 2008
21,840
There is also the issue that without the neutral wire any other path is high resistance (enough to kill you, but not enough to run an appliance, Murphy's Law in action). Generally house wireing has three wires, Hot, Neutral (source and return), and Ground (for safety).

The 220VAC going into homes is a 2 phase system (the L1 and L2 referred to earlier), each leg by itself is actually a 120VAC system. Ground is usually created where the power company introduces power into your home, and is connected to neutral at this point.
 

SgtWookie

Joined Jul 17, 2007
22,201
In a typical split-phase AC circuit, one leg is grounded. I understand why it is.
Actually, I think you're talking about home wiring.
From the power company's transformer, you get three wires; L1, L2, and Neutral, the latter of which is the center tap of the transformer's secondary, while L1 and L2 are opposite ends of the secondary winding. Between L1 or L2 and Neutral, you'll measure 120VAC, but between L1 and L2 you'll measure 240V. That is because L1 and L2 are equal in voltage, but opposite in phase (180° out of phase).

Neutral is connected to earth ground at the breaker/distribution panel for safety. This is to ensure that the voltages don't float at some arbitrary high level. The breaker/distribution panel is the ONLY place where ground and Neutral should be connected.

What I don't understand is why grounding one leg of an AC circuit doesn't short circuit the entire circuit. In other words, why don't all available electrons in the circuit, including from the hot leg, rush to the nearest ground electrode?
If you happened to place a short circuit in one of your outlets, the power would go from the hot lead through to neutral, and rapidly trip the circuit breaker due to excess current.
Why would they continue to flow through the circuit, through the house and back to the ground tap on the distribution transformer? There are other grounding opportunities in the house for electrons to travel to. These grounding opportunities offer a low impedance to the flow of electrons.
You mean light bulbs, appliances, etc? Well, they limit the current flow through themselves according to their power rating.
Second, what would happen in your basic household AC circuit if both legs were grounded instead of just one? How would that affect the operation of appliances? Since we ground one for safety, why not ground both?
We ground Neutral for safety.

In a normal household electrical system, L1 is used for half of the circuits, and L2 is used for the other half. Hopefully, this results in a fairly well-balanced load overall.

For 240v appliances (such as electric dryers, ovens/stoves, perhaps air conditioners and heaters) L1 and L2 are used, and the chassis are connected to earth ground to protect the users in case there is a fault.
 

PRS

Joined Aug 24, 2008
989
I think both L1 and L2 are in phase with one another. Tha way, adding them doubles the voltage. If they were 180 degrees out of phase with each other they would add to zero volts.
 

blocco a spirale

Joined Jun 18, 2008
1,535
I think both L1 and L2 are in phase with one another. Tha way, adding them doubles the voltage. If they were 180 degrees out of phase with each other they would add to zero volts.
If L1 and L2 were in-phase with each other there would be no potential difference between them.
 

jpanhalt

Joined Jan 18, 2008
7,698
It is called split-phase --a term that has caused endless discussion and confusion. I have heard knowledgeable people call it "essentially" two phase, which to me makes it easier to grasp.

All About Circuits said:
Here is where the (+) and (-) polarity markings really become important. This notation is often used to reference the phasings of multiple AC voltage sources, so it is clear whether they are aiding (“boosting”) each other or opposing (“bucking”) each other. If not for these polarity markings, phase relations between multiple AC sources might be very confusing. Note that the split-phase sources in the schematic (each one 120 volts ∠ 0o), with polarity marks (+) to (-) just like series-aiding batteries can alternatively be represented as such: (Figure below)



If we mark the two sources' common connection point (the neutral wire) with the same polarity mark (-), we must express their relative phase shifts as being 180 ° apart(emplasis added). Otherwise, we'd be denoting two voltage sources in direct opposition with each other, which would give 0 volts between the two “hot” conductors.
John
 
In a typical split-phase AC circuit, one leg is grounded. I understand why it is. What I don't understand is why grounding one leg of an AC circuit doesn't short circuit the entire circuit. In other words, why don't all available electrons in the circuit, including from the hot leg, rush to the nearest ground electrode? Why would they continue to flow through the circuit, through the house and back to the ground tap on the distribution transformer? There are other grounding opportunities in the house for electrons to travel to. These grounding opportunities offer a low impedance to the flow of electrons.
In House wireing there are normally three holes in a socket, and appliances usually have three prongs. The two prongs at the top are for the power to an appliance. The third prong at the bottom is to a G.F.I (Ground Fault Interuptor). Basicly if there is a short in an appliance where the case of a power supply for instance is energized, electricity will flow to the (GFI) and trip a breaker to that plug so you don't die. :eek:
 

PRS

Joined Aug 24, 2008
989
I was taught that sinusoids of the same frequency add algebraically only when they are in phase, if out of phase you use vector analysis. If 180 degrees out of phase their maginitudes subtract. For example if one source was 120 volts and it was added to a source of 120 degrees and both are in phase, then you get 240 volts, and if out of phase you get zero volts.
 

PRS

Joined Aug 24, 2008
989
Your analysis is gimicky? put plus signs at the top of each source, negative at the bottom. Without splitting them yet, we have 240 volts between the positive terminal of the upper source and the negative terminal of the lower source. This is how we get 240 volts in the home. To get 120 volts we split them between the sources with a ground. This gives us two legs of 120 volts each.
 

studiot

Joined Nov 9, 2007
5,003
PRS, perhaps you are thinking of the Power equation

P = IV cosθ, where θ is the phase angle between current and voltage.
 

PRS

Joined Aug 24, 2008
989
No, I'm not a guru, but I think I'm right in this particular case. By the way, are you the same studiot as at CARM? I'm Paul Solberg there.
 

PRS

Joined Aug 24, 2008
989
Did you draw those sinusoids? Something tells me we are all at odds about definitions. I would draw the first example as correct.

Do you know how to add functions graphically? If so, just look at the second diagram. It adds to zero everywhere. On the other hand the first one doubles the voltage everywhere if both voltages are equal in magnitude.
 

Wendy

Joined Mar 24, 2008
21,840
The potential between T1 on the first peak is the same, 0V difference. You can't get work from that. Two sine waves side by side, depending on phase, can have no difference between them, or be 180 out of phase. This would create either 0V potential, or double the RMS value. Remember, these aren't two signals on one wire, but two wires each with it's own signal.

Yes, I drew them.

In an ungrounded system, if you were to hold the top example you wouldn't feel a thing.

Using the second example you would have the 240VAC we've been talking about, but each leg compared to neutral is 120VAC.

This is off topic, but you can do the same with solid state electronics to get an apparent increase if power out of two H bridges if they are out of phase.
 
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studiot

Joined Nov 9, 2007
5,003
I was taught that sinusoids of the same frequency add algebraically only when they are in phase,
PRS,

Bill is quite correct. You will need to get a hold of these concepts to progress in electricity.

Yes your teacher was also correct in your quote above.

But

Your teacher was talking about a different situation from Bill or the original poster.

The addition you refer to happens when the voltages are at a single point.

In this case we are talking about the voltage difference between two points.

What Bill is saying is that if two points (not one ) are at the same voltage there is no voltage difference between them.

What your teacher was saying was if two voltage generators are connected to the same point the resultant voltage, at that point, is the algebraic (or vector) sum of the generator voltages.

Incidentally look again at John Panhalt's explanation
It is good. Very good. Very very good.

He is referring to split phase domestic mains supplies. This is a very efficient distribution method used in some parts of the USA. Other parts are more like the UK, where we do not use this system.
 

Wendy

Joined Mar 24, 2008
21,840
I think I see where some of the confusion is, looking at the batteries in the drawing they are facing the same way, but compared to ground they are reversed. If the same polarity was connected to ground then there would not be any voltage out.

 

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floomdoggle

Joined Sep 1, 2008
217
KDcouture,
The bottom prong on an outlet is to ground. Only the hot is connected to a GFI, or GFCI. Or breaker, or fuse. And switches.
Dan
 

leftyretro

Joined Nov 25, 2008
394
KDcouture,
The bottom prong on an outlet is to ground. Only the hot is connected to a GFI, or GFCI. Or breaker, or fuse. And switches.
Dan
Off thread note:

Well actually a GFI has to be wired and see both hot and neutral conductors (but your right, not the ground terminal) to work.

It works by sensing if there is a unbalance between hot and neutral wire currents, and if unbalanced it implies that some hot current must be finding a alternate route through any grounded path.

On Thread note: L1 and L2 most certainly have a 180 degrees phase difference, else there would be no 240 volt to run my oven ;)

Lefty
 
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